Just-in-time background writing strategy. This code avoids re-scanning

[postgresql] / src / backend / postmaster / pgstat.c

/* ----------
 * pgstat.c
 *
 *      All the statistics collector stuff hacked up in one big, ugly file.
 *
 *      TODO:   - Separate collector, postmaster and backend stuff
 *                        into different files.
 *
 *                      - Add some automatic call for pgstat vacuuming.
 *
 *                      - Add a pgstat config column to pg_database, so this
 *                        entire thing can be enabled/disabled on a per db basis.
 *
 *      Copyright (c) 2001-2007, PostgreSQL Global Development Group
 *
 *      $PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.166 2007/09/25 20:03:37 tgl Exp $
 * ----------
 */
#include "postgres.h"

#include <unistd.h>
#include <fcntl.h>
#include <sys/param.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <time.h>
#ifdef HAVE_POLL_H
#include <poll.h>
#endif
#ifdef HAVE_SYS_POLL_H
#include <sys/poll.h>
#endif

#include "pgstat.h"

#include "access/heapam.h"
#include "access/transam.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "catalog/pg_database.h"
#include "libpq/ip.h"
#include "libpq/libpq.h"
#include "libpq/pqsignal.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "postmaster/fork_process.h"
#include "postmaster/postmaster.h"
#include "storage/backendid.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"
#include "storage/pmsignal.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"


/* ----------
 * Paths for the statistics files (relative to installation's $PGDATA).
 * ----------
 */
#define PGSTAT_STAT_FILENAME    "global/pgstat.stat"
#define PGSTAT_STAT_TMPFILE             "global/pgstat.tmp"

/* ----------
 * Timer definitions.
 * ----------
 */
#define PGSTAT_STAT_INTERVAL    500             /* How often to write the status file;
                                                                                 * in milliseconds. */

#define PGSTAT_RESTART_INTERVAL 60              /* How often to attempt to restart a
                                                                                 * failed statistics collector; in
                                                                                 * seconds. */

#define PGSTAT_SELECT_TIMEOUT   2               /* How often to check for postmaster
                                                                                 * death; in seconds. */


/* ----------
 * The initial size hints for the hash tables used in the collector.
 * ----------
 */
#define PGSTAT_DB_HASH_SIZE             16
#define PGSTAT_TAB_HASH_SIZE    512


/* ----------
 * GUC parameters
 * ----------
 */
bool            pgstat_track_activities = false;
bool            pgstat_track_counts = false;

/*
 * BgWriter global statistics counters (unused in other processes).
 * Stored directly in a stats message structure so it can be sent
 * without needing to copy things around.  We assume this inits to zeroes.
 */
PgStat_MsgBgWriter BgWriterStats;

/* ----------
 * Local data
 * ----------
 */
NON_EXEC_STATIC int pgStatSock = -1;

static struct sockaddr_storage pgStatAddr;

static time_t last_pgstat_start_time;

static bool pgStatRunningInCollector = false;

/*
 * Structures in which backends store per-table info that's waiting to be
 * sent to the collector.
 *
 * NOTE: once allocated, TabStatusArray structures are never moved or deleted
 * for the life of the backend.  Also, we zero out the t_id fields of the
 * contained PgStat_TableStatus structs whenever they are not actively in use.
 * This allows relcache pgstat_info pointers to be treated as long-lived data,
 * avoiding repeated searches in pgstat_initstats() when a relation is
 * repeatedly opened during a transaction.
 */
#define TABSTAT_QUANTUM         100                     /* we alloc this many at a time */

typedef struct TabStatusArray
{
        struct TabStatusArray *tsa_next;        /* link to next array, if any */
        int                     tsa_used;                               /* # entries currently used */
        PgStat_TableStatus tsa_entries[TABSTAT_QUANTUM];        /* per-table data */
} TabStatusArray;

static TabStatusArray *pgStatTabList = NULL;

/*
 * Tuple insertion/deletion counts for an open transaction can't be propagated
 * into PgStat_TableStatus counters until we know if it is going to commit
 * or abort.  Hence, we keep these counts in per-subxact structs that live
 * in TopTransactionContext.  This data structure is designed on the assumption
 * that subxacts won't usually modify very many tables.
 */
typedef struct PgStat_SubXactStatus
{
        int                     nest_level;                             /* subtransaction nest level */
        struct PgStat_SubXactStatus *prev;      /* higher-level subxact if any */
        PgStat_TableXactStatus *first;          /* head of list for this subxact */
} PgStat_SubXactStatus;

static PgStat_SubXactStatus *pgStatXactStack = NULL;

static int      pgStatXactCommit = 0;
static int      pgStatXactRollback = 0;

/* Record that's written to 2PC state file when pgstat state is persisted */
typedef struct TwoPhasePgStatRecord
{
        PgStat_Counter tuples_inserted; /* tuples inserted in xact */
        PgStat_Counter tuples_deleted;  /* tuples deleted in xact */
        Oid                     t_id;                           /* table's OID */
        bool            t_shared;                       /* is it a shared catalog? */
} TwoPhasePgStatRecord;

/*
 * Info about current "snapshot" of stats file
 */
static MemoryContext pgStatLocalContext = NULL;
static HTAB *pgStatDBHash = NULL;
static PgBackendStatus *localBackendStatusTable = NULL;
static int      localNumBackends = 0;

/*
 * Cluster wide statistics, kept in the stats collector.
 * Contains statistics that are not collected per database
 * or per table.
 */
static PgStat_GlobalStats globalStats;

static volatile bool need_exit = false;
static volatile bool need_statwrite = false;


/* ----------
 * Local function forward declarations
 * ----------
 */
#ifdef EXEC_BACKEND
static pid_t pgstat_forkexec(void);
#endif

NON_EXEC_STATIC void PgstatCollectorMain(int argc, char *argv[]);
static void pgstat_exit(SIGNAL_ARGS);
static void force_statwrite(SIGNAL_ARGS);
static void pgstat_beshutdown_hook(int code, Datum arg);

static PgStat_StatDBEntry *pgstat_get_db_entry(Oid databaseid, bool create);
static void pgstat_write_statsfile(void);
static HTAB *pgstat_read_statsfile(Oid onlydb);
static void backend_read_statsfile(void);
static void pgstat_read_current_status(void);

static void pgstat_send_tabstat(PgStat_MsgTabstat *tsmsg);
static HTAB *pgstat_collect_oids(Oid catalogid);

static PgStat_TableStatus *get_tabstat_entry(Oid rel_id, bool isshared);

static void pgstat_setup_memcxt(void);

static void pgstat_setheader(PgStat_MsgHdr *hdr, StatMsgType mtype);
static void pgstat_send(void *msg, int len);

static void pgstat_recv_tabstat(PgStat_MsgTabstat *msg, int len);
static void pgstat_recv_tabpurge(PgStat_MsgTabpurge *msg, int len);
static void pgstat_recv_dropdb(PgStat_MsgDropdb *msg, int len);
static void pgstat_recv_resetcounter(PgStat_MsgResetcounter *msg, int len);
static void pgstat_recv_autovac(PgStat_MsgAutovacStart *msg, int len);
static void pgstat_recv_vacuum(PgStat_MsgVacuum *msg, int len);
static void pgstat_recv_analyze(PgStat_MsgAnalyze *msg, int len);
static void pgstat_recv_bgwriter(PgStat_MsgBgWriter *msg, int len);


/* ------------------------------------------------------------
 * Public functions called from postmaster follow
 * ------------------------------------------------------------
 */

/* ----------
 * pgstat_init() -
 *
 *      Called from postmaster at startup. Create the resources required
 *      by the statistics collector process.  If unable to do so, do not
 *      fail --- better to let the postmaster start with stats collection
 *      disabled.
 * ----------
 */
void
pgstat_init(void)
{
        ACCEPT_TYPE_ARG3 alen;
        struct addrinfo *addrs = NULL,
                           *addr,
                                hints;
        int                     ret;
        fd_set          rset;
        struct timeval tv;
        char            test_byte;
        int                     sel_res;
        int                     tries = 0;

#define TESTBYTEVAL ((char) 199)

        /*
         * Create the UDP socket for sending and receiving statistic messages
         */
        hints.ai_flags = AI_PASSIVE;
        hints.ai_family = PF_UNSPEC;
        hints.ai_socktype = SOCK_DGRAM;
        hints.ai_protocol = 0;
        hints.ai_addrlen = 0;
        hints.ai_addr = NULL;
        hints.ai_canonname = NULL;
        hints.ai_next = NULL;
        ret = pg_getaddrinfo_all("localhost", NULL, &hints, &addrs);
        if (ret || !addrs)
        {
                ereport(LOG,
                                (errmsg("could not resolve \"localhost\": %s",
                                                gai_strerror(ret))));
                goto startup_failed;
        }

        /*
         * On some platforms, pg_getaddrinfo_all() may return multiple addresses
         * only one of which will actually work (eg, both IPv6 and IPv4 addresses
         * when kernel will reject IPv6).  Worse, the failure may occur at the
         * bind() or perhaps even connect() stage.      So we must loop through the
         * results till we find a working combination. We will generate LOG
         * messages, but no error, for bogus combinations.
         */
        for (addr = addrs; addr; addr = addr->ai_next)
        {
#ifdef HAVE_UNIX_SOCKETS
                /* Ignore AF_UNIX sockets, if any are returned. */
                if (addr->ai_family == AF_UNIX)
                        continue;
#endif

                if (++tries > 1)
                        ereport(LOG,
                        (errmsg("trying another address for the statistics collector")));

                /*
                 * Create the socket.
                 */
                if ((pgStatSock = socket(addr->ai_family, SOCK_DGRAM, 0)) < 0)
                {
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                        errmsg("could not create socket for statistics collector: %m")));
                        continue;
                }

                /*
                 * Bind it to a kernel assigned port on localhost and get the assigned
                 * port via getsockname().
                 */
                if (bind(pgStatSock, addr->ai_addr, addr->ai_addrlen) < 0)
                {
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                          errmsg("could not bind socket for statistics collector: %m")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                alen = sizeof(pgStatAddr);
                if (getsockname(pgStatSock, (struct sockaddr *) & pgStatAddr, &alen) < 0)
                {
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                                         errmsg("could not get address of socket for statistics collector: %m")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                /*
                 * Connect the socket to its own address.  This saves a few cycles by
                 * not having to respecify the target address on every send. This also
                 * provides a kernel-level check that only packets from this same
                 * address will be received.
                 */
                if (connect(pgStatSock, (struct sockaddr *) & pgStatAddr, alen) < 0)
                {
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                        errmsg("could not connect socket for statistics collector: %m")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                /*
                 * Try to send and receive a one-byte test message on the socket. This
                 * is to catch situations where the socket can be created but will not
                 * actually pass data (for instance, because kernel packet filtering
                 * rules prevent it).
                 */
                test_byte = TESTBYTEVAL;

retry1:
                if (send(pgStatSock, &test_byte, 1, 0) != 1)
                {
                        if (errno == EINTR)
                                goto retry1;    /* if interrupted, just retry */
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                                         errmsg("could not send test message on socket for statistics collector: %m")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                /*
                 * There could possibly be a little delay before the message can be
                 * received.  We arbitrarily allow up to half a second before deciding
                 * it's broken.
                 */
                for (;;)                                /* need a loop to handle EINTR */
                {
                        FD_ZERO(&rset);
                        FD_SET(pgStatSock, &rset);
                        tv.tv_sec = 0;
                        tv.tv_usec = 500000;
                        sel_res = select(pgStatSock + 1, &rset, NULL, NULL, &tv);
                        if (sel_res >= 0 || errno != EINTR)
                                break;
                }
                if (sel_res < 0)
                {
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                                         errmsg("select() failed in statistics collector: %m")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }
                if (sel_res == 0 || !FD_ISSET(pgStatSock, &rset))
                {
                        /*
                         * This is the case we actually think is likely, so take pains to
                         * give a specific message for it.
                         *
                         * errno will not be set meaningfully here, so don't use it.
                         */
                        ereport(LOG,
                                        (errcode(ERRCODE_CONNECTION_FAILURE),
                                         errmsg("test message did not get through on socket for statistics collector")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                test_byte++;                    /* just make sure variable is changed */

retry2:
                if (recv(pgStatSock, &test_byte, 1, 0) != 1)
                {
                        if (errno == EINTR)
                                goto retry2;    /* if interrupted, just retry */
                        ereport(LOG,
                                        (errcode_for_socket_access(),
                                         errmsg("could not receive test message on socket for statistics collector: %m")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                if (test_byte != TESTBYTEVAL)   /* strictly paranoia ... */
                {
                        ereport(LOG,
                                        (errcode(ERRCODE_INTERNAL_ERROR),
                                         errmsg("incorrect test message transmission on socket for statistics collector")));
                        closesocket(pgStatSock);
                        pgStatSock = -1;
                        continue;
                }

                /* If we get here, we have a working socket */
                break;
        }

        /* Did we find a working address? */
        if (!addr || pgStatSock < 0)
                goto startup_failed;

        /*
         * Set the socket to non-blocking IO.  This ensures that if the collector
         * falls behind, statistics messages will be discarded; backends won't
         * block waiting to send messages to the collector.
         */
        if (!pg_set_noblock(pgStatSock))
        {
                ereport(LOG,
                                (errcode_for_socket_access(),
                                 errmsg("could not set statistics collector socket to nonblocking mode: %m")));
                goto startup_failed;
        }

        pg_freeaddrinfo_all(hints.ai_family, addrs);

        return;

startup_failed:
        ereport(LOG,
          (errmsg("disabling statistics collector for lack of working socket")));

        if (addrs)
                pg_freeaddrinfo_all(hints.ai_family, addrs);

        if (pgStatSock >= 0)
                closesocket(pgStatSock);
        pgStatSock = -1;

        /*
         * Adjust GUC variables to suppress useless activity, and for debugging
         * purposes (seeing track_counts off is a clue that we failed here).
         * We use PGC_S_OVERRIDE because there is no point in trying to turn it
         * back on from postgresql.conf without a restart.
         */
        SetConfigOption("track_counts", "off", PGC_INTERNAL, PGC_S_OVERRIDE);
}

/*
 * pgstat_reset_all() -
 *
 * Remove the stats file.  This is currently used only if WAL
 * recovery is needed after a crash.
 */
void
pgstat_reset_all(void)
{
        unlink(PGSTAT_STAT_FILENAME);
}

#ifdef EXEC_BACKEND

/*
 * pgstat_forkexec() -
 *
 * Format up the arglist for, then fork and exec, statistics collector process
 */
static pid_t
pgstat_forkexec(void)
{
        char       *av[10];
        int                     ac = 0;

        av[ac++] = "postgres";
        av[ac++] = "--forkcol";
        av[ac++] = NULL;                        /* filled in by postmaster_forkexec */

        av[ac] = NULL;
        Assert(ac < lengthof(av));

        return postmaster_forkexec(ac, av);
}
#endif   /* EXEC_BACKEND */


/*
 * pgstat_start() -
 *
 *      Called from postmaster at startup or after an existing collector
 *      died.  Attempt to fire up a fresh statistics collector.
 *
 *      Returns PID of child process, or 0 if fail.
 *
 *      Note: if fail, we will be called again from the postmaster main loop.
 */
int
pgstat_start(void)
{
        time_t          curtime;
        pid_t           pgStatPid;

        /*
         * Check that the socket is there, else pgstat_init failed and we can
         * do nothing useful.
         */
        if (pgStatSock < 0)
                return 0;

        /*
         * Do nothing if too soon since last collector start.  This is a safety
         * valve to protect against continuous respawn attempts if the collector
         * is dying immediately at launch.      Note that since we will be re-called
         * from the postmaster main loop, we will get another chance later.
         */
        curtime = time(NULL);
        if ((unsigned int) (curtime - last_pgstat_start_time) <
                (unsigned int) PGSTAT_RESTART_INTERVAL)
                return 0;
        last_pgstat_start_time = curtime;

        /*
         * Okay, fork off the collector.
         */
#ifdef EXEC_BACKEND
        switch ((pgStatPid = pgstat_forkexec()))
#else
        switch ((pgStatPid = fork_process()))
#endif
        {
                case -1:
                        ereport(LOG,
                                        (errmsg("could not fork statistics collector: %m")));
                        return 0;

#ifndef EXEC_BACKEND
                case 0:
                        /* in postmaster child ... */
                        /* Close the postmaster's sockets */
                        ClosePostmasterPorts(false);

                        /* Lose the postmaster's on-exit routines */
                        on_exit_reset();

                        /* Drop our connection to postmaster's shared memory, as well */
                        PGSharedMemoryDetach();

                        PgstatCollectorMain(0, NULL);
                        break;
#endif

                default:
                        return (int) pgStatPid;
        }

        /* shouldn't get here */
        return 0;
}

void allow_immediate_pgstat_restart(void)
{
                last_pgstat_start_time = 0;
}

/* ------------------------------------------------------------
 * Public functions used by backends follow
 *------------------------------------------------------------
 */


/* ----------
 * pgstat_report_tabstat() -
 *
 *      Called from tcop/postgres.c to send the so far collected per-table
 *      access statistics to the collector.  Note that this is called only
 *      when not within a transaction, so it is fair to use transaction stop
 *      time as an approximation of current time.
 * ----------
 */
void
pgstat_report_tabstat(bool force)
{
        /* we assume this inits to all zeroes: */
        static const PgStat_TableCounts all_zeroes;
        static TimestampTz last_report = 0;     

        TimestampTz now;
        PgStat_MsgTabstat regular_msg;
        PgStat_MsgTabstat shared_msg;
        TabStatusArray *tsa;
        int                     i;

        /* Don't expend a clock check if nothing to do */
        if (pgStatTabList == NULL ||
                pgStatTabList->tsa_used == 0)
                return;

        /*
         * Don't send a message unless it's been at least PGSTAT_STAT_INTERVAL
         * msec since we last sent one, or the caller wants to force stats out.
         */
        now = GetCurrentTransactionStopTimestamp();
        if (!force &&
                !TimestampDifferenceExceeds(last_report, now, PGSTAT_STAT_INTERVAL))
                return;
        last_report = now;

        /*
         * Scan through the TabStatusArray struct(s) to find tables that actually
         * have counts, and build messages to send.  We have to separate shared
         * relations from regular ones because the databaseid field in the
         * message header has to depend on that.
         */
        regular_msg.m_databaseid = MyDatabaseId;
        shared_msg.m_databaseid = InvalidOid;
        regular_msg.m_nentries = 0;
        shared_msg.m_nentries = 0;

        for (tsa = pgStatTabList; tsa != NULL; tsa = tsa->tsa_next)
        {
                for (i = 0; i < tsa->tsa_used; i++)
                {
                        PgStat_TableStatus *entry = &tsa->tsa_entries[i];
                        PgStat_MsgTabstat *this_msg;
                        PgStat_TableEntry *this_ent;

                        /* Shouldn't have any pending transaction-dependent counts */
                        Assert(entry->trans == NULL);

                        /*
                         * Ignore entries that didn't accumulate any actual counts,
                         * such as indexes that were opened by the planner but not used.
                         */
                        if (memcmp(&entry->t_counts, &all_zeroes,
                                           sizeof(PgStat_TableCounts)) == 0)
                                continue;
                        /*
                         * OK, insert data into the appropriate message, and send if full.
                         */
                        this_msg = entry->t_shared ? &shared_msg : &regular_msg;
                        this_ent = &this_msg->m_entry[this_msg->m_nentries];
                        this_ent->t_id = entry->t_id;
                        memcpy(&this_ent->t_counts, &entry->t_counts,
                                   sizeof(PgStat_TableCounts));
                        if (++this_msg->m_nentries >= PGSTAT_NUM_TABENTRIES)
                        {
                                pgstat_send_tabstat(this_msg);
                                this_msg->m_nentries = 0;
                        }
                }
                /* zero out TableStatus structs after use */
                MemSet(tsa->tsa_entries, 0,
                           tsa->tsa_used * sizeof(PgStat_TableStatus));
                tsa->tsa_used = 0;
        }

        /*
         * Send partial messages.  If force is true, make sure that any pending
         * xact commit/abort gets counted, even if no table stats to send.
         */
        if (regular_msg.m_nentries > 0 ||
                (force && (pgStatXactCommit > 0 || pgStatXactRollback > 0)))
                pgstat_send_tabstat(&regular_msg);
        if (shared_msg.m_nentries > 0)
                pgstat_send_tabstat(&shared_msg);
}

/*
 * Subroutine for pgstat_report_tabstat: finish and send a tabstat message
 */
static void
pgstat_send_tabstat(PgStat_MsgTabstat *tsmsg)
{
        int                     n;
        int                     len;

        /* It's unlikely we'd get here with no socket, but maybe not impossible */
        if (pgStatSock < 0)
                return;

        /*
         * Report accumulated xact commit/rollback whenever we send a normal
         * tabstat message
         */
        if (OidIsValid(tsmsg->m_databaseid))
        {
                tsmsg->m_xact_commit = pgStatXactCommit;
                tsmsg->m_xact_rollback = pgStatXactRollback;
                pgStatXactCommit = 0;
                pgStatXactRollback = 0;
        }
        else
        {
                tsmsg->m_xact_commit = 0;
                tsmsg->m_xact_rollback = 0;
        }

        n = tsmsg->m_nentries;
        len = offsetof(PgStat_MsgTabstat, m_entry[0]) +
                n * sizeof(PgStat_TableEntry);

        pgstat_setheader(&tsmsg->m_hdr, PGSTAT_MTYPE_TABSTAT);
        pgstat_send(tsmsg, len);
}


/* ----------
 * pgstat_vacuum_tabstat() -
 *
 *      Will tell the collector about objects he can get rid of.
 * ----------
 */
void
pgstat_vacuum_tabstat(void)
{
        HTAB       *htab;
        PgStat_MsgTabpurge msg;
        HASH_SEQ_STATUS hstat;
        PgStat_StatDBEntry *dbentry;
        PgStat_StatTabEntry *tabentry;
        int                     len;

        if (pgStatSock < 0)
                return;

        /*
         * If not done for this transaction, read the statistics collector stats
         * file into some hash tables.
         */
        backend_read_statsfile();

        /*
         * Read pg_database and make a list of OIDs of all existing databases
         */
        htab = pgstat_collect_oids(DatabaseRelationId);

        /*
         * Search the database hash table for dead databases and tell the
         * collector to drop them.
         */
        hash_seq_init(&hstat, pgStatDBHash);
        while ((dbentry = (PgStat_StatDBEntry *) hash_seq_search(&hstat)) != NULL)
        {
                Oid                     dbid = dbentry->databaseid;

                CHECK_FOR_INTERRUPTS();

                /* the DB entry for shared tables (with InvalidOid) is never dropped */
                if (OidIsValid(dbid) &&
                        hash_search(htab, (void *) &dbid, HASH_FIND, NULL) == NULL)
                        pgstat_drop_database(dbid);
        }

        /* Clean up */
        hash_destroy(htab);

        /*
         * Lookup our own database entry; if not found, nothing more to do.
         */
        dbentry = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                                                                 (void *) &MyDatabaseId,
                                                                                                 HASH_FIND, NULL);
        if (dbentry == NULL || dbentry->tables == NULL)
                return;

        /*
         * Similarly to above, make a list of all known relations in this DB.
         */
        htab = pgstat_collect_oids(RelationRelationId);

        /*
         * Initialize our messages table counter to zero
         */
        msg.m_nentries = 0;

        /*
         * Check for all tables listed in stats hashtable if they still exist.
         */
        hash_seq_init(&hstat, dbentry->tables);
        while ((tabentry = (PgStat_StatTabEntry *) hash_seq_search(&hstat)) != NULL)
        {
                Oid                     tabid = tabentry->tableid;

                CHECK_FOR_INTERRUPTS();

                if (hash_search(htab, (void *) &tabid, HASH_FIND, NULL) != NULL)
                        continue;

                /*
                 * Not there, so add this table's Oid to the message
                 */
                msg.m_tableid[msg.m_nentries++] = tabid;

                /*
                 * If the message is full, send it out and reinitialize to empty
                 */
                if (msg.m_nentries >= PGSTAT_NUM_TABPURGE)
                {
                        len = offsetof(PgStat_MsgTabpurge, m_tableid[0])
                                +msg.m_nentries * sizeof(Oid);

                        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
                        msg.m_databaseid = MyDatabaseId;
                        pgstat_send(&msg, len);

                        msg.m_nentries = 0;
                }
        }

        /*
         * Send the rest
         */
        if (msg.m_nentries > 0)
        {
                len = offsetof(PgStat_MsgTabpurge, m_tableid[0])
                        +msg.m_nentries * sizeof(Oid);

                pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
                msg.m_databaseid = MyDatabaseId;
                pgstat_send(&msg, len);
        }

        /* Clean up */
        hash_destroy(htab);
}


/* ----------
 * pgstat_collect_oids() -
 *
 *      Collect the OIDs of either all databases or all tables, according to
 *      the parameter, into a temporary hash table.  Caller should hash_destroy
 *      the result when done with it.
 * ----------
 */
static HTAB *
pgstat_collect_oids(Oid catalogid)
{
        HTAB       *htab;
        HASHCTL         hash_ctl;
        Relation        rel;
        HeapScanDesc scan;
        HeapTuple       tup;

        memset(&hash_ctl, 0, sizeof(hash_ctl));
        hash_ctl.keysize = sizeof(Oid);
        hash_ctl.entrysize = sizeof(Oid);
        hash_ctl.hash = oid_hash;
        htab = hash_create("Temporary table of OIDs",
                                           PGSTAT_TAB_HASH_SIZE,
                                           &hash_ctl,
                                           HASH_ELEM | HASH_FUNCTION);

        rel = heap_open(catalogid, AccessShareLock);
        scan = heap_beginscan(rel, SnapshotNow, 0, NULL);
        while ((tup = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Oid             thisoid = HeapTupleGetOid(tup);

                CHECK_FOR_INTERRUPTS();

                (void) hash_search(htab, (void *) &thisoid, HASH_ENTER, NULL);
        }
        heap_endscan(scan);
        heap_close(rel, AccessShareLock);

        return htab;
}


/* ----------
 * pgstat_drop_database() -
 *
 *      Tell the collector that we just dropped a database.
 *      (If the message gets lost, we will still clean the dead DB eventually
 *      via future invocations of pgstat_vacuum_tabstat().)
 * ----------
 */
void
pgstat_drop_database(Oid databaseid)
{
        PgStat_MsgDropdb msg;

        if (pgStatSock < 0)
                return;

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_DROPDB);
        msg.m_databaseid = databaseid;
        pgstat_send(&msg, sizeof(msg));
}


/* ----------
 * pgstat_drop_relation() -
 *
 *      Tell the collector that we just dropped a relation.
 *      (If the message gets lost, we will still clean the dead entry eventually
 *      via future invocations of pgstat_vacuum_tabstat().)
 *
 *      Currently not used for lack of any good place to call it; we rely
 *      entirely on pgstat_vacuum_tabstat() to clean out stats for dead rels.
 * ----------
 */
#ifdef NOT_USED
void
pgstat_drop_relation(Oid relid)
{
        PgStat_MsgTabpurge msg;
        int                     len;

        if (pgStatSock < 0)
                return;

        msg.m_tableid[0] = relid;
        msg.m_nentries = 1;

        len = offsetof(PgStat_MsgTabpurge, m_tableid[0]) +sizeof(Oid);

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_TABPURGE);
        msg.m_databaseid = MyDatabaseId;
        pgstat_send(&msg, len);
}
#endif /* NOT_USED */


/* ----------
 * pgstat_reset_counters() -
 *
 *      Tell the statistics collector to reset counters for our database.
 * ----------
 */
void
pgstat_reset_counters(void)
{
        PgStat_MsgResetcounter msg;

        if (pgStatSock < 0)
                return;

        if (!superuser())
                ereport(ERROR,
                                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                                 errmsg("must be superuser to reset statistics counters")));

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_RESETCOUNTER);
        msg.m_databaseid = MyDatabaseId;
        pgstat_send(&msg, sizeof(msg));
}


/* ----------
 * pgstat_report_autovac() -
 *
 *      Called from autovacuum.c to report startup of an autovacuum process.
 *      We are called before InitPostgres is done, so can't rely on MyDatabaseId;
 *      the db OID must be passed in, instead.
 * ----------
 */
void
pgstat_report_autovac(Oid dboid)
{
        PgStat_MsgAutovacStart msg;

        if (pgStatSock < 0)
                return;

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_AUTOVAC_START);
        msg.m_databaseid = dboid;
        msg.m_start_time = GetCurrentTimestamp();

        pgstat_send(&msg, sizeof(msg));
}


/* ---------
 * pgstat_report_vacuum() -
 *
 *      Tell the collector about the table we just vacuumed.
 * ---------
 */
void
pgstat_report_vacuum(Oid tableoid, bool shared,
                                         bool analyze, PgStat_Counter tuples)
{
        PgStat_MsgVacuum msg;

        if (pgStatSock < 0 || !pgstat_track_counts)
                return;

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_VACUUM);
        msg.m_databaseid = shared ? InvalidOid : MyDatabaseId;
        msg.m_tableoid = tableoid;
        msg.m_analyze = analyze;
        msg.m_autovacuum = IsAutoVacuumWorkerProcess(); /* is this autovacuum? */
        msg.m_vacuumtime = GetCurrentTimestamp();
        msg.m_tuples = tuples;
        pgstat_send(&msg, sizeof(msg));
}

/* --------
 * pgstat_report_analyze() -
 *
 *      Tell the collector about the table we just analyzed.
 * --------
 */
void
pgstat_report_analyze(Oid tableoid, bool shared, PgStat_Counter livetuples,
                                          PgStat_Counter deadtuples)
{
        PgStat_MsgAnalyze msg;

        if (pgStatSock < 0 || !pgstat_track_counts)
                return;

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_ANALYZE);
        msg.m_databaseid = shared ? InvalidOid : MyDatabaseId;
        msg.m_tableoid = tableoid;
        msg.m_autovacuum = IsAutoVacuumWorkerProcess(); /* is this autovacuum? */
        msg.m_analyzetime = GetCurrentTimestamp();
        msg.m_live_tuples = livetuples;
        msg.m_dead_tuples = deadtuples;
        pgstat_send(&msg, sizeof(msg));
}


/* ----------
 * pgstat_ping() -
 *
 *      Send some junk data to the collector to increase traffic.
 * ----------
 */
void
pgstat_ping(void)
{
        PgStat_MsgDummy msg;

        if (pgStatSock < 0)
                return;

        pgstat_setheader(&msg.m_hdr, PGSTAT_MTYPE_DUMMY);
        pgstat_send(&msg, sizeof(msg));
}


/* ----------
 * pgstat_initstats() -
 *
 *      Initialize a relcache entry to count access statistics.
 *      Called whenever a relation is opened.
 *
 *      We assume that a relcache entry's pgstat_info field is zeroed by
 *      relcache.c when the relcache entry is made; thereafter it is long-lived
 *      data.  We can avoid repeated searches of the TabStatus arrays when the
 *      same relation is touched repeatedly within a transaction.
 * ----------
 */
void
pgstat_initstats(Relation rel)
{
        Oid                     rel_id = rel->rd_id;
        char            relkind = rel->rd_rel->relkind;

        /* We only count stats for things that have storage */
        if (!(relkind == RELKIND_RELATION ||
                  relkind == RELKIND_INDEX ||
                  relkind == RELKIND_TOASTVALUE))
        {
                rel->pgstat_info = NULL;
                return;
        }

        if (pgStatSock < 0 || !pgstat_track_counts)
        {
                /* We're not counting at all */
                rel->pgstat_info = NULL;
                return;
        }

        /*
         * If we already set up this relation in the current transaction,
         * nothing to do.
         */
        if (rel->pgstat_info != NULL &&
                rel->pgstat_info->t_id == rel_id)
                return;

        /* Else find or make the PgStat_TableStatus entry, and update link */
        rel->pgstat_info = get_tabstat_entry(rel_id, rel->rd_rel->relisshared);
}

/*
 * get_tabstat_entry - find or create a PgStat_TableStatus entry for rel
 */
static PgStat_TableStatus *
get_tabstat_entry(Oid rel_id, bool isshared)
{
        PgStat_TableStatus *entry;
        TabStatusArray *tsa;
        TabStatusArray *prev_tsa;
        int                     i;

        /*
         * Search the already-used tabstat slots for this relation.
         */
        prev_tsa = NULL;
        for (tsa = pgStatTabList; tsa != NULL; prev_tsa = tsa, tsa = tsa->tsa_next)
        {
                for (i = 0; i < tsa->tsa_used; i++)
                {
                        entry = &tsa->tsa_entries[i];
                        if (entry->t_id == rel_id)
                                return entry;
                }

                if (tsa->tsa_used < TABSTAT_QUANTUM)
                {
                        /*
                         * It must not be present, but we found a free slot instead.
                         * Fine, let's use this one.  We assume the entry was already
                         * zeroed, either at creation or after last use.
                         */
                        entry = &tsa->tsa_entries[tsa->tsa_used++];
                        entry->t_id = rel_id;
                        entry->t_shared = isshared;
                        return entry;
                }
        }

        /*
         * We ran out of tabstat slots, so allocate more.  Be sure they're zeroed.
         */
        tsa = (TabStatusArray *) MemoryContextAllocZero(TopMemoryContext,
                                                                                                        sizeof(TabStatusArray));
        if (prev_tsa)
                prev_tsa->tsa_next = tsa;
        else
                pgStatTabList = tsa;

        /*
         * Use the first entry of the new TabStatusArray.
         */
        entry = &tsa->tsa_entries[tsa->tsa_used++];
        entry->t_id = rel_id;
        entry->t_shared = isshared;
        return entry;
}

/*
 * get_tabstat_stack_level - add a new (sub)transaction stack entry if needed
 */
static PgStat_SubXactStatus *
get_tabstat_stack_level(int nest_level)
{
        PgStat_SubXactStatus *xact_state;

        xact_state = pgStatXactStack;
        if (xact_state == NULL || xact_state->nest_level != nest_level)
        {
                xact_state = (PgStat_SubXactStatus *)
                        MemoryContextAlloc(TopTransactionContext,
                                                           sizeof(PgStat_SubXactStatus));
                xact_state->nest_level = nest_level;
                xact_state->prev = pgStatXactStack;
                xact_state->first = NULL;
                pgStatXactStack = xact_state;
        }
        return xact_state;
}

/*
 * add_tabstat_xact_level - add a new (sub)transaction state record
 */
static void
add_tabstat_xact_level(PgStat_TableStatus *pgstat_info, int nest_level)
{
        PgStat_SubXactStatus *xact_state;
        PgStat_TableXactStatus *trans;

        /*
         * If this is the first rel to be modified at the current nest level,
         * we first have to push a transaction stack entry.
         */
        xact_state = get_tabstat_stack_level(nest_level);

        /* Now make a per-table stack entry */
        trans = (PgStat_TableXactStatus *)
                MemoryContextAllocZero(TopTransactionContext,
                                                           sizeof(PgStat_TableXactStatus));
        trans->nest_level = nest_level;
        trans->upper = pgstat_info->trans;
        trans->parent = pgstat_info;
        trans->next = xact_state->first;
        xact_state->first = trans;
        pgstat_info->trans = trans;
}

/*
 * pgstat_count_heap_insert - count a tuple insertion
 */
void
pgstat_count_heap_insert(Relation rel)
{
        PgStat_TableStatus *pgstat_info = rel->pgstat_info;

        if (pgstat_track_counts && pgstat_info != NULL)
        {
                int             nest_level = GetCurrentTransactionNestLevel();

                /* t_tuples_inserted is nontransactional, so just advance it */
                pgstat_info->t_counts.t_tuples_inserted++;

                /* We have to log the transactional effect at the proper level */
                if (pgstat_info->trans == NULL ||
                        pgstat_info->trans->nest_level != nest_level)
                        add_tabstat_xact_level(pgstat_info, nest_level);

                pgstat_info->trans->tuples_inserted++;
        }
}

/*
 * pgstat_count_heap_update - count a tuple update
 */
void
pgstat_count_heap_update(Relation rel, bool hot)
{
        PgStat_TableStatus *pgstat_info = rel->pgstat_info;

        if (pgstat_track_counts && pgstat_info != NULL)
        {
                int             nest_level = GetCurrentTransactionNestLevel();

                /* t_tuples_updated is nontransactional, so just advance it */
                pgstat_info->t_counts.t_tuples_updated++;
                /* ditto for the hot_update counter */
                if (hot)
                        pgstat_info->t_counts.t_tuples_hot_updated++;

                /* We have to log the transactional effect at the proper level */
                if (pgstat_info->trans == NULL ||
                        pgstat_info->trans->nest_level != nest_level)
                        add_tabstat_xact_level(pgstat_info, nest_level);

                /* An UPDATE both inserts a new tuple and deletes the old */
                pgstat_info->trans->tuples_inserted++;
                pgstat_info->trans->tuples_deleted++;
        }
}

/*
 * pgstat_count_heap_delete - count a tuple deletion
 */
void
pgstat_count_heap_delete(Relation rel)
{
        PgStat_TableStatus *pgstat_info = rel->pgstat_info;

        if (pgstat_track_counts && pgstat_info != NULL)
        {
                int             nest_level = GetCurrentTransactionNestLevel();

                /* t_tuples_deleted is nontransactional, so just advance it */
                pgstat_info->t_counts.t_tuples_deleted++;

                /* We have to log the transactional effect at the proper level */
                if (pgstat_info->trans == NULL ||
                        pgstat_info->trans->nest_level != nest_level)
                        add_tabstat_xact_level(pgstat_info, nest_level);

                pgstat_info->trans->tuples_deleted++;
        }
}

/*
 * pgstat_update_heap_dead_tuples - update dead-tuples count
 *
 * The semantics of this are that we are reporting the nontransactional
 * recovery of "delta" dead tuples; so t_new_dead_tuples decreases
 * rather than increasing, and the change goes straight into the per-table
 * counter, not into transactional state.
 */
void
pgstat_update_heap_dead_tuples(Relation rel, int delta)
{
        PgStat_TableStatus *pgstat_info = rel->pgstat_info;

        if (pgstat_track_counts && pgstat_info != NULL)
                pgstat_info->t_counts.t_new_dead_tuples -= delta;
}


/* ----------
 * AtEOXact_PgStat
 *
 *      Called from access/transam/xact.c at top-level transaction commit/abort.
 * ----------
 */
void
AtEOXact_PgStat(bool isCommit)
{
        PgStat_SubXactStatus *xact_state;

        /*
         * Count transaction commit or abort.  (We use counters, not just bools,
         * in case the reporting message isn't sent right away.)
         */
        if (isCommit)
                pgStatXactCommit++;
        else
                pgStatXactRollback++;

        /*
         * Transfer transactional insert/update counts into the base tabstat
         * entries.  We don't bother to free any of the transactional state,
         * since it's all in TopTransactionContext and will go away anyway.
         */
        xact_state = pgStatXactStack;
        if (xact_state != NULL)
        {
                PgStat_TableXactStatus *trans;

                Assert(xact_state->nest_level == 1);
                Assert(xact_state->prev == NULL);
                for (trans = xact_state->first; trans != NULL; trans = trans->next)
                {
                        PgStat_TableStatus *tabstat;

                        Assert(trans->nest_level == 1);
                        Assert(trans->upper == NULL);
                        tabstat = trans->parent;
                        Assert(tabstat->trans == trans);
                        if (isCommit)
                        {
                                tabstat->t_counts.t_new_live_tuples +=
                                        trans->tuples_inserted - trans->tuples_deleted;
                                tabstat->t_counts.t_new_dead_tuples += trans->tuples_deleted;
                        }
                        else
                        {
                                /* inserted tuples are dead, deleted tuples are unaffected */
                                tabstat->t_counts.t_new_dead_tuples += trans->tuples_inserted;
                        }
                        tabstat->trans = NULL;
                }
        }
        pgStatXactStack = NULL;

        /* Make sure any stats snapshot is thrown away */
        pgstat_clear_snapshot();
}

/* ----------
 * AtEOSubXact_PgStat
 *
 *      Called from access/transam/xact.c at subtransaction commit/abort.
 * ----------
 */
void
AtEOSubXact_PgStat(bool isCommit, int nestDepth)
{
        PgStat_SubXactStatus *xact_state;

        /*
         * Transfer transactional insert/update counts into the next higher
         * subtransaction state.
         */
        xact_state = pgStatXactStack;
        if (xact_state != NULL &&
                xact_state->nest_level >= nestDepth)
        {
                PgStat_TableXactStatus *trans;
                PgStat_TableXactStatus *next_trans;

                /* delink xact_state from stack immediately to simplify reuse case */
                pgStatXactStack = xact_state->prev;

                for (trans = xact_state->first; trans != NULL; trans = next_trans)
                {
                        PgStat_TableStatus *tabstat;

                        next_trans = trans->next;
                        Assert(trans->nest_level == nestDepth);
                        tabstat = trans->parent;
                        Assert(tabstat->trans == trans);
                        if (isCommit)
                        {
                                if (trans->upper && trans->upper->nest_level == nestDepth - 1)
                                {
                                        trans->upper->tuples_inserted += trans->tuples_inserted;
                                        trans->upper->tuples_deleted += trans->tuples_deleted;
                                        tabstat->trans = trans->upper;
                                        pfree(trans);
                                }
                                else
                                {
                                        /*
                                         * When there isn't an immediate parent state, we can
                                         * just reuse the record instead of going through a
                                         * palloc/pfree pushup (this works since it's all in
                                         * TopTransactionContext anyway).  We have to re-link
                                         * it into the parent level, though, and that might mean
                                         * pushing a new entry into the pgStatXactStack.
                                         */
                                        PgStat_SubXactStatus *upper_xact_state;

                                        upper_xact_state = get_tabstat_stack_level(nestDepth - 1);
                                        trans->next = upper_xact_state->first;
                                        upper_xact_state->first = trans;
                                        trans->nest_level = nestDepth - 1;
                                }
                        }
                        else
                        {
                                /*
                                 * On abort, inserted tuples are dead (and can be bounced out
                                 * to the top-level tabstat), deleted tuples are unaffected
                                 */
                                tabstat->t_counts.t_new_dead_tuples += trans->tuples_inserted;
                                tabstat->trans = trans->upper;
                                pfree(trans);
                        }
                }
                pfree(xact_state);
        }
}


/*
 * AtPrepare_PgStat
 *              Save the transactional stats state at 2PC transaction prepare.
 *
 * In this phase we just generate 2PC records for all the pending
 * transaction-dependent stats work.
 */
void
AtPrepare_PgStat(void)
{
        PgStat_SubXactStatus *xact_state;

        xact_state = pgStatXactStack;
        if (xact_state != NULL)
        {
                PgStat_TableXactStatus *trans;

                Assert(xact_state->nest_level == 1);
                Assert(xact_state->prev == NULL);
                for (trans = xact_state->first; trans != NULL; trans = trans->next)
                {
                        PgStat_TableStatus *tabstat;
                        TwoPhasePgStatRecord record;

                        Assert(trans->nest_level == 1);
                        Assert(trans->upper == NULL);
                        tabstat = trans->parent;
                        Assert(tabstat->trans == trans);

                        record.tuples_inserted = trans->tuples_inserted;
                        record.tuples_deleted = trans->tuples_deleted;
                        record.t_id = tabstat->t_id;
                        record.t_shared = tabstat->t_shared;

                        RegisterTwoPhaseRecord(TWOPHASE_RM_PGSTAT_ID, 0,
                                                                   &record, sizeof(TwoPhasePgStatRecord));
                }
        }
}

/*
 * PostPrepare_PgStat
 *              Clean up after successful PREPARE.
 *
 * All we need do here is unlink the transaction stats state from the
 * nontransactional state.  The nontransactional action counts will be
 * reported to the stats collector immediately, while the effects on live
 * and dead tuple counts are preserved in the 2PC state file.
 *
 * Note: AtEOXact_PgStat is not called during PREPARE.
 */
void
PostPrepare_PgStat(void)
{
        PgStat_SubXactStatus *xact_state;

        /*
         * We don't bother to free any of the transactional state,
         * since it's all in TopTransactionContext and will go away anyway.
         */
        xact_state = pgStatXactStack;
        if (xact_state != NULL)
        {
                PgStat_TableXactStatus *trans;

                for (trans = xact_state->first; trans != NULL; trans = trans->next)
                {
                        PgStat_TableStatus *tabstat;

                        tabstat = trans->parent;
                        tabstat->trans = NULL;
                }
        }
        pgStatXactStack = NULL;

        /* Make sure any stats snapshot is thrown away */
        pgstat_clear_snapshot();
}

/*
 * 2PC processing routine for COMMIT PREPARED case.
 *
 * Load the saved counts into our local pgstats state.
 */
void
pgstat_twophase_postcommit(TransactionId xid, uint16 info,
                                                   void *recdata, uint32 len)
{
        TwoPhasePgStatRecord *rec = (TwoPhasePgStatRecord *) recdata;
        PgStat_TableStatus *pgstat_info;

        /* Find or create a tabstat entry for the rel */
        pgstat_info = get_tabstat_entry(rec->t_id, rec->t_shared);

        pgstat_info->t_counts.t_new_live_tuples +=
                rec->tuples_inserted - rec->tuples_deleted;
        pgstat_info->t_counts.t_new_dead_tuples += rec->tuples_deleted;
}

/*
 * 2PC processing routine for ROLLBACK PREPARED case.
 *
 * Load the saved counts into our local pgstats state, but treat them
 * as aborted.
 */
void
pgstat_twophase_postabort(TransactionId xid, uint16 info,
                                                  void *recdata, uint32 len)
{
        TwoPhasePgStatRecord *rec = (TwoPhasePgStatRecord *) recdata;
        PgStat_TableStatus *pgstat_info;

        /* Find or create a tabstat entry for the rel */
        pgstat_info = get_tabstat_entry(rec->t_id, rec->t_shared);

        /* inserted tuples are dead, deleted tuples are no-ops */
        pgstat_info->t_counts.t_new_dead_tuples += rec->tuples_inserted;
}


/* ----------
 * pgstat_fetch_stat_dbentry() -
 *
 *      Support function for the SQL-callable pgstat* functions. Returns
 *      the collected statistics for one database or NULL. NULL doesn't mean
 *      that the database doesn't exist, it is just not yet known by the
 *      collector, so the caller is better off to report ZERO instead.
 * ----------
 */
PgStat_StatDBEntry *
pgstat_fetch_stat_dbentry(Oid dbid)
{
        /*
         * If not done for this transaction, read the statistics collector stats
         * file into some hash tables.
         */
        backend_read_statsfile();

        /*
         * Lookup the requested database; return NULL if not found
         */
        return (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                                                          (void *) &dbid,
                                                                                          HASH_FIND, NULL);
}


/* ----------
 * pgstat_fetch_stat_tabentry() -
 *
 *      Support function for the SQL-callable pgstat* functions. Returns
 *      the collected statistics for one table or NULL. NULL doesn't mean
 *      that the table doesn't exist, it is just not yet known by the
 *      collector, so the caller is better off to report ZERO instead.
 * ----------
 */
PgStat_StatTabEntry *
pgstat_fetch_stat_tabentry(Oid relid)
{
        Oid                     dbid;
        PgStat_StatDBEntry *dbentry;
        PgStat_StatTabEntry *tabentry;

        /*
         * If not done for this transaction, read the statistics collector stats
         * file into some hash tables.
         */
        backend_read_statsfile();

        /*
         * Lookup our database, then look in its table hash table.
         */
        dbid = MyDatabaseId;
        dbentry = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                                                                 (void *) &dbid,
                                                                                                 HASH_FIND, NULL);
        if (dbentry != NULL && dbentry->tables != NULL)
        {
                tabentry = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                                                                           (void *) &relid,
                                                                                                           HASH_FIND, NULL);
                if (tabentry)
                        return tabentry;
        }

        /*
         * If we didn't find it, maybe it's a shared table.
         */
        dbid = InvalidOid;
        dbentry = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                                                                 (void *) &dbid,
                                                                                                 HASH_FIND, NULL);
        if (dbentry != NULL && dbentry->tables != NULL)
        {
                tabentry = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                                                                           (void *) &relid,
                                                                                                           HASH_FIND, NULL);
                if (tabentry)
                        return tabentry;
        }

        return NULL;
}


/* ----------
 * pgstat_fetch_stat_beentry() -
 *
 *      Support function for the SQL-callable pgstat* functions. Returns
 *      our local copy of the current-activity entry for one backend.
 *
 *      NB: caller is responsible for a check if the user is permitted to see
 *      this info (especially the querystring).
 * ----------
 */
PgBackendStatus *
pgstat_fetch_stat_beentry(int beid)
{
        pgstat_read_current_status();

        if (beid < 1 || beid > localNumBackends)
                return NULL;

        return &localBackendStatusTable[beid - 1];
}


/* ----------
 * pgstat_fetch_stat_numbackends() -
 *
 *      Support function for the SQL-callable pgstat* functions. Returns
 *      the maximum current backend id.
 * ----------
 */
int
pgstat_fetch_stat_numbackends(void)
{
        pgstat_read_current_status();

        return localNumBackends;
}

/*
 * ---------
 * pgstat_fetch_global() -
 *
 *  Support function for the SQL-callable pgstat* functions. Returns
 *  a pointer to the global statistics struct.
 * ---------
 */
PgStat_GlobalStats *
pgstat_fetch_global(void)
{
        backend_read_statsfile();

        return &globalStats;
}


/* ------------------------------------------------------------
 * Functions for management of the shared-memory PgBackendStatus array
 * ------------------------------------------------------------
 */

static PgBackendStatus *BackendStatusArray = NULL;
static PgBackendStatus *MyBEEntry = NULL;


/*
 * Report shared-memory space needed by CreateSharedBackendStatus.
 */
Size
BackendStatusShmemSize(void)
{
        Size            size;

        size = mul_size(sizeof(PgBackendStatus), MaxBackends);
        return size;
}

/*
 * Initialize the shared status array during postmaster startup.
 */
void
CreateSharedBackendStatus(void)
{
        Size            size = BackendStatusShmemSize();
        bool            found;

        /* Create or attach to the shared array */
        BackendStatusArray = (PgBackendStatus *)
                ShmemInitStruct("Backend Status Array", size, &found);

        if (!found)
        {
                /*
                 * We're the first - initialize.
                 */
                MemSet(BackendStatusArray, 0, size);
        }
}


/* ----------
 * pgstat_initialize() -
 *
 *      Initialize pgstats state, and set up our on-proc-exit hook.
 *      Called from InitPostgres.  MyBackendId must be set,
 *      but we must not have started any transaction yet (since the
 *      exit hook must run after the last transaction exit).
 * ----------
 */
void
pgstat_initialize(void)
{
        /* Initialize MyBEEntry */
        Assert(MyBackendId >= 1 && MyBackendId <= MaxBackends);
        MyBEEntry = &BackendStatusArray[MyBackendId - 1];

        /* Set up a process-exit hook to clean up */
        on_shmem_exit(pgstat_beshutdown_hook, 0);
}

/* ----------
 * pgstat_bestart() -
 *
 *      Initialize this backend's entry in the PgBackendStatus array.
 *      Called from InitPostgres.  MyDatabaseId and session userid must be set
 *      (hence, this cannot be combined with pgstat_initialize).
 * ----------
 */
void
pgstat_bestart(void)
{
        TimestampTz proc_start_timestamp;
        Oid                     userid;
        SockAddr        clientaddr;
        volatile PgBackendStatus *beentry;

        /*
         * To minimize the time spent modifying the PgBackendStatus entry,
         * fetch all the needed data first.
         *
         * If we have a MyProcPort, use its session start time (for consistency,
         * and to save a kernel call).
         */
        if (MyProcPort)
                proc_start_timestamp = MyProcPort->SessionStartTime;
        else
                proc_start_timestamp = GetCurrentTimestamp();
        userid = GetSessionUserId();

        /*
         * We may not have a MyProcPort (eg, if this is the autovacuum process).
         * If so, use all-zeroes client address, which is dealt with specially in
         * pg_stat_get_backend_client_addr and pg_stat_get_backend_client_port.
         */
        if (MyProcPort)
                memcpy(&clientaddr, &MyProcPort->raddr, sizeof(clientaddr));
        else
                MemSet(&clientaddr, 0, sizeof(clientaddr));

        /*
         * Initialize my status entry, following the protocol of bumping
         * st_changecount before and after; and make sure it's even afterwards. We
         * use a volatile pointer here to ensure the compiler doesn't try to get
         * cute.
         */
        beentry = MyBEEntry;
        do
        {
                beentry->st_changecount++;
        } while ((beentry->st_changecount & 1) == 0);

        beentry->st_procpid = MyProcPid;
        beentry->st_proc_start_timestamp = proc_start_timestamp;
        beentry->st_activity_start_timestamp = 0;
        beentry->st_xact_start_timestamp = 0;
        beentry->st_databaseid = MyDatabaseId;
        beentry->st_userid = userid;
        beentry->st_clientaddr = clientaddr;
        beentry->st_waiting = false;
        beentry->st_activity[0] = '\0';
        /* Also make sure the last byte in the string area is always 0 */
        beentry->st_activity[PGBE_ACTIVITY_SIZE - 1] = '\0';

        beentry->st_changecount++;
        Assert((beentry->st_changecount & 1) == 0);
}

/*
 * Shut down a single backend's statistics reporting at process exit.
 *
 * Flush any remaining statistics counts out to the collector.
 * Without this, operations triggered during backend exit (such as
 * temp table deletions) won't be counted.
 *
 * Lastly, clear out our entry in the PgBackendStatus array.
 */
static void
pgstat_beshutdown_hook(int code, Datum arg)
{
        volatile PgBackendStatus *beentry = MyBEEntry;

        pgstat_report_tabstat(true);

        /*
         * Clear my status entry, following the protocol of bumping st_changecount
         * before and after.  We use a volatile pointer here to ensure the
         * compiler doesn't try to get cute.
         */
        beentry->st_changecount++;

        beentry->st_procpid = 0;        /* mark invalid */

        beentry->st_changecount++;
        Assert((beentry->st_changecount & 1) == 0);
}


/* ----------
 * pgstat_report_activity() -
 *
 *      Called from tcop/postgres.c to report what the backend is actually doing
 *      (usually "<IDLE>" or the start of the query to be executed).
 * ----------
 */
void
pgstat_report_activity(const char *cmd_str)
{
        volatile PgBackendStatus *beentry = MyBEEntry;
        TimestampTz start_timestamp;
        int                     len;

        if (!pgstat_track_activities || !beentry)
                return;

        /*
         * To minimize the time spent modifying the entry, fetch all the needed
         * data first.
         */
        start_timestamp = GetCurrentStatementStartTimestamp();

        len = strlen(cmd_str);
        len = pg_mbcliplen(cmd_str, len, PGBE_ACTIVITY_SIZE - 1);

        /*
         * Update my status entry, following the protocol of bumping
         * st_changecount before and after.  We use a volatile pointer here to
         * ensure the compiler doesn't try to get cute.
         */
        beentry->st_changecount++;

        beentry->st_activity_start_timestamp = start_timestamp;
        memcpy((char *) beentry->st_activity, cmd_str, len);
        beentry->st_activity[len] = '\0';

        beentry->st_changecount++;
        Assert((beentry->st_changecount & 1) == 0);
}

/*
 * Report current transaction start timestamp as the specified value.
 * Zero means there is no active transaction.
 */
void
pgstat_report_xact_timestamp(TimestampTz tstamp)
{
        volatile PgBackendStatus *beentry = MyBEEntry;

        if (!pgstat_track_activities || !beentry)
                return;

        /*
         * Update my status entry, following the protocol of bumping
         * st_changecount before and after.  We use a volatile pointer
         * here to ensure the compiler doesn't try to get cute.
         */
        beentry->st_changecount++;
        beentry->st_xact_start_timestamp = tstamp;
        beentry->st_changecount++;
        Assert((beentry->st_changecount & 1) == 0);
}

/* ----------
 * pgstat_report_waiting() -
 *
 *      Called from lock manager to report beginning or end of a lock wait.
 *
 * NB: this *must* be able to survive being called before MyBEEntry has been
 * initialized.
 * ----------
 */
void
pgstat_report_waiting(bool waiting)
{
        volatile PgBackendStatus *beentry = MyBEEntry;

        if (!pgstat_track_activities || !beentry)
                return;

        /*
         * Since this is a single-byte field in a struct that only this process
         * may modify, there seems no need to bother with the st_changecount
         * protocol.  The update must appear atomic in any case.
         */
        beentry->st_waiting = waiting;
}


/* ----------
 * pgstat_read_current_status() -
 *
 *      Copy the current contents of the PgBackendStatus array to local memory,
 *      if not already done in this transaction.
 * ----------
 */
static void
pgstat_read_current_status(void)
{
        volatile PgBackendStatus *beentry;
        PgBackendStatus *localtable;
        PgBackendStatus *localentry;
        int                     i;

        Assert(!pgStatRunningInCollector);
        if (localBackendStatusTable)
                return;                                 /* already done */

        pgstat_setup_memcxt();

        localtable = (PgBackendStatus *)
                MemoryContextAlloc(pgStatLocalContext,
                                                   sizeof(PgBackendStatus) * MaxBackends);
        localNumBackends = 0;

        beentry = BackendStatusArray;
        localentry = localtable;
        for (i = 1; i <= MaxBackends; i++)
        {
                /*
                 * Follow the protocol of retrying if st_changecount changes while we
                 * copy the entry, or if it's odd.  (The check for odd is needed to
                 * cover the case where we are able to completely copy the entry while
                 * the source backend is between increment steps.)      We use a volatile
                 * pointer here to ensure the compiler doesn't try to get cute.
                 */
                for (;;)
                {
                        int                     save_changecount = beentry->st_changecount;

                        /*
                         * XXX if PGBE_ACTIVITY_SIZE is really large, it might be best to
                         * use strcpy not memcpy for copying the activity string?
                         */
                        memcpy(localentry, (char *) beentry, sizeof(PgBackendStatus));

                        if (save_changecount == beentry->st_changecount &&
                                (save_changecount & 1) == 0)
                                break;

                        /* Make sure we can break out of loop if stuck... */
                        CHECK_FOR_INTERRUPTS();
                }

                beentry++;
                /* Only valid entries get included into the local array */
                if (localentry->st_procpid > 0)
                {
                        localentry++;
                        localNumBackends++;
                }
        }

        /* Set the pointer only after completion of a valid table */
        localBackendStatusTable = localtable;
}


/* ------------------------------------------------------------
 * Local support functions follow
 * ------------------------------------------------------------
 */


/* ----------
 * pgstat_setheader() -
 *
 *              Set common header fields in a statistics message
 * ----------
 */
static void
pgstat_setheader(PgStat_MsgHdr *hdr, StatMsgType mtype)
{
        hdr->m_type = mtype;
}


/* ----------
 * pgstat_send() -
 *
 *              Send out one statistics message to the collector
 * ----------
 */
static void
pgstat_send(void *msg, int len)
{
        int                     rc;

        if (pgStatSock < 0)
                return;

        ((PgStat_MsgHdr *) msg)->m_size = len;

        /* We'll retry after EINTR, but ignore all other failures */
        do
        {
                rc = send(pgStatSock, msg, len, 0);
        } while (rc < 0 && errno == EINTR);

#ifdef USE_ASSERT_CHECKING
        /* In debug builds, log send failures ... */
        if (rc < 0)
                elog(LOG, "could not send to statistics collector: %m");
#endif
}

/* ----------
 * pgstat_send_bgwriter() -
 *
 *      Send bgwriter statistics to the collector
 * ----------
 */
void
pgstat_send_bgwriter(void)
{
        /* We assume this initializes to zeroes */
        static const PgStat_MsgBgWriter all_zeroes;

        /*
         * This function can be called even if nothing at all has happened.
         * In this case, avoid sending a completely empty message to
         * the stats collector.
         */
        if (memcmp(&BgWriterStats, &all_zeroes, sizeof(PgStat_MsgBgWriter)) == 0)
                return;

        /*
         * Prepare and send the message
         */
        pgstat_setheader(&BgWriterStats.m_hdr, PGSTAT_MTYPE_BGWRITER);
        pgstat_send(&BgWriterStats, sizeof(BgWriterStats));

        /*
         * Clear out the statistics buffer, so it can be re-used.
         */
        MemSet(&BgWriterStats, 0, sizeof(BgWriterStats));
}


/* ----------
 * PgstatCollectorMain() -
 *
 *      Start up the statistics collector process.      This is the body of the
 *      postmaster child process.
 *
 *      The argc/argv parameters are valid only in EXEC_BACKEND case.
 * ----------
 */
NON_EXEC_STATIC void
PgstatCollectorMain(int argc, char *argv[])
{
        struct itimerval write_timeout;
        bool            need_timer = false;
        int                     len;
        PgStat_Msg      msg;

#ifndef WIN32
#ifdef HAVE_POLL
        struct pollfd input_fd;
#else
        struct timeval sel_timeout;
        fd_set          rfds;
#endif
#endif

        IsUnderPostmaster = true;       /* we are a postmaster subprocess now */

        MyProcPid = getpid();           /* reset MyProcPid */

        MyStartTime = time(NULL);       /* record Start Time for logging */

        /*
         * If possible, make this process a group leader, so that the postmaster
         * can signal any child processes too.  (pgstat probably never has
         * any child processes, but for consistency we make all postmaster
         * child processes do this.)
         */
#ifdef HAVE_SETSID
        if (setsid() < 0)
                elog(FATAL, "setsid() failed: %m");
#endif

        /*
         * Ignore all signals usually bound to some action in the postmaster,
         * except SIGQUIT and SIGALRM.
         */
        pqsignal(SIGHUP, SIG_IGN);
        pqsignal(SIGINT, SIG_IGN);
        pqsignal(SIGTERM, SIG_IGN);
        pqsignal(SIGQUIT, pgstat_exit);
        pqsignal(SIGALRM, force_statwrite);
        pqsignal(SIGPIPE, SIG_IGN);
        pqsignal(SIGUSR1, SIG_IGN);
        pqsignal(SIGUSR2, SIG_IGN);
        pqsignal(SIGCHLD, SIG_DFL);
        pqsignal(SIGTTIN, SIG_DFL);
        pqsignal(SIGTTOU, SIG_DFL);
        pqsignal(SIGCONT, SIG_DFL);
        pqsignal(SIGWINCH, SIG_DFL);
        PG_SETMASK(&UnBlockSig);

        /*
         * Identify myself via ps
         */
        init_ps_display("stats collector process", "", "", "");

        /*
         * Arrange to write the initial status file right away
         */
        need_statwrite = true;

        /* Preset the delay between status file writes */
        MemSet(&write_timeout, 0, sizeof(struct itimerval));
        write_timeout.it_value.tv_sec = PGSTAT_STAT_INTERVAL / 1000;
        write_timeout.it_value.tv_usec = (PGSTAT_STAT_INTERVAL % 1000) * 1000;

        /*
         * Read in an existing statistics stats file or initialize the stats to
         * zero.
         */
        pgStatRunningInCollector = true;
        pgStatDBHash = pgstat_read_statsfile(InvalidOid);

        /*
         * Setup the descriptor set for select(2).      Since only one bit in the set
         * ever changes, we need not repeat FD_ZERO each time.
         */
#if !defined(HAVE_POLL) && !defined(WIN32)
        FD_ZERO(&rfds);
#endif

        /*
         * Loop to process messages until we get SIGQUIT or detect ungraceful
         * death of our parent postmaster.
         *
         * For performance reasons, we don't want to do a PostmasterIsAlive() test
         * after every message; instead, do it at statwrite time and if
         * select()/poll() is interrupted by timeout.
         */
        for (;;)
        {
                int                     got_data;

                /*
                 * Quit if we get SIGQUIT from the postmaster.
                 */
                if (need_exit)
                        break;

                /*
                 * If time to write the stats file, do so.      Note that the alarm
                 * interrupt isn't re-enabled immediately, but only after we next
                 * receive a stats message; so no cycles are wasted when there is
                 * nothing going on.
                 */
                if (need_statwrite)
                {
                        /* Check for postmaster death; if so we'll write file below */
                        if (!PostmasterIsAlive(true))
                                break;

                        pgstat_write_statsfile();
                        need_statwrite = false;
                        need_timer = true;
                }

                /*
                 * Wait for a message to arrive; but not for more than
                 * PGSTAT_SELECT_TIMEOUT seconds. (This determines how quickly we will
                 * shut down after an ungraceful postmaster termination; so it needn't
                 * be very fast.  However, on some systems SIGQUIT won't interrupt the
                 * poll/select call, so this also limits speed of response to SIGQUIT,
                 * which is more important.)
                 *
                 * We use poll(2) if available, otherwise select(2).
                 * Win32 has its own implementation.
                 */
#ifndef WIN32
#ifdef HAVE_POLL
                input_fd.fd = pgStatSock;
                input_fd.events = POLLIN | POLLERR;
                input_fd.revents = 0;

                if (poll(&input_fd, 1, PGSTAT_SELECT_TIMEOUT * 1000) < 0)
                {
                        if (errno == EINTR)
                                continue;
                        ereport(ERROR,
                                        (errcode_for_socket_access(),
                                         errmsg("poll() failed in statistics collector: %m")));
                }

                got_data = (input_fd.revents != 0);
#else                                                   /* !HAVE_POLL */

                FD_SET(pgStatSock, &rfds);

                /*
                 * timeout struct is modified by select() on some operating systems,
                 * so re-fill it each time.
                 */
                sel_timeout.tv_sec = PGSTAT_SELECT_TIMEOUT;
                sel_timeout.tv_usec = 0;

                if (select(pgStatSock + 1, &rfds, NULL, NULL, &sel_timeout) < 0)
                {
                        if (errno == EINTR)
                                continue;
                        ereport(ERROR,
                                        (errcode_for_socket_access(),
                                         errmsg("select() failed in statistics collector: %m")));
                }

                got_data = FD_ISSET(pgStatSock, &rfds);
#endif   /* HAVE_POLL */
#else /* WIN32 */
                got_data = pgwin32_waitforsinglesocket(pgStatSock, FD_READ,
                                                                                           PGSTAT_SELECT_TIMEOUT*1000);
#endif

                /*
                 * If there is a message on the socket, read it and check for
                 * validity.
                 */
                if (got_data)
                {
                        len = recv(pgStatSock, (char *) &msg,
                                           sizeof(PgStat_Msg), 0);
                        if (len < 0)
                        {
                                if (errno == EINTR)
                                        continue;
                                ereport(ERROR,
                                                (errcode_for_socket_access(),
                                                 errmsg("could not read statistics message: %m")));
                        }

                        /*
                         * We ignore messages that are smaller than our common header
                         */
                        if (len < sizeof(PgStat_MsgHdr))
                                continue;

                        /*
                         * The received length must match the length in the header
                         */
                        if (msg.msg_hdr.m_size != len)
                                continue;

                        /*
                         * O.K. - we accept this message.  Process it.
                         */
                        switch (msg.msg_hdr.m_type)
                        {
                                case PGSTAT_MTYPE_DUMMY:
                                        break;

                                case PGSTAT_MTYPE_TABSTAT:
                                        pgstat_recv_tabstat((PgStat_MsgTabstat *) &msg, len);
                                        break;

                                case PGSTAT_MTYPE_TABPURGE:
                                        pgstat_recv_tabpurge((PgStat_MsgTabpurge *) &msg, len);
                                        break;

                                case PGSTAT_MTYPE_DROPDB:
                                        pgstat_recv_dropdb((PgStat_MsgDropdb *) &msg, len);
                                        break;

                                case PGSTAT_MTYPE_RESETCOUNTER:
                                        pgstat_recv_resetcounter((PgStat_MsgResetcounter *) &msg,
                                                                                         len);
                                        break;

                                case PGSTAT_MTYPE_AUTOVAC_START:
                                        pgstat_recv_autovac((PgStat_MsgAutovacStart *) &msg, len);
                                        break;

                                case PGSTAT_MTYPE_VACUUM:
                                        pgstat_recv_vacuum((PgStat_MsgVacuum *) &msg, len);
                                        break;

                                case PGSTAT_MTYPE_ANALYZE:
                                        pgstat_recv_analyze((PgStat_MsgAnalyze *) &msg, len);
                                        break;

                                case PGSTAT_MTYPE_BGWRITER:
                                        pgstat_recv_bgwriter((PgStat_MsgBgWriter *) &msg, len);
                                        break;

                                default:
                                        break;
                        }

                        /*
                         * If this is the first message after we wrote the stats file the
                         * last time, enable the alarm interrupt to make it be written
                         * again later.
                         */
                        if (need_timer)
                        {
                                if (setitimer(ITIMER_REAL, &write_timeout, NULL))
                                        ereport(ERROR,
                                        (errmsg("could not set statistics collector timer: %m")));
                                need_timer = false;
                        }
                }
                else
                {
                        /*
                         * We can only get here if the select/poll timeout elapsed. Check
                         * for postmaster death.
                         */
                        if (!PostmasterIsAlive(true))
                                break;
                }
        }                                                       /* end of message-processing loop */

        /*
         * Save the final stats to reuse at next startup.
         */
        pgstat_write_statsfile();

        exit(0);
}


/* SIGQUIT signal handler for collector process */
static void
pgstat_exit(SIGNAL_ARGS)
{
        need_exit = true;
}

/* SIGALRM signal handler for collector process */
static void
force_statwrite(SIGNAL_ARGS)
{
        need_statwrite = true;
}


/*
 * Lookup the hash table entry for the specified database. If no hash
 * table entry exists, initialize it, if the create parameter is true.
 * Else, return NULL.
 */
static PgStat_StatDBEntry *
pgstat_get_db_entry(Oid databaseid, bool create)
{
        PgStat_StatDBEntry *result;
        bool            found;
        HASHACTION      action = (create ? HASH_ENTER : HASH_FIND);

        /* Lookup or create the hash table entry for this database */
        result = (PgStat_StatDBEntry *) hash_search(pgStatDBHash,
                                                                                                &databaseid,
                                                                                                action, &found);

        if (!create && !found)
                return NULL;

        /* If not found, initialize the new one. */
        if (!found)
        {
                HASHCTL         hash_ctl;

                result->tables = NULL;
                result->n_xact_commit = 0;
                result->n_xact_rollback = 0;
                result->n_blocks_fetched = 0;
                result->n_blocks_hit = 0;
                result->n_tuples_returned = 0;
                result->n_tuples_fetched = 0;
                result->n_tuples_inserted = 0;
                result->n_tuples_updated = 0;
                result->n_tuples_deleted = 0;
                result->last_autovac_time = 0;

                memset(&hash_ctl, 0, sizeof(hash_ctl));
                hash_ctl.keysize = sizeof(Oid);
                hash_ctl.entrysize = sizeof(PgStat_StatTabEntry);
                hash_ctl.hash = oid_hash;
                result->tables = hash_create("Per-database table",
                                                                         PGSTAT_TAB_HASH_SIZE,
                                                                         &hash_ctl,
                                                                         HASH_ELEM | HASH_FUNCTION);
        }

        return result;
}


/* ----------
 * pgstat_write_statsfile() -
 *
 *      Tell the news.
 * ----------
 */
static void
pgstat_write_statsfile(void)
{
        HASH_SEQ_STATUS hstat;
        HASH_SEQ_STATUS tstat;
        PgStat_StatDBEntry *dbentry;
        PgStat_StatTabEntry *tabentry;
        FILE       *fpout;
        int32           format_id;

        /*
         * Open the statistics temp file to write out the current values.
         */
        fpout = fopen(PGSTAT_STAT_TMPFILE, PG_BINARY_W);
        if (fpout == NULL)
        {
                ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not open temporary statistics file \"%s\": %m",
                                                PGSTAT_STAT_TMPFILE)));
                return;
        }

        /*
         * Write the file header --- currently just a format ID.
         */
        format_id = PGSTAT_FILE_FORMAT_ID;
        fwrite(&format_id, sizeof(format_id), 1, fpout);

        /*
         * Write global stats struct
         */
        fwrite(&globalStats, sizeof(globalStats), 1, fpout);

        /*
         * Walk through the database table.
         */
        hash_seq_init(&hstat, pgStatDBHash);
        while ((dbentry = (PgStat_StatDBEntry *) hash_seq_search(&hstat)) != NULL)
        {
                /*
                 * Write out the DB entry including the number of live backends. We
                 * don't write the tables pointer since it's of no use to any other
                 * process.
                 */
                fputc('D', fpout);
                fwrite(dbentry, offsetof(PgStat_StatDBEntry, tables), 1, fpout);

                /*
                 * Walk through the database's access stats per table.
                 */
                hash_seq_init(&tstat, dbentry->tables);
                while ((tabentry = (PgStat_StatTabEntry *) hash_seq_search(&tstat)) != NULL)
                {
                        fputc('T', fpout);
                        fwrite(tabentry, sizeof(PgStat_StatTabEntry), 1, fpout);
                }

                /*
                 * Mark the end of this DB
                 */
                fputc('d', fpout);
        }

        /*
         * No more output to be done. Close the temp file and replace the old
         * pgstat.stat with it.  The ferror() check replaces testing for error
         * after each individual fputc or fwrite above.
         */
        fputc('E', fpout);

        if (ferror(fpout))
        {
                ereport(LOG,
                                (errcode_for_file_access(),
                           errmsg("could not write temporary statistics file \"%s\": %m",
                                          PGSTAT_STAT_TMPFILE)));
                fclose(fpout);
                unlink(PGSTAT_STAT_TMPFILE);
        }
        else if (fclose(fpout) < 0)
        {
                ereport(LOG,
                                (errcode_for_file_access(),
                           errmsg("could not close temporary statistics file \"%s\": %m",
                                          PGSTAT_STAT_TMPFILE)));
                unlink(PGSTAT_STAT_TMPFILE);
        }
        else if (rename(PGSTAT_STAT_TMPFILE, PGSTAT_STAT_FILENAME) < 0)
        {
                ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not rename temporary statistics file \"%s\" to \"%s\": %m",
                                                PGSTAT_STAT_TMPFILE, PGSTAT_STAT_FILENAME)));
                unlink(PGSTAT_STAT_TMPFILE);
        }
}


/* ----------
 * pgstat_read_statsfile() -
 *
 *      Reads in an existing statistics collector file and initializes the
 *      databases' hash table (whose entries point to the tables' hash tables).
 * ----------
 */
static HTAB *
pgstat_read_statsfile(Oid onlydb)
{
        PgStat_StatDBEntry *dbentry;
        PgStat_StatDBEntry dbbuf;
        PgStat_StatTabEntry *tabentry;
        PgStat_StatTabEntry tabbuf;
        HASHCTL         hash_ctl;
        HTAB       *dbhash;
        HTAB       *tabhash = NULL;
        FILE       *fpin;
        int32           format_id;
        bool            found;

        /*
         * The tables will live in pgStatLocalContext.
         */
        pgstat_setup_memcxt();

        /*
         * Create the DB hashtable
         */
        memset(&hash_ctl, 0, sizeof(hash_ctl));
        hash_ctl.keysize = sizeof(Oid);
        hash_ctl.entrysize = sizeof(PgStat_StatDBEntry);
        hash_ctl.hash = oid_hash;
        hash_ctl.hcxt = pgStatLocalContext;
        dbhash = hash_create("Databases hash", PGSTAT_DB_HASH_SIZE, &hash_ctl,
                                                 HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);

        /*
         * Clear out global statistics so they start from zero in case we can't
         * load an existing statsfile.
         */
        memset(&globalStats, 0, sizeof(globalStats));

        /*
         * Try to open the status file. If it doesn't exist, the backends simply
         * return zero for anything and the collector simply starts from scratch
         * with empty counters.
         */
        if ((fpin = AllocateFile(PGSTAT_STAT_FILENAME, PG_BINARY_R)) == NULL)
                return dbhash;

        /*
         * Verify it's of the expected format.
         */
        if (fread(&format_id, 1, sizeof(format_id), fpin) != sizeof(format_id)
                || format_id != PGSTAT_FILE_FORMAT_ID)
        {
                ereport(pgStatRunningInCollector ? LOG : WARNING,
                                (errmsg("corrupted pgstat.stat file")));
                goto done;
        }

        /*
         * Read global stats struct
         */
        if (fread(&globalStats, 1, sizeof(globalStats), fpin) != sizeof(globalStats))
        {
                ereport(pgStatRunningInCollector ? LOG : WARNING,
                                (errmsg("corrupted pgstat.stat file")));
                goto done;
        }

        /*
         * We found an existing collector stats file. Read it and put all the
         * hashtable entries into place.
         */
        for (;;)
        {
                switch (fgetc(fpin))
                {
                                /*
                                 * 'D'  A PgStat_StatDBEntry struct describing a database
                                 * follows. Subsequently, zero to many 'T' entries will follow
                                 * until a 'd' is encountered.
                                 */
                        case 'D':
                                if (fread(&dbbuf, 1, offsetof(PgStat_StatDBEntry, tables),
                                                  fpin) != offsetof(PgStat_StatDBEntry, tables))
                                {
                                        ereport(pgStatRunningInCollector ? LOG : WARNING,
                                                        (errmsg("corrupted pgstat.stat file")));
                                        goto done;
                                }

                                /*
                                 * Add to the DB hash
                                 */
                                dbentry = (PgStat_StatDBEntry *) hash_search(dbhash,
                                                                                                  (void *) &dbbuf.databaseid,
                                                                                                                         HASH_ENTER,
                                                                                                                         &found);
                                if (found)
                                {
                                        ereport(pgStatRunningInCollector ? LOG : WARNING,
                                                        (errmsg("corrupted pgstat.stat file")));
                                        goto done;
                                }

                                memcpy(dbentry, &dbbuf, sizeof(PgStat_StatDBEntry));
                                dbentry->tables = NULL;

                                /*
                                 * Don't collect tables if not the requested DB (or the
                                 * shared-table info)
                                 */
                                if (onlydb != InvalidOid)
                                {
                                        if (dbbuf.databaseid != onlydb &&
                                                dbbuf.databaseid != InvalidOid)
                                                break;
                                }

                                memset(&hash_ctl, 0, sizeof(hash_ctl));
                                hash_ctl.keysize = sizeof(Oid);
                                hash_ctl.entrysize = sizeof(PgStat_StatTabEntry);
                                hash_ctl.hash = oid_hash;
                                hash_ctl.hcxt = pgStatLocalContext;
                                dbentry->tables = hash_create("Per-database table",
                                                                                          PGSTAT_TAB_HASH_SIZE,
                                                                                          &hash_ctl,
                                                                         HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);

                                /*
                                 * Arrange that following 'T's add entries to this database's
                                 * tables hash table.
                                 */
                                tabhash = dbentry->tables;
                                break;

                                /*
                                 * 'd'  End of this database.
                                 */
                        case 'd':
                                tabhash = NULL;
                                break;

                                /*
                                 * 'T'  A PgStat_StatTabEntry follows.
                                 */
                        case 'T':
                                if (fread(&tabbuf, 1, sizeof(PgStat_StatTabEntry),
                                                  fpin) != sizeof(PgStat_StatTabEntry))
                                {
                                        ereport(pgStatRunningInCollector ? LOG : WARNING,
                                                        (errmsg("corrupted pgstat.stat file")));
                                        goto done;
                                }

                                /*
                                 * Skip if table belongs to a not requested database.
                                 */
                                if (tabhash == NULL)
                                        break;

                                tabentry = (PgStat_StatTabEntry *) hash_search(tabhash,
                                                                                                        (void *) &tabbuf.tableid,
                                                                                                                 HASH_ENTER, &found);

                                if (found)
                                {
                                        ereport(pgStatRunningInCollector ? LOG : WARNING,
                                                        (errmsg("corrupted pgstat.stat file")));
                                        goto done;
                                }

                                memcpy(tabentry, &tabbuf, sizeof(tabbuf));
                                break;

                                /*
                                 * 'E'  The EOF marker of a complete stats file.
                                 */
                        case 'E':
                                goto done;

                        default:
                                ereport(pgStatRunningInCollector ? LOG : WARNING,
                                                (errmsg("corrupted pgstat.stat file")));
                                goto done;
                }
        }

done:
        FreeFile(fpin);

        return dbhash;
}

/*
 * If not already done, read the statistics collector stats file into
 * some hash tables.  The results will be kept until pgstat_clear_snapshot()
 * is called (typically, at end of transaction).
 */
static void
backend_read_statsfile(void)
{
        /* already read it? */
        if (pgStatDBHash)
                return;
        Assert(!pgStatRunningInCollector);

        /* Autovacuum launcher wants stats about all databases */
        if (IsAutoVacuumLauncherProcess())
                pgStatDBHash = pgstat_read_statsfile(InvalidOid);
        else
                pgStatDBHash = pgstat_read_statsfile(MyDatabaseId);
}


/* ----------
 * pgstat_setup_memcxt() -
 *
 *      Create pgStatLocalContext, if not already done.
 * ----------
 */
static void
pgstat_setup_memcxt(void)
{
        if (!pgStatLocalContext)
                pgStatLocalContext = AllocSetContextCreate(TopMemoryContext,
                                                                                                   "Statistics snapshot",
                                                                                                   ALLOCSET_SMALL_MINSIZE,
                                                                                                   ALLOCSET_SMALL_INITSIZE,
                                                                                                   ALLOCSET_SMALL_MAXSIZE);
}


/* ----------
 * pgstat_clear_snapshot() -
 *
 *      Discard any data collected in the current transaction.  Any subsequent
 *      request will cause new snapshots to be read.
 *
 *      This is also invoked during transaction commit or abort to discard
 *      the no-longer-wanted snapshot.
 * ----------
 */
void
pgstat_clear_snapshot(void)
{
        /* Release memory, if any was allocated */
        if (pgStatLocalContext)
                MemoryContextDelete(pgStatLocalContext);

        /* Reset variables */
        pgStatLocalContext = NULL;
        pgStatDBHash = NULL;
        localBackendStatusTable = NULL;
        localNumBackends = 0;
}


/* ----------
 * pgstat_recv_tabstat() -
 *
 *      Count what the backend has done.
 * ----------
 */
static void
pgstat_recv_tabstat(PgStat_MsgTabstat *msg, int len)
{
        PgStat_TableEntry *tabmsg = &(msg->m_entry[0]);
        PgStat_StatDBEntry *dbentry;
        PgStat_StatTabEntry *tabentry;
        int                     i;
        bool            found;

        dbentry = pgstat_get_db_entry(msg->m_databaseid, true);

        /*
         * Update database-wide stats.
         */
        dbentry->n_xact_commit += (PgStat_Counter) (msg->m_xact_commit);
        dbentry->n_xact_rollback += (PgStat_Counter) (msg->m_xact_rollback);

        /*
         * Process all table entries in the message.
         */
        for (i = 0; i < msg->m_nentries; i++)
        {
                tabentry = (PgStat_StatTabEntry *) hash_search(dbentry->tables,
                                                                                                  (void *) &(tabmsg[i].t_id),
                                                                                                           HASH_ENTER, &found);

                if (!found)
                {
                        /*
                         * If it's a new table entry, initialize counters to the values we
                         * just got.
                         */
                        tabentry->numscans = tabmsg[i].t_counts.t_numscans;
                        tabentry->tuples_returned = tabmsg[i].t_counts.t_tuples_returned;
                        tabentry->tuples_fetched = tabmsg[i].t_counts.t_tuples_fetched;
                        tabentry->tuples_inserted = tabmsg[i].t_counts.t_tuples_inserted;
                        tabentry->tuples_updated = tabmsg[i].t_counts.t_tuples_updated;
                        tabentry->tuples_deleted = tabmsg[i].t_counts.t_tuples_deleted;
                        tabentry->tuples_hot_updated = tabmsg[i].t_counts.t_tuples_hot_updated;
                        tabentry->n_live_tuples = tabmsg[i].t_counts.t_new_live_tuples;
                        tabentry->n_dead_tuples = tabmsg[i].t_counts.t_new_dead_tuples;
                        tabentry->blocks_fetched = tabmsg[i].t_counts.t_blocks_fetched;
                        tabentry->blocks_hit = tabmsg[i].t_counts.t_blocks_hit;

                        tabentry->last_anl_tuples = 0;
                        tabentry->vacuum_timestamp = 0;
                        tabentry->autovac_vacuum_timestamp = 0;
                        tabentry->analyze_timestamp = 0;
                        tabentry->autovac_analyze_timestamp = 0;
                }
                else
                {
                        /*
                         * Otherwise add the values to the existing entry.
                         */
                        tabentry->numscans += tabmsg[i].t_counts.t_numscans;
                        tabentry->tuples_returned += tabmsg[i].t_counts.t_tuples_returned;
                        tabentry->tuples_fetched += tabmsg[i].t_counts.t_tuples_fetched;
                        tabentry->tuples_inserted += tabmsg[i].t_counts.t_tuples_inserted;
                        tabentry->tuples_updated += tabmsg[i].t_counts.t_tuples_updated;
                        tabentry->tuples_deleted += tabmsg[i].t_counts.t_tuples_deleted;
                        tabentry->tuples_hot_updated += tabmsg[i].t_counts.t_tuples_hot_updated;
                        tabentry->n_live_tuples += tabmsg[i].t_counts.t_new_live_tuples;
                        tabentry->n_dead_tuples += tabmsg[i].t_counts.t_new_dead_tuples;
                        tabentry->blocks_fetched += tabmsg[i].t_counts.t_blocks_fetched;
                        tabentry->blocks_hit += tabmsg[i].t_counts.t_blocks_hit;
                }

                /* Clamp n_live_tuples in case of negative new_live_tuples */
                tabentry->n_live_tuples = Max(tabentry->n_live_tuples, 0);
                /* Likewise for n_dead_tuples */
                tabentry->n_dead_tuples = Max(tabentry->n_dead_tuples, 0);

                /*
                 * Add per-table stats to the per-database entry, too.
                 */
                dbentry->n_tuples_returned += tabmsg[i].t_counts.t_tuples_returned;
                dbentry->n_tuples_fetched += tabmsg[i].t_counts.t_tuples_fetched;
                dbentry->n_tuples_inserted += tabmsg[i].t_counts.t_tuples_inserted;
                dbentry->n_tuples_updated += tabmsg[i].t_counts.t_tuples_updated;
                dbentry->n_tuples_deleted += tabmsg[i].t_counts.t_tuples_deleted;
                dbentry->n_blocks_fetched += tabmsg[i].t_counts.t_blocks_fetched;
                dbentry->n_blocks_hit += tabmsg[i].t_counts.t_blocks_hit;
        }
}


/* ----------
 * pgstat_recv_tabpurge() -
 *
 *      Arrange for dead table removal.
 * ----------
 */
static void
pgstat_recv_tabpurge(PgStat_MsgTabpurge *msg, int len)
{
        PgStat_StatDBEntry *dbentry;
        int                     i;

        dbentry = pgstat_get_db_entry(msg->m_databaseid, false);

        /*
         * No need to purge if we don't even know the database.
         */
        if (!dbentry || !dbentry->tables)
                return;

        /*
         * Process all table entries in the message.
         */
        for (i = 0; i < msg->m_nentries; i++)
        {
                /* Remove from hashtable if present; we don't care if it's not. */
                (void) hash_search(dbentry->tables,
                                                   (void *) &(msg->m_tableid[i]),
                                                   HASH_REMOVE, NULL);
        }
}


/* ----------
 * pgstat_recv_dropdb() -
 *
 *      Arrange for dead database removal
 * ----------
 */
static void
pgstat_recv_dropdb(PgStat_MsgDropdb *msg, int len)
{
        PgStat_StatDBEntry *dbentry;

        /*
         * Lookup the database in the hashtable.
         */
        dbentry = pgstat_get_db_entry(msg->m_databaseid, false);

        /*
         * If found, remove it.
         */
        if (dbentry)
        {
                if (dbentry->tables != NULL)
                        hash_destroy(dbentry->tables);

                if (hash_search(pgStatDBHash,
                                                (void *) &(dbentry->databaseid),
                                                HASH_REMOVE, NULL) == NULL)
                        ereport(ERROR,
                                        (errmsg("database hash table corrupted "
                                                        "during cleanup --- abort")));
        }
}


/* ----------
 * pgstat_recv_resetcounter() -
 *
 *      Reset the statistics for the specified database.
 * ----------
 */
static void
pgstat_recv_resetcounter(PgStat_MsgResetcounter *msg, int len)
{
        HASHCTL         hash_ctl;
        PgStat_StatDBEntry *dbentry;

        /*
         * Lookup the database in the hashtable.  Nothing to do if not there.
         */
        dbentry = pgstat_get_db_entry(msg->m_databaseid, false);

        if (!dbentry)
                return;

        /*
         * We simply throw away all the database's table entries by recreating a
         * new hash table for them.
         */
        if (dbentry->tables != NULL)
                hash_destroy(dbentry->tables);

        dbentry->tables = NULL;
        dbentry->n_xact_commit = 0;
        dbentry->n_xact_rollback = 0;
        dbentry->n_blocks_fetched = 0;
        dbentry->n_blocks_hit = 0;

        memset(&hash_ctl, 0, sizeof(hash_ctl));
        hash_ctl.keysize = sizeof(Oid);
        hash_ctl.entrysize = sizeof(PgStat_StatTabEntry);
        hash_ctl.hash = oid_hash;
        dbentry->tables = hash_create("Per-database table",
                                                                  PGSTAT_TAB_HASH_SIZE,
                                                                  &hash_ctl,
                                                                  HASH_ELEM | HASH_FUNCTION);
}

/* ----------
 * pgstat_recv_autovac() -
 *
 *      Process an autovacuum signalling message.
 * ----------
 */
static void
pgstat_recv_autovac(PgStat_MsgAutovacStart *msg, int len)
{
        PgStat_StatDBEntry *dbentry;

        /*
         * Lookup the database in the hashtable.  Don't create the entry if it
         * doesn't exist, because autovacuum may be processing a template
         * database.  If this isn't the case, the database is most likely to have
         * an entry already.  (If it doesn't, not much harm is done anyway --
         * it'll get created as soon as somebody actually uses the database.)
         */
        dbentry = pgstat_get_db_entry(msg->m_databaseid, false);
        if (dbentry == NULL)
                return;

        /*
         * Store the last autovacuum time in the database entry.
         */
        dbentry->last_autovac_time = msg->m_start_time;
}

/* ----------
 * pgstat_recv_vacuum() -
 *
 *      Process a VACUUM message.
 * ----------
 */
static void
pgstat_recv_vacuum(PgStat_MsgVacuum *msg, int len)
{
        PgStat_StatDBEntry *dbentry;
        PgStat_StatTabEntry *tabentry;

        /*
         * Don't create either the database or table entry if it doesn't already
         * exist.  This avoids bloating the stats with entries for stuff that is
         * only touched by vacuum and not by live operations.
         */
        dbentry = pgstat_get_db_entry(msg->m_databaseid, false);
        if (dbentry == NULL)
                return;

        tabentry = hash_search(dbentry->tables, &(msg->m_tableoid),
                                                   HASH_FIND, NULL);
        if (tabentry == NULL)
                return;

        if (msg->m_autovacuum)
                tabentry->autovac_vacuum_timestamp = msg->m_vacuumtime;
        else
                tabentry->vacuum_timestamp = msg->m_vacuumtime;
        tabentry->n_live_tuples = msg->m_tuples;
        /* Resetting dead_tuples to 0 is an approximation ... */
        tabentry->n_dead_tuples = 0;
        if (msg->m_analyze)
        {
                tabentry->last_anl_tuples = msg->m_tuples;
                if (msg->m_autovacuum)
                        tabentry->autovac_analyze_timestamp = msg->m_vacuumtime;
                else
                        tabentry->analyze_timestamp = msg->m_vacuumtime;
        }
        else
        {
                /* last_anl_tuples must never exceed n_live_tuples+n_dead_tuples */
                tabentry->last_anl_tuples = Min(tabentry->last_anl_tuples,
                                                                                msg->m_tuples);
        }
}

/* ----------
 * pgstat_recv_analyze() -
 *
 *      Process an ANALYZE message.
 * ----------
 */
static void
pgstat_recv_analyze(PgStat_MsgAnalyze *msg, int len)
{
        PgStat_StatDBEntry *dbentry;
        PgStat_StatTabEntry *tabentry;

        /*
         * Don't create either the database or table entry if it doesn't already
         * exist.  This avoids bloating the stats with entries for stuff that is
         * only touched by analyze and not by live operations.
         */
        dbentry = pgstat_get_db_entry(msg->m_databaseid, false);
        if (dbentry == NULL)
                return;

        tabentry = hash_search(dbentry->tables, &(msg->m_tableoid),
                                                   HASH_FIND, NULL);
        if (tabentry == NULL)
                return;

        if (msg->m_autovacuum)
                tabentry->autovac_analyze_timestamp = msg->m_analyzetime;
        else
                tabentry->analyze_timestamp = msg->m_analyzetime;
        tabentry->n_live_tuples = msg->m_live_tuples;
        tabentry->n_dead_tuples = msg->m_dead_tuples;
        tabentry->last_anl_tuples = msg->m_live_tuples + msg->m_dead_tuples;
}


/* ----------
 * pgstat_recv_bgwriter() -
 *
 *      Process a BGWRITER message.
 * ----------
 */
static void
pgstat_recv_bgwriter(PgStat_MsgBgWriter *msg, int len)
{
        globalStats.timed_checkpoints += msg->m_timed_checkpoints;
        globalStats.requested_checkpoints += msg->m_requested_checkpoints;
        globalStats.buf_written_checkpoints += msg->m_buf_written_checkpoints;
        globalStats.buf_written_clean += msg->m_buf_written_clean;
        globalStats.maxwritten_clean += msg->m_maxwritten_clean;
        globalStats.buf_written_backend += msg->m_buf_written_backend;
        globalStats.buf_alloc += msg->m_buf_alloc;
}