Arrange to squeeze out the MINIMAL_TUPLE_PADDING in the tuple representation

[postgresql] / src / backend / utils / sort / tuplestore.c

/*-------------------------------------------------------------------------
 *
 * tuplestore.c
 *        Generalized routines for temporary tuple storage.
 *
 * This module handles temporary storage of tuples for purposes such
 * as Materialize nodes, hashjoin batch files, etc.  It is essentially
 * a dumbed-down version of tuplesort.c; it does no sorting of tuples
 * but can only store and regurgitate a sequence of tuples.  However,
 * because no sort is required, it is allowed to start reading the sequence
 * before it has all been written.      This is particularly useful for cursors,
 * because it allows random access within the already-scanned portion of
 * a query without having to process the underlying scan to completion.
 * Also, it is possible to support multiple independent read pointers.
 *
 * A temporary file is used to handle the data if it exceeds the
 * space limit specified by the caller.
 *
 * The (approximate) amount of memory allowed to the tuplestore is specified
 * in kilobytes by the caller.  We absorb tuples and simply store them in an
 * in-memory array as long as we haven't exceeded maxKBytes.  If we do exceed
 * maxKBytes, we dump all the tuples into a temp file and then read from that
 * when needed.
 *
 * Upon creation, a tuplestore supports a single read pointer, numbered 0.
 * Additional read pointers can be created using tuplestore_alloc_read_pointer.
 * Mark/restore behavior is supported by copying read pointers.
 *
 * When the caller requests backward-scan capability, we write the temp file
 * in a format that allows either forward or backward scan.  Otherwise, only
 * forward scan is allowed.  A request for backward scan must be made before
 * putting any tuples into the tuplestore.  Rewind is normally allowed but
 * can be turned off via tuplestore_set_eflags; turning off both backward
 * scan and rewind for all read pointers enables truncation of the tuplestore
 * at the oldest read point for minimal memory usage.
 *
 * Note: in TSS_WRITEFILE state, the temp file's seek position is the
 * current write position, and the write-position variables in the tuplestore
 * aren't kept up to date.  Similarly, in TSS_READFILE state the temp file's
 * seek position is the active read pointer's position, and that read pointer
 * isn't kept up to date.  We update the appropriate variables using ftell()
 * before switching to the other state or activating a different read pointer.
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/utils/sort/tuplestore.c,v 1.43 2008/10/28 15:51:03 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "commands/tablespace.h"
#include "executor/executor.h"
#include "storage/buffile.h"
#include "utils/memutils.h"
#include "utils/tuplestore.h"


/*
 * Possible states of a Tuplestore object.      These denote the states that
 * persist between calls of Tuplestore routines.
 */
typedef enum
{
        TSS_INMEM,                                      /* Tuples still fit in memory */
        TSS_WRITEFILE,                          /* Writing to temp file */
        TSS_READFILE                            /* Reading from temp file */
} TupStoreStatus;

/*
 * State for a single read pointer.  If we are in state INMEM then all the
 * read pointers' "current" fields denote the read positions.  In state
 * WRITEFILE, the file/offset fields denote the read positions.  In state
 * READFILE, inactive read pointers have valid file/offset, but the active
 * read pointer implicitly has position equal to the temp file's seek position.
 *
 * Special case: if eof_reached is true, then the pointer's read position is
 * implicitly equal to the write position, and current/file/offset aren't
 * maintained.  This way we need not update all the read pointers each time
 * we write.
 */
typedef struct
{
        int                     eflags;                 /* capability flags */
        bool            eof_reached;    /* read has reached EOF */
        int                     current;                /* next array index to read */
        int                     file;                   /* temp file# */
        off_t           offset;                 /* byte offset in file */
} TSReadPointer;

/*
 * Private state of a Tuplestore operation.
 */
struct Tuplestorestate
{
        TupStoreStatus status;          /* enumerated value as shown above */
        int                     eflags;                 /* capability flags (OR of pointers' flags) */
        bool            backward;               /* store extra length words in file? */
        bool            interXact;              /* keep open through transactions? */
        long            availMem;               /* remaining memory available, in bytes */
        BufFile    *myfile;                     /* underlying file, or NULL if none */

        /*
         * These function pointers decouple the routines that must know what kind
         * of tuple we are handling from the routines that don't need to know it.
         * They are set up by the tuplestore_begin_xxx routines.
         *
         * (Although tuplestore.c currently only supports heap tuples, I've copied
         * this part of tuplesort.c so that extension to other kinds of objects
         * will be easy if it's ever needed.)
         *
         * Function to copy a supplied input tuple into palloc'd space. (NB: we
         * assume that a single pfree() is enough to release the tuple later, so
         * the representation must be "flat" in one palloc chunk.) state->availMem
         * must be decreased by the amount of space used.
         */
        void       *(*copytup) (Tuplestorestate *state, void *tup);

        /*
         * Function to write a stored tuple onto tape.  The representation of the
         * tuple on tape need not be the same as it is in memory; requirements on
         * the tape representation are given below.  After writing the tuple,
         * pfree() it, and increase state->availMem by the amount of memory space
         * thereby released.
         */
        void            (*writetup) (Tuplestorestate *state, void *tup);

        /*
         * Function to read a stored tuple from tape back into memory. 'len' is
         * the already-read length of the stored tuple.  Create and return a
         * palloc'd copy, and decrease state->availMem by the amount of memory
         * space consumed.
         */
        void       *(*readtup) (Tuplestorestate *state, unsigned int len);

        /*
         * This array holds pointers to tuples in memory if we are in state INMEM.
         * In states WRITEFILE and READFILE it's not used.
         */
        void      **memtuples;          /* array of pointers to palloc'd tuples */
        int                     memtupcount;    /* number of tuples currently present */
        int                     memtupsize;             /* allocated length of memtuples array */

        /*
         * These variables are used to keep track of the current positions.
         *
         * In state WRITEFILE, the current file seek position is the write point;
         * in state READFILE, the write position is remembered in writepos_xxx.
         * (The write position is the same as EOF, but since BufFileSeek doesn't
         * currently implement SEEK_END, we have to remember it explicitly.)
         */
        TSReadPointer *readptrs;        /* array of read pointers */
        int                     activeptr;              /* index of the active read pointer */
        int                     readptrcount;   /* number of pointers currently valid */
        int                     readptrsize;    /* allocated length of readptrs array */

        int                     writepos_file;  /* file# (valid if READFILE state) */
        off_t           writepos_offset; /* offset (valid if READFILE state) */
};

#define COPYTUP(state,tup)      ((*(state)->copytup) (state, tup))
#define WRITETUP(state,tup) ((*(state)->writetup) (state, tup))
#define READTUP(state,len)      ((*(state)->readtup) (state, len))
#define LACKMEM(state)          ((state)->availMem < 0)
#define USEMEM(state,amt)       ((state)->availMem -= (amt))
#define FREEMEM(state,amt)      ((state)->availMem += (amt))

/*--------------------
 *
 * NOTES about on-tape representation of tuples:
 *
 * We require the first "unsigned int" of a stored tuple to be the total size
 * on-tape of the tuple, including itself (so it is never zero).
 * The remainder of the stored tuple
 * may or may not match the in-memory representation of the tuple ---
 * any conversion needed is the job of the writetup and readtup routines.
 *
 * If state->backward is true, then the stored representation of
 * the tuple must be followed by another "unsigned int" that is a copy of the
 * length --- so the total tape space used is actually sizeof(unsigned int)
 * more than the stored length value.  This allows read-backwards.      When
 * state->backward is not set, the write/read routines may omit the extra
 * length word.
 *
 * writetup is expected to write both length words as well as the tuple
 * data.  When readtup is called, the tape is positioned just after the
 * front length word; readtup must read the tuple data and advance past
 * the back length word (if present).
 *
 * The write/read routines can make use of the tuple description data
 * stored in the Tuplestorestate record, if needed. They are also expected
 * to adjust state->availMem by the amount of memory space (not tape space!)
 * released or consumed.  There is no error return from either writetup
 * or readtup; they should ereport() on failure.
 *
 *
 * NOTES about memory consumption calculations:
 *
 * We count space allocated for tuples against the maxKBytes limit,
 * plus the space used by the variable-size array memtuples.
 * Fixed-size space (primarily the BufFile I/O buffer) is not counted.
 * We don't worry about the size of the read pointer array, either.
 *
 * Note that we count actual space used (as shown by GetMemoryChunkSpace)
 * rather than the originally-requested size.  This is important since
 * palloc can add substantial overhead.  It's not a complete answer since
 * we won't count any wasted space in palloc allocation blocks, but it's
 * a lot better than what we were doing before 7.3.
 *
 *--------------------
 */


static Tuplestorestate *tuplestore_begin_common(int eflags,
                                                bool interXact,
                                                int maxKBytes);
static void tuplestore_puttuple_common(Tuplestorestate *state, void *tuple);
static void dumptuples(Tuplestorestate *state);
static void tuplestore_trim(Tuplestorestate *state);
static unsigned int getlen(Tuplestorestate *state, bool eofOK);
static void *copytup_heap(Tuplestorestate *state, void *tup);
static void writetup_heap(Tuplestorestate *state, void *tup);
static void *readtup_heap(Tuplestorestate *state, unsigned int len);


/*
 *              tuplestore_begin_xxx
 *
 * Initialize for a tuple store operation.
 */
static Tuplestorestate *
tuplestore_begin_common(int eflags, bool interXact, int maxKBytes)
{
        Tuplestorestate *state;

        state = (Tuplestorestate *) palloc0(sizeof(Tuplestorestate));

        state->status = TSS_INMEM;
        state->eflags = eflags;
        state->interXact = interXact;
        state->availMem = maxKBytes * 1024L;
        state->myfile = NULL;

        state->memtupcount = 0;
        state->memtupsize = 1024;       /* initial guess */
        state->memtuples = (void **) palloc(state->memtupsize * sizeof(void *));

        USEMEM(state, GetMemoryChunkSpace(state->memtuples));

        state->activeptr = 0;
        state->readptrcount = 1;
        state->readptrsize = 8;         /* arbitrary */
        state->readptrs = (TSReadPointer *)
                palloc(state->readptrsize * sizeof(TSReadPointer));

        state->readptrs[0].eflags = eflags;
        state->readptrs[0].eof_reached = false;
        state->readptrs[0].current = 0;

        return state;
}

/*
 * tuplestore_begin_heap
 *
 * Create a new tuplestore; other types of tuple stores (other than
 * "heap" tuple stores, for heap tuples) are possible, but not presently
 * implemented.
 *
 * randomAccess: if true, both forward and backward accesses to the
 * tuple store are allowed.
 *
 * interXact: if true, the files used for on-disk storage persist beyond the
 * end of the current transaction.      NOTE: It's the caller's responsibility to
 * create such a tuplestore in a memory context that will also survive
 * transaction boundaries, and to ensure the tuplestore is closed when it's
 * no longer wanted.
 *
 * maxKBytes: how much data to store in memory (any data beyond this
 * amount is paged to disk).  When in doubt, use work_mem.
 */
Tuplestorestate *
tuplestore_begin_heap(bool randomAccess, bool interXact, int maxKBytes)
{
        Tuplestorestate *state;
        int                     eflags;

        /*
         * This interpretation of the meaning of randomAccess is compatible with
         * the pre-8.3 behavior of tuplestores.
         */
        eflags = randomAccess ?
                (EXEC_FLAG_BACKWARD | EXEC_FLAG_REWIND) :
                (EXEC_FLAG_REWIND);

        state = tuplestore_begin_common(eflags, interXact, maxKBytes);

        state->copytup = copytup_heap;
        state->writetup = writetup_heap;
        state->readtup = readtup_heap;

        return state;
}

/*
 * tuplestore_set_eflags
 *
 * Set the capability flags for read pointer 0 at a finer grain than is
 * allowed by tuplestore_begin_xxx.  This must be called before inserting
 * any data into the tuplestore.
 *
 * eflags is a bitmask following the meanings used for executor node
 * startup flags (see executor.h).      tuplestore pays attention to these bits:
 *              EXEC_FLAG_REWIND                need rewind to start
 *              EXEC_FLAG_BACKWARD              need backward fetch
 * If tuplestore_set_eflags is not called, REWIND is allowed, and BACKWARD
 * is set per "randomAccess" in the tuplestore_begin_xxx call.
 */
void
tuplestore_set_eflags(Tuplestorestate *state, int eflags)
{
        int                     i;

        if (state->status != TSS_INMEM || state->memtupcount != 0)
                elog(ERROR, "too late to call tuplestore_set_eflags");

        state->readptrs[0].eflags = eflags;
        for (i = 1; i < state->readptrcount; i++)
                eflags |= state->readptrs[i].eflags;
        state->eflags = eflags;
}

/*
 * tuplestore_alloc_read_pointer - allocate another read pointer.
 *
 * Returns the pointer's index.
 *
 * The new pointer initially copies the position of read pointer 0.
 * It can have its own eflags, but if any data has been inserted into
 * the tuplestore, these eflags must not represent an increase in
 * requirements.
 */
int
tuplestore_alloc_read_pointer(Tuplestorestate *state, int eflags)
{
        /* Check for possible increase of requirements */
        if (state->status != TSS_INMEM || state->memtupcount != 0)
        {
                if ((state->eflags | eflags) != state->eflags)
                        elog(ERROR, "too late to require new tuplestore eflags");
        }

        /* Make room for another read pointer if needed */
        if (state->readptrcount >= state->readptrsize)
        {
                int             newcnt = state->readptrsize * 2;

                state->readptrs = (TSReadPointer *)
                        repalloc(state->readptrs, newcnt * sizeof(TSReadPointer));
                state->readptrsize = newcnt;
        }

        /* And set it up */
        state->readptrs[state->readptrcount] = state->readptrs[0];
        state->readptrs[state->readptrcount].eflags = eflags;

        state->eflags |= eflags;

        return state->readptrcount++;
}

/*
 * tuplestore_clear
 *
 *      Delete all the contents of a tuplestore, and reset its read pointers
 *      to the start.
 */
void
tuplestore_clear(Tuplestorestate *state)
{
        int                     i;
        TSReadPointer *readptr;

        if (state->myfile)
                BufFileClose(state->myfile);
        state->myfile = NULL;
        if (state->memtuples)
        {
                for (i = 0; i < state->memtupcount; i++)
                {
                        FREEMEM(state, GetMemoryChunkSpace(state->memtuples[i]));
                        pfree(state->memtuples[i]);
                }
        }
        state->status = TSS_INMEM;
        state->memtupcount = 0;
        readptr = state->readptrs;
        for (i = 0; i < state->readptrcount; readptr++, i++)
        {
                readptr->eof_reached = false;
                readptr->current = 0;
        }
}

/*
 * tuplestore_end
 *
 *      Release resources and clean up.
 */
void
tuplestore_end(Tuplestorestate *state)
{
        int                     i;

        if (state->myfile)
                BufFileClose(state->myfile);
        if (state->memtuples)
        {
                for (i = 0; i < state->memtupcount; i++)
                        pfree(state->memtuples[i]);
                pfree(state->memtuples);
        }
        pfree(state->readptrs);
        pfree(state);
}

/*
 * tuplestore_select_read_pointer - make the specified read pointer active
 */
void
tuplestore_select_read_pointer(Tuplestorestate *state, int ptr)
{
        TSReadPointer *readptr;
        TSReadPointer *oldptr;

        Assert(ptr >= 0 && ptr < state->readptrcount);

        /* No work if already active */
        if (ptr == state->activeptr)
                return;

        readptr = &state->readptrs[ptr];
        oldptr = &state->readptrs[state->activeptr];

        switch (state->status)
        {
                case TSS_INMEM:
                case TSS_WRITEFILE:
                        /* no work */
                        break;
                case TSS_READFILE:
                        /*
                         * First, save the current read position in the pointer about
                         * to become inactive.
                         */
                        if (!oldptr->eof_reached)
                                BufFileTell(state->myfile,
                                                        &oldptr->file,
                                                        &oldptr->offset);

                        /*
                         * We have to make the temp file's seek position equal to the
                         * logical position of the new read pointer.  In eof_reached
                         * state, that's the EOF, which we have available from the saved
                         * write position.
                         */
                        if (readptr->eof_reached)
                        {
                                if (BufFileSeek(state->myfile,
                                                                state->writepos_file,
                                                                state->writepos_offset,
                                                                SEEK_SET) != 0)
                                        elog(ERROR, "tuplestore seek failed");
                        }
                        else
                        {
                                if (BufFileSeek(state->myfile,
                                                                readptr->file,
                                                                readptr->offset,
                                                                SEEK_SET) != 0)
                                        elog(ERROR, "tuplestore seek failed");
                        }
                        break;
                default:
                        elog(ERROR, "invalid tuplestore state");
                        break;
        }

        state->activeptr = ptr;
}

/*
 * tuplestore_ateof
 *
 * Returns the active read pointer's eof_reached state.
 */
bool
tuplestore_ateof(Tuplestorestate *state)
{
        return state->readptrs[state->activeptr].eof_reached;
}

/*
 * Accept one tuple and append it to the tuplestore.
 *
 * Note that the input tuple is always copied; the caller need not save it.
 *
 * If the active read pointer is currently "at EOF", it remains so (the read
 * pointer implicitly advances along with the write pointer); otherwise the
 * read pointer is unchanged.  Non-active read pointers do not move, which
 * means they are certain to not be "at EOF" immediately after puttuple.
 * This curious-seeming behavior is for the convenience of nodeMaterial.c and
 * nodeCtescan.c, which would otherwise need to do extra pointer repositioning
 * steps.
 *
 * tuplestore_puttupleslot() is a convenience routine to collect data from
 * a TupleTableSlot without an extra copy operation.
 */
void
tuplestore_puttupleslot(Tuplestorestate *state,
                                                TupleTableSlot *slot)
{
        MinimalTuple tuple;

        /*
         * Form a MinimalTuple in working memory
         */
        tuple = ExecCopySlotMinimalTuple(slot);
        USEMEM(state, GetMemoryChunkSpace(tuple));

        tuplestore_puttuple_common(state, (void *) tuple);
}

/*
 * "Standard" case to copy from a HeapTuple.  This is actually now somewhat
 * deprecated, but not worth getting rid of in view of the number of callers.
 */
void
tuplestore_puttuple(Tuplestorestate *state, HeapTuple tuple)
{
        /*
         * Copy the tuple.      (Must do this even in WRITEFILE case.)
         */
        tuple = COPYTUP(state, tuple);

        tuplestore_puttuple_common(state, (void *) tuple);
}

/*
 * Similar to tuplestore_puttuple(), but start from the values + nulls
 * array. This avoids requiring that the caller construct a HeapTuple,
 * saving a copy.
 */
void
tuplestore_putvalues(Tuplestorestate *state, TupleDesc tdesc,
                                         Datum *values, bool *isnull)
{
        MinimalTuple tuple;

        tuple = heap_form_minimal_tuple(tdesc, values, isnull);

        tuplestore_puttuple_common(state, (void *) tuple);
}

static void
tuplestore_puttuple_common(Tuplestorestate *state, void *tuple)
{
        TSReadPointer *readptr;
        int                     i;

        switch (state->status)
        {
                case TSS_INMEM:

                        /*
                         * Update read pointers as needed; see API spec above.
                         */
                        readptr = state->readptrs;
                        for (i = 0; i < state->readptrcount; readptr++, i++)
                        {
                                if (readptr->eof_reached && i != state->activeptr)
                                {
                                        readptr->eof_reached = false;
                                        readptr->current = state->memtupcount;
                                }
                        }

                        /*
                         * Grow the array as needed.  Note that we try to grow the array
                         * when there is still one free slot remaining --- if we fail,
                         * there'll still be room to store the incoming tuple, and then
                         * we'll switch to tape-based operation.
                         */
                        if (state->memtupcount >= state->memtupsize - 1)
                        {
                                /*
                                 * See grow_memtuples() in tuplesort.c for the rationale
                                 * behind these two tests.
                                 */
                                if (state->availMem > (long) (state->memtupsize * sizeof(void *)) &&
                                        (Size) (state->memtupsize * 2) < MaxAllocSize / sizeof(void *))
                                {
                                        FREEMEM(state, GetMemoryChunkSpace(state->memtuples));
                                        state->memtupsize *= 2;
                                        state->memtuples = (void **)
                                                repalloc(state->memtuples,
                                                                 state->memtupsize * sizeof(void *));
                                        USEMEM(state, GetMemoryChunkSpace(state->memtuples));
                                }
                        }

                        /* Stash the tuple in the in-memory array */
                        state->memtuples[state->memtupcount++] = tuple;

                        /*
                         * Done if we still fit in available memory and have array slots.
                         */
                        if (state->memtupcount < state->memtupsize && !LACKMEM(state))
                                return;

                        /*
                         * Nope; time to switch to tape-based operation.  Make sure that
                         * the temp file(s) are created in suitable temp tablespaces.
                         */
                        PrepareTempTablespaces();
                        state->myfile = BufFileCreateTemp(state->interXact);
                        /*
                         * Freeze the decision about whether trailing length words
                         * will be used.  We can't change this choice once data is on
                         * tape, even though callers might drop the requirement.
                         */
                        state->backward = (state->eflags & EXEC_FLAG_BACKWARD) != 0;
                        state->status = TSS_WRITEFILE;
                        dumptuples(state);
                        break;
                case TSS_WRITEFILE:

                        /*
                         * Update read pointers as needed; see API spec above.
                         * Note: BufFileTell is quite cheap, so not worth trying
                         * to avoid multiple calls.
                         */
                        readptr = state->readptrs;
                        for (i = 0; i < state->readptrcount; readptr++, i++)
                        {
                                if (readptr->eof_reached && i != state->activeptr)
                                {
                                        readptr->eof_reached = false;
                                        BufFileTell(state->myfile,
                                                                &readptr->file,
                                                                &readptr->offset);
                                }
                        }

                        WRITETUP(state, tuple);
                        break;
                case TSS_READFILE:

                        /*
                         * Switch from reading to writing.
                         */
                        if (!state->readptrs[state->activeptr].eof_reached)
                                BufFileTell(state->myfile,
                                                        &state->readptrs[state->activeptr].file,
                                                        &state->readptrs[state->activeptr].offset);
                        if (BufFileSeek(state->myfile,
                                                        state->writepos_file, state->writepos_offset,
                                                        SEEK_SET) != 0)
                                elog(ERROR, "tuplestore seek to EOF failed");
                        state->status = TSS_WRITEFILE;

                        /*
                         * Update read pointers as needed; see API spec above.
                         */
                        readptr = state->readptrs;
                        for (i = 0; i < state->readptrcount; readptr++, i++)
                        {
                                if (readptr->eof_reached && i != state->activeptr)
                                {
                                        readptr->eof_reached = false;
                                        readptr->file = state->writepos_file;
                                        readptr->offset = state->writepos_offset;
                                }
                        }

                        WRITETUP(state, tuple);
                        break;
                default:
                        elog(ERROR, "invalid tuplestore state");
                        break;
        }
}

/*
 * Fetch the next tuple in either forward or back direction.
 * Returns NULL if no more tuples.      If should_free is set, the
 * caller must pfree the returned tuple when done with it.
 *
 * Backward scan is only allowed if randomAccess was set true or
 * EXEC_FLAG_BACKWARD was specified to tuplestore_set_eflags().
 */
static void *
tuplestore_gettuple(Tuplestorestate *state, bool forward,
                                        bool *should_free)
{
        TSReadPointer *readptr = &state->readptrs[state->activeptr];
        unsigned int tuplen;
        void       *tup;

        Assert(forward || (readptr->eflags & EXEC_FLAG_BACKWARD));

        switch (state->status)
        {
                case TSS_INMEM:
                        *should_free = false;
                        if (forward)
                        {
                                if (readptr->eof_reached)
                                        return NULL;
                                if (readptr->current < state->memtupcount)
                                {
                                        /*
                                         * We have another tuple, so return it.  Note: in
                                         * principle we could try tuplestore_trim() here after
                                         * advancing current, but this would cost cycles with
                                         * little chance of success, so we don't bother.
                                         */
                                        return state->memtuples[readptr->current++];
                                }
                                readptr->eof_reached = true;
                                return NULL;
                        }
                        else
                        {
                                /*
                                 * if all tuples are fetched already then we return last
                                 * tuple, else - tuple before last returned.
                                 */
                                if (readptr->eof_reached)
                                {
                                        readptr->current = state->memtupcount;
                                        readptr->eof_reached = false;
                                }
                                else
                                {
                                        if (readptr->current <= 0)
                                                return NULL;
                                        readptr->current--;     /* last returned tuple */
                                }
                                if (readptr->current <= 0)
                                        return NULL;
                                return state->memtuples[readptr->current - 1];
                        }
                        break;

                case TSS_WRITEFILE:
                        /* Skip state change if we'll just return NULL */
                        if (readptr->eof_reached && forward)
                                return NULL;

                        /*
                         * Switch from writing to reading.
                         */
                        BufFileTell(state->myfile,
                                                &state->writepos_file, &state->writepos_offset);
                        if (!readptr->eof_reached)
                                if (BufFileSeek(state->myfile,
                                                                readptr->file, readptr->offset,
                                                                SEEK_SET) != 0)
                                        elog(ERROR, "tuplestore seek failed");
                        state->status = TSS_READFILE;
                        /* FALL THRU into READFILE case */

                case TSS_READFILE:
                        *should_free = true;
                        if (forward)
                        {
                                if ((tuplen = getlen(state, true)) != 0)
                                {
                                        tup = READTUP(state, tuplen);
                                        return tup;
                                }
                                else
                                {
                                        readptr->eof_reached = true;
                                        return NULL;
                                }
                        }

                        /*
                         * Backward.
                         *
                         * if all tuples are fetched already then we return last tuple,
                         * else - tuple before last returned.
                         *
                         * Back up to fetch previously-returned tuple's ending length
                         * word. If seek fails, assume we are at start of file.
                         */
                        if (BufFileSeek(state->myfile, 0, -(long) sizeof(unsigned int),
                                                        SEEK_CUR) != 0)
                        {
                                /* even a failed backwards fetch gets you out of eof state */
                                readptr->eof_reached = false;
                                return NULL;
                        }
                        tuplen = getlen(state, false);

                        if (readptr->eof_reached)
                        {
                                readptr->eof_reached = false;
                                /* We will return the tuple returned before returning NULL */
                        }
                        else
                        {
                                /*
                                 * Back up to get ending length word of tuple before it.
                                 */
                                if (BufFileSeek(state->myfile, 0,
                                                                -(long) (tuplen + 2 * sizeof(unsigned int)),
                                                                SEEK_CUR) != 0)
                                {
                                        /*
                                         * If that fails, presumably the prev tuple is the first
                                         * in the file.  Back up so that it becomes next to read
                                         * in forward direction (not obviously right, but that is
                                         * what in-memory case does).
                                         */
                                        if (BufFileSeek(state->myfile, 0,
                                                                        -(long) (tuplen + sizeof(unsigned int)),
                                                                        SEEK_CUR) != 0)
                                                elog(ERROR, "bogus tuple length in backward scan");
                                        return NULL;
                                }
                                tuplen = getlen(state, false);
                        }

                        /*
                         * Now we have the length of the prior tuple, back up and read it.
                         * Note: READTUP expects we are positioned after the initial
                         * length word of the tuple, so back up to that point.
                         */
                        if (BufFileSeek(state->myfile, 0,
                                                        -(long) tuplen,
                                                        SEEK_CUR) != 0)
                                elog(ERROR, "bogus tuple length in backward scan");
                        tup = READTUP(state, tuplen);
                        return tup;

                default:
                        elog(ERROR, "invalid tuplestore state");
                        return NULL;            /* keep compiler quiet */
        }
}

/*
 * tuplestore_gettupleslot - exported function to fetch a MinimalTuple
 *
 * If successful, put tuple in slot and return TRUE; else, clear the slot
 * and return FALSE.
 */
bool
tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
                                                TupleTableSlot *slot)
{
        MinimalTuple tuple;
        bool            should_free;

        tuple = (MinimalTuple) tuplestore_gettuple(state, forward, &should_free);

        if (tuple)
        {
                ExecStoreMinimalTuple(tuple, slot, should_free);
                return true;
        }
        else
        {
                ExecClearTuple(slot);
                return false;
        }
}

/*
 * tuplestore_advance - exported function to adjust position without fetching
 *
 * We could optimize this case to avoid palloc/pfree overhead, but for the
 * moment it doesn't seem worthwhile.
 */
bool
tuplestore_advance(Tuplestorestate *state, bool forward)
{
        void       *tuple;
        bool            should_free;

        tuple = tuplestore_gettuple(state, forward, &should_free);

        if (tuple)
        {
                if (should_free)
                        pfree(tuple);
                return true;
        }
        else
        {
                return false;
        }
}

/*
 * dumptuples - remove tuples from memory and write to tape
 *
 * As a side effect, we must convert each read pointer's position from
 * "current" to file/offset format.  But eof_reached pointers don't
 * need to change state.
 */
static void
dumptuples(Tuplestorestate *state)
{
        int                     i;

        for (i = 0;; i++)
        {
                TSReadPointer *readptr = state->readptrs;
                int                     j;

                for (j = 0; j < state->readptrcount; readptr++, j++)
                {
                        if (i == readptr->current && !readptr->eof_reached)
                                BufFileTell(state->myfile,
                                                        &readptr->file, &readptr->offset);
                }
                if (i >= state->memtupcount)
                        break;
                WRITETUP(state, state->memtuples[i]);
        }
        state->memtupcount = 0;
}

/*
 * tuplestore_rescan            - rewind the active read pointer to start
 */
void
tuplestore_rescan(Tuplestorestate *state)
{
        TSReadPointer *readptr = &state->readptrs[state->activeptr];

        Assert(readptr->eflags & EXEC_FLAG_REWIND);

        switch (state->status)
        {
                case TSS_INMEM:
                        readptr->eof_reached = false;
                        readptr->current = 0;
                        break;
                case TSS_WRITEFILE:
                        readptr->eof_reached = false;
                        readptr->file = 0;
                        readptr->offset = 0L;
                        break;
                case TSS_READFILE:
                        readptr->eof_reached = false;
                        if (BufFileSeek(state->myfile, 0, 0L, SEEK_SET) != 0)
                                elog(ERROR, "tuplestore seek to start failed");
                        break;
                default:
                        elog(ERROR, "invalid tuplestore state");
                        break;
        }
}

/*
 * tuplestore_copy_read_pointer - copy a read pointer's state to another
 */
void
tuplestore_copy_read_pointer(Tuplestorestate *state,
                                                         int srcptr, int destptr)
{
        TSReadPointer *sptr = &state->readptrs[srcptr];
        TSReadPointer *dptr = &state->readptrs[destptr];

        Assert(srcptr >= 0 && srcptr < state->readptrcount);
        Assert(destptr >= 0 && destptr < state->readptrcount);

        /* Assigning to self is a no-op */
        if (srcptr == destptr)
                return;

        if (dptr->eflags != sptr->eflags)
        {
                /* Possible change of overall eflags, so copy and then recompute */
                int             eflags;
                int             i;

                *dptr = *sptr;
                eflags = state->readptrs[0].eflags;
                for (i = 1; i < state->readptrcount; i++)
                        eflags |= state->readptrs[i].eflags;
                state->eflags = eflags;
        }
        else
                *dptr = *sptr;

        switch (state->status)
        {
                case TSS_INMEM:
                        /* We might be able to truncate the tuplestore */
                        tuplestore_trim(state);
                        break;
                case TSS_WRITEFILE:
                        break;
                case TSS_READFILE:
                        /*
                         * This case is a bit tricky since the active read pointer's
                         * position corresponds to the seek point, not what is in its
                         * variables.  Assigning to the active requires a seek, and
                         * assigning from the active requires a tell, except when
                         * eof_reached.
                         */
                        if (destptr == state->activeptr)
                        {
                                if (dptr->eof_reached)
                                {
                                        if (BufFileSeek(state->myfile,
                                                                        state->writepos_file,
                                                                        state->writepos_offset,
                                                                        SEEK_SET) != 0)
                                                elog(ERROR, "tuplestore seek failed");
                                }
                                else
                                {
                                        if (BufFileSeek(state->myfile,
                                                                        dptr->file, dptr->offset,
                                                                        SEEK_SET) != 0)
                                                elog(ERROR, "tuplestore seek failed");
                                }
                        }
                        else if (srcptr == state->activeptr)
                        {
                                if (!dptr->eof_reached)
                                        BufFileTell(state->myfile,
                                                                &dptr->file,
                                                                &dptr->offset);
                        }
                        break;
                default:
                        elog(ERROR, "invalid tuplestore state");
                        break;
        }
}

/*
 * tuplestore_trim      - remove all no-longer-needed tuples
 */
static void
tuplestore_trim(Tuplestorestate *state)
{
        int                     oldest;
        int                     nremove;
        int                     i;

        /*
         * We can truncate the tuplestore if neither backward scan nor
         * rewind capability are required by any read pointer.
         */
        if (state->eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_REWIND))
                return;

        /*
         * We don't bother trimming temp files since it usually would mean more
         * work than just letting them sit in kernel buffers until they age out.
         */
        if (state->status != TSS_INMEM)
                return;

        /* Find the oldest read pointer */
        oldest = state->memtupcount;
        for (i = 0; i < state->readptrcount; i++)
        {
                if (!state->readptrs[i].eof_reached)
                        oldest = Min(oldest, state->readptrs[i].current);
        }

        /*
         * Note: you might think we could remove all the tuples before the oldest
         * "current", since that one is the next to be returned.  However,
         * since tuplestore_gettuple returns a direct pointer to our
         * internal copy of the tuple, it's likely that the caller has
         * still got the tuple just before "current" referenced in a slot.
         * So we keep one extra tuple before the oldest "current".
         */
        nremove = oldest - 1;
        if (nremove <= 0)
                return;                                 /* nothing to do */
        Assert(nremove <= state->memtupcount);

        /* Release no-longer-needed tuples */
        for (i = 0; i < nremove; i++)
        {
                FREEMEM(state, GetMemoryChunkSpace(state->memtuples[i]));
                pfree(state->memtuples[i]);
        }

        /*
         * Slide the array down and readjust pointers.  This may look pretty
         * stupid, but we expect that there will usually not be very many
         * tuple-pointers to move, so this isn't that expensive; and it keeps a
         * lot of other logic simple.
         *
         * In fact, in the current usage for merge joins, it's demonstrable that
         * there will always be exactly one non-removed tuple; so optimize that
         * case.
         */
        if (nremove + 1 == state->memtupcount)
                state->memtuples[0] = state->memtuples[nremove];
        else
                memmove(state->memtuples, state->memtuples + nremove,
                                (state->memtupcount - nremove) * sizeof(void *));

        state->memtupcount -= nremove;
        for (i = 0; i < state->readptrcount; i++)
        {
                if (!state->readptrs[i].eof_reached)
                        state->readptrs[i].current -= nremove;
        }
}


/*
 * Tape interface routines
 */

static unsigned int
getlen(Tuplestorestate *state, bool eofOK)
{
        unsigned int len;
        size_t          nbytes;

        nbytes = BufFileRead(state->myfile, (void *) &len, sizeof(len));
        if (nbytes == sizeof(len))
                return len;
        if (nbytes != 0)
                elog(ERROR, "unexpected end of tape");
        if (!eofOK)
                elog(ERROR, "unexpected end of data");
        return 0;
}


/*
 * Routines specialized for HeapTuple case
 *
 * The stored form is actually a MinimalTuple, but for largely historical
 * reasons we allow COPYTUP to work from a HeapTuple.
 *
 * Since MinimalTuple already has length in its first word, we don't need
 * to write that separately.
 */

static void *
copytup_heap(Tuplestorestate *state, void *tup)
{
        MinimalTuple tuple;

        tuple = minimal_tuple_from_heap_tuple((HeapTuple) tup);
        USEMEM(state, GetMemoryChunkSpace(tuple));
        return (void *) tuple;
}

static void
writetup_heap(Tuplestorestate *state, void *tup)
{
        MinimalTuple tuple = (MinimalTuple) tup;
        /* the part of the MinimalTuple we'll write: */
        char       *tupbody = (char *) tuple + MINIMAL_TUPLE_DATA_OFFSET;
        unsigned int tupbodylen = tuple->t_len - MINIMAL_TUPLE_DATA_OFFSET;
        /* total on-disk footprint: */
        unsigned int tuplen = tupbodylen + sizeof(int);

        if (BufFileWrite(state->myfile, (void *) &tuplen,
                                         sizeof(tuplen)) != sizeof(tuplen))
                elog(ERROR, "write failed");
        if (BufFileWrite(state->myfile, (void *) tupbody,
                                         tupbodylen) != (size_t) tupbodylen)
                elog(ERROR, "write failed");
        if (state->backward)            /* need trailing length word? */
                if (BufFileWrite(state->myfile, (void *) &tuplen,
                                                 sizeof(tuplen)) != sizeof(tuplen))
                        elog(ERROR, "write failed");

        FREEMEM(state, GetMemoryChunkSpace(tuple));
        heap_free_minimal_tuple(tuple);
}

static void *
readtup_heap(Tuplestorestate *state, unsigned int len)
{
        unsigned int tupbodylen = len - sizeof(int);
        unsigned int tuplen = tupbodylen + MINIMAL_TUPLE_DATA_OFFSET;
        MinimalTuple tuple = (MinimalTuple) palloc(tuplen);
        char       *tupbody = (char *) tuple + MINIMAL_TUPLE_DATA_OFFSET;

        USEMEM(state, GetMemoryChunkSpace(tuple));
        /* read in the tuple proper */
        tuple->t_len = tuplen;
        if (BufFileRead(state->myfile, (void *) tupbody,
                                         tupbodylen) != (size_t) tupbodylen)
                elog(ERROR, "unexpected end of data");
        if (state->backward)            /* need trailing length word? */
                if (BufFileRead(state->myfile, (void *) &tuplen,
                                                sizeof(tuplen)) != sizeof(tuplen))
                        elog(ERROR, "unexpected end of data");
        return (void *) tuple;
}