Improve performance of find_tabstat_entry()/get_tabstat_entry()

Patch introduces a hash map reloid -> PgStat_TableStatus which improves
performance in case of large number of tables/partitions.

Author: Aleksander Alekseev
Reviewed-by: Andres Freund, Anastasia Lubennikova, Tels, me
https://commitfest.postgresql.org/13/1058/

diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 869afd4c4a5703d0df70542acf24f9d82f8fc9f4..56a8bf2d17f7582554128b4cfe1df9ddf955887b 100644 (file)
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -173,6 +173,20 @@ typedef struct TabStatusArray
 
 static TabStatusArray *pgStatTabList = NULL;
 
+/*
+ * pgStatTabHash entry
+ */
+typedef struct TabStatHashEntry
+{
+       Oid t_id;
+       PgStat_TableStatus* tsa_entry;
+} TabStatHashEntry;
+
+/*
+ * Hash table for O(1) t_id -> tsa_entry lookup
+ */
+static HTAB *pgStatTabHash = NULL;
+
 /*
  * Backends store per-function info that's waiting to be sent to the collector
  * in this hash table (indexed by function OID).
@@ -840,6 +854,14 @@ pgstat_report_stat(bool force)
                tsa->tsa_used = 0;
        }
 
+       /*
+        * pgStatTabHash is outdated on this point so we have to clean it,
+        * hash_destroy() will remove hash memory context, allocated in
+        * make_sure_stat_tab_initialized()
+        */
+       hash_destroy(pgStatTabHash);
+       pgStatTabHash = NULL;
+
        /*
         * Send partial messages.  Make sure that any pending xact commit/abort
         * gets counted, even if there are no table stats to send.
@@ -1684,60 +1706,88 @@ pgstat_initstats(Relation rel)
        rel->pgstat_info = get_tabstat_entry(rel_id, rel->rd_rel->relisshared);
 }
 
+/*
+ * Make sure pgStatTabList and pgStatTabHash are initialized.
+ */
+static void
+make_sure_stat_tab_initialized()
+{
+       HASHCTL                 ctl;
+       MemoryContext   new_ctx;
+
+       if(!pgStatTabList)
+       {
+               /* This is first time procedure is called */
+               pgStatTabList = (TabStatusArray *) MemoryContextAllocZero(TopMemoryContext,
+                                                                                               sizeof(TabStatusArray));
+       }
+
+       if(pgStatTabHash)
+               return;
+
+       /* Hash table was freed or never existed.  */
+
+       new_ctx = AllocSetContextCreate(
+               TopMemoryContext,
+               "PGStatLookupHashTableContext",
+               ALLOCSET_DEFAULT_SIZES);
+
+       memset(&ctl, 0, sizeof(ctl));
+       ctl.keysize = sizeof(Oid);
+       ctl.entrysize = sizeof(TabStatHashEntry);
+       ctl.hcxt = new_ctx;
+
+       pgStatTabHash = hash_create("pgstat t_id to tsa_entry lookup hash table",
+               TABSTAT_QUANTUM, &ctl, HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+}
+
 /*
  * get_tabstat_entry - find or create a PgStat_TableStatus entry for rel
  */
 static PgStat_TableStatus *
 get_tabstat_entry(Oid rel_id, bool isshared)
 {
+       TabStatHashEntry* hash_entry;
        PgStat_TableStatus *entry;
        TabStatusArray *tsa;
-       TabStatusArray *prev_tsa;
-       int                     i;
+       bool found;
+
+       make_sure_stat_tab_initialized();
 
        /*
-        * Search the already-used tabstat slots for this relation.
+        * Find an entry or create a new one.
         */
-       prev_tsa = NULL;
-       for (tsa = pgStatTabList; tsa != NULL; prev_tsa = tsa, tsa = tsa->tsa_next)
+       hash_entry = hash_search(pgStatTabHash, &rel_id, HASH_ENTER, &found);
+       if(found)
+               return hash_entry->tsa_entry;
+
+       /*
+        * `hash_entry` was just created and now we have to fill it.
+        * First make sure there is a free space in a last element of pgStatTabList.
+        */
+       tsa = pgStatTabList;
+       while(tsa->tsa_used == TABSTAT_QUANTUM)
        {
-               for (i = 0; i < tsa->tsa_used; i++)
+               if(tsa->tsa_next == NULL)
                {
-                       entry = &tsa->tsa_entries[i];
-                       if (entry->t_id == rel_id)
-                               return entry;
+                       tsa->tsa_next = (TabStatusArray *) MemoryContextAllocZero(TopMemoryContext,
+                                                                                                               sizeof(TabStatusArray));
                }
 
-               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;
-               }
+               tsa = tsa->tsa_next;
        }
 
        /*
-        * 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.
+        * Add an entry.
         */
        entry = &tsa->tsa_entries[tsa->tsa_used++];
        entry->t_id = rel_id;
        entry->t_shared = isshared;
+
+       /*
+        * Add a corresponding entry to pgStatTabHash.
+        */
+       hash_entry->tsa_entry = entry;
        return entry;
 }
 
@@ -1749,22 +1799,19 @@ get_tabstat_entry(Oid rel_id, bool isshared)
 PgStat_TableStatus *
 find_tabstat_entry(Oid rel_id)
 {
-       PgStat_TableStatus *entry;
-       TabStatusArray *tsa;
-       int                     i;
+       TabStatHashEntry* hash_entry;
 
-       for (tsa = pgStatTabList; tsa != NULL; 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;
-               }
-       }
+       /*
+        * There are no entries at all.
+        */
+       if(!pgStatTabHash)
+               return NULL;
 
-       /* Not present */
-       return NULL;
+       hash_entry = hash_search(pgStatTabHash, &rel_id, HASH_FIND, NULL);
+       if(!hash_entry)
+               return NULL;
+
+       return hash_entry->tsa_entry;
 }
 
 /*