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+    * pgStatTabHash is outdated on this point so we have to clean it,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;
 }
 
 /*