Improve performance of find_all_inheritors()
Previous coding uses three nested loops which obviously were a pain for large number of table's children. Patch replaces inner loop with a hashmap. Author: Aleksander Alekseev Reviewed-by: me https://commitfest.postgresql.org/13/1058/
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@ -31,7 +31,16 @@
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#include "utils/fmgroids.h"
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#include "utils/syscache.h"
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#include "utils/tqual.h"
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#include "utils/memutils.h"
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/*
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* Entry of a hash table used in find_all_inheritors. See below.
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*/
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typedef struct SeenRelsEntry
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{
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Oid rel_id; /* relation oid */
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ListCell *numparents_cell; /* corresponding list cell */
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} SeenRelsEntry;
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/*
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* find_inheritance_children
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@ -157,10 +166,33 @@ find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
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List *
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find_all_inheritors(Oid parentrelId, LOCKMODE lockmode, List **numparents)
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{
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/* hash table for O(1) rel_oid -> rel_numparents cell lookup */
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HTAB *seen_rels;
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HASHCTL ctl;
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MemoryContext new_ctx;
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List *rels_list,
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*rel_numparents;
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ListCell *l;
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/*
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* We need a separate memory context for a hash table. This is because
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* hash table is used only in this procedure. To free a memory we need to
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* call hash_destroy which is just a wrapper around MemoryContextDelete.
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*/
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new_ctx = AllocSetContextCreate(CurrentMemoryContext,
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"FindAllInheritorsSeenRelsContext",
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ALLOCSET_DEFAULT_SIZES);
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memset(&ctl, 0, sizeof(ctl));
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ctl.keysize = sizeof(Oid);
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ctl.entrysize = sizeof(SeenRelsEntry);
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ctl.hcxt = new_ctx;
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seen_rels = hash_create(
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"find_all_inheritors temporary table",
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32, /* start small and extend */
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&ctl, HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
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/*
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* We build a list starting with the given rel and adding all direct and
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* indirect children. We can use a single list as both the record of
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@ -190,26 +222,21 @@ find_all_inheritors(Oid parentrelId, LOCKMODE lockmode, List **numparents)
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foreach(lc, currentchildren)
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{
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Oid child_oid = lfirst_oid(lc);
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bool found = false;
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ListCell *lo;
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ListCell *li;
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bool found;
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SeenRelsEntry *hash_entry;
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hash_entry = hash_search(seen_rels, &child_oid, HASH_ENTER, &found);
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if (found)
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{
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/* if the rel is already there, bump number-of-parents counter */
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forboth(lo, rels_list, li, rel_numparents)
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{
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if (lfirst_oid(lo) == child_oid)
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{
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lfirst_int(li)++;
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found = true;
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break;
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lfirst_int(hash_entry->numparents_cell)++;
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}
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}
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else
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{
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/* if it's not there, add it. expect 1 parent, initially. */
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if (!found)
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{
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rels_list = lappend_oid(rels_list, child_oid);
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rel_numparents = lappend_int(rel_numparents, 1);
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hash_entry->numparents_cell = rel_numparents->tail;
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}
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}
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}
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@ -218,6 +245,9 @@ find_all_inheritors(Oid parentrelId, LOCKMODE lockmode, List **numparents)
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*numparents = rel_numparents;
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else
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list_free(rel_numparents);
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hash_destroy(seen_rels);
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return rels_list;
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}
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