Avoid catalog lookups in RelationAllowsEarlyPruning().

RelationAllowsEarlyPruning() performed a catalog scan, but is used
in two contexts where that was a bad idea:

1.  In heap_page_prune_opt(), which runs very frequently in some large
    scans.  This caused major performance problems in a field report
    that was easy to reproduce.

2.  In TestForOldSnapshot(), which runs while we hold a buffer content
    lock.  It's not clear if this was guaranteed to be free of buffer
    deadlock risk.

The check was introduced in commit 2cc41acd8 and defended against a
real problem: 9.6's hash indexes have no page LSN and so we can't
allow early pruning (ie the snapshot-too-old feature).  We can remove
the check from all later releases though: hash indexes are now logged,
and there is no way to create UNLOGGED indexes on regular logged
tables.

If a future release allows such a combination, it might need to put
a similar check in place, but it'll need some more thought.

Back-patch to 10.

Author: Thomas Munro
Reviewed-by: Tom Lane, who spotted the second problem
Discussion: https://postgr.es/m/CA%2BhUKGKT8oTkp5jw_U4p0S-7UG9zsvtw_M47Y285bER6a2gD%2Bg%40mail.gmail.com
Discussion: https://postgr.es/m/CAA4eK1%2BWy%2BN4eE5zPm765h68LrkWc3Biu_8rzzi%2BOYX4j%2BiHRw%40mail.gmail.com

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 65b74cb6d8788c4975c8ae0076f09e8e0293625b..24e830a39c19788984be0e11197b69f98c94a8a7 100644 (file)
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -6068,48 +6068,6 @@ RelationIdIsInInitFile(Oid relationId)
        return RelationSupportsSysCache(relationId);
 }
 
-/*
- * Tells whether any index for the relation is unlogged.
- *
- * Note: There doesn't seem to be any way to have an unlogged index attached
- * to a permanent table, but it seems best to keep this general so that it
- * returns sensible results even when they seem obvious (like for an unlogged
- * table) and to handle possible future unlogged indexes on permanent tables.
- */
-bool
-RelationHasUnloggedIndex(Relation rel)
-{
-       List       *indexoidlist;
-       ListCell   *indexoidscan;
-       bool            result = false;
-
-       indexoidlist = RelationGetIndexList(rel);
-
-       foreach(indexoidscan, indexoidlist)
-       {
-               Oid                     indexoid = lfirst_oid(indexoidscan);
-               HeapTuple       tp;
-               Form_pg_class reltup;
-
-               tp = SearchSysCache1(RELOID, ObjectIdGetDatum(indexoid));
-               if (!HeapTupleIsValid(tp))
-                       elog(ERROR, "cache lookup failed for relation %u", indexoid);
-               reltup = (Form_pg_class) GETSTRUCT(tp);
-
-               if (reltup->relpersistence == RELPERSISTENCE_UNLOGGED)
-                       result = true;
-
-               ReleaseSysCache(tp);
-
-               if (result == true)
-                       break;
-       }
-
-       list_free(indexoidlist);
-
-       return result;
-}
-
 /*
  * Invalidate (remove) the init file during commit of a transaction that
  * changed one or more of the relation cache entries that are kept in the

diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index 58877e7f1cf0f6d830a523497c99eae6769a91cf..c6b8f4fb47f7e974479639f38783d4072f5c18a3 100644 (file)
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -638,7 +638,6 @@ get_partition_col_typmod(PartitionKey key, int col)
 /* routines in utils/cache/relcache.c */
 extern void RelationIncrementReferenceCount(Relation rel);
 extern void RelationDecrementReferenceCount(Relation rel);
-extern bool RelationHasUnloggedIndex(Relation rel);
 extern List *RelationGetRepsetList(Relation rel);
 
 #endif                                                 /* REL_H */

diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 8585194e78544eeedafb47f6bd13c967fbf0765f..7b428fe9c01dffdb53f506da7f6129546bde7cdd 100644 (file)
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -40,7 +40,6 @@
         RelationNeedsWAL(rel) \
   && !IsCatalogRelation(rel) \
   && !RelationIsAccessibleInLogicalDecoding(rel) \
-  && !RelationHasUnloggedIndex(rel) \
 )
 
 #define EarlyPruningEnabled(rel) (old_snapshot_threshold >= 0 && RelationAllowsEarlyPruning(rel))