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 204e1eb7aade44b1985d603f09a3996e0a5665f2..e8d11a1d0eef4746ddaa222436bf43fab126b5d3 100644 (file)
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -5941,48 +5941,6 @@ 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 9080a3c16097b1ee793d3f2ce498497761ca55a8..1385ff3d04c7fa30c62320ee7f08c8b1cce89946 100644 (file)
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -606,7 +606,6 @@ typedef struct ViewOptions
 /* 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 6641ee510a1db05c6ba735ba69b49011478af425..62351a06bdf4dd9fd518d73c56cce058fd073cec 100644 (file)
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -41,7 +41,6 @@
     RelationNeedsWAL(rel) \
   && !IsCatalogRelation(rel) \
   && !RelationIsAccessibleInLogicalDecoding(rel) \
-  && !RelationHasUnloggedIndex(rel) \
 )
 
 #define EarlyPruningEnabled(rel) (old_snapshot_threshold >= 0 && RelationAllowsEarlyPruning(rel))