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

src/backend/utils/cache/relcache.c

@@ -5913,48 +5913,6 @@ 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

src/include/utils/rel.h

@@ -605,6 +605,5 @@ typedef struct ViewOptions
 /* routines in utils/cache/relcache.c */
 extern void RelationIncrementReferenceCount(Relation rel);
 extern void RelationDecrementReferenceCount(Relation rel);
-extern bool RelationHasUnloggedIndex(Relation rel);
 
 #endif							/* REL_H */

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))