Fix hash index vs "snapshot too old" problemms

Hash indexes are not WAL-logged, and so do not maintain the LSN of
index pages.  Since the "snapshot too old" feature counts on
detecting error conditions using the LSN of a table and all indexes
on it, this makes it impossible to safely do early vacuuming on any
table with a hash index, so add this to the tests for whether the
xid used to vacuum a table can be adjusted based on
old_snapshot_threshold.

While at it, add a paragraph to the docs for old_snapshot_threshold
which specifically mentions this and other aspects of the feature
which may otherwise surprise users.

Problem reported and patch reviewed by Amit Kapila

1 files changed, 46 insertions, 0 deletions

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 432feefa609..79cc3df590a 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -5313,6 +5313,52 @@ RelationIdIsInInitFile(Oid relationId)
 }
 
 /*
+ * Tells whether any index for the relation is unlogged.
+ *
+ * Any index using the hash AM is implicitly unlogged.
+ *
+ * Note: There doesn't seem to be any way to have an unlogged index attached
+ * to a permanent table except to create a hash index, 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
+			|| reltup->relam == HASH_AM_OID)
+			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
  * local init file.