While looking at this:
https://stackoverflow.com/q/50413623/6464308
I realized that "LOCK TABLE <unlogged table>" puts a
Standby/LOCK into the WAL stream, which causes a log flush
at COMMIT time.
That hurts performance, and I doubt that it is necessary.
At any rate, DROP TABLE on an unlogged table logs nothing.
The attached patch would take care of it, but I'm not sure
if that's the right place to check.
Yours,
Laurenz Albe
From d474e7b41298944e43bb9141eb33adbdd9ea1098 Mon Sep 17 00:00:00 2001
From: Laurenz Albe <laurenz.a...@cybertec.at>
Date: Tue, 22 May 2018 18:13:31 +0200
Subject: [PATCH] Don't log locks on unlogged tables
---
src/backend/storage/lmgr/lock.c | 28 +++++++++++++++++++++-------
1 file changed, 21 insertions(+), 7 deletions(-)
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index dc3d8d9817..70cac47ab3 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -37,6 +37,7 @@
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "access/xlog.h"
+#include "catalog/pg_class.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
@@ -47,6 +48,7 @@
#include "storage/standby.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
+#include "utils/rel.h"
#include "utils/resowner_private.h"
@@ -1041,13 +1043,25 @@ LockAcquireExtended(const LOCKTAG *locktag,
*/
if (log_lock)
{
- /*
- * Decode the locktag back to the original values, to avoid sending
- * lots of empty bytes with every message. See lock.h to check how a
- * locktag is defined for LOCKTAG_RELATION
- */
- LogAccessExclusiveLock(locktag->locktag_field1,
- locktag->locktag_field2);
+ bool unlogged_rel = false;
+
+ if (locktag->locktag_type == LOCKTAG_RELATION)
+ {
+ Relation r = RelationIdGetRelation(locktag->locktag_field2);
+ unlogged_rel = r->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED;
+ RelationClose(r);
+ }
+
+ if (!unlogged_rel)
+ {
+ /*
+ * Decode the locktag back to the original values, to avoid sending
+ * lots of empty bytes with every message. See lock.h to check how a
+ * locktag is defined for LOCKTAG_RELATION
+ */
+ LogAccessExclusiveLock(locktag->locktag_field1,
+ locktag->locktag_field2);
+ }
}
return LOCKACQUIRE_OK;
--
2.14.3
