From f6a0863e3cb72763490ceca2c558d5ef2dddd5f2 Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Mon, 20 Dec 2010 12:59:33 -0500
Subject: [PATCH] Allow transactions that don't write WAL to commit
asynchronously.
This case can arise if a transaction has written data, but only to
temporary tables. Loss of the commit record in case of a crash won't
matter, because the temporary tables will be lost anyway.
Reviewed by Heikki Linnakangas and Simon Riggs.
---
src/backend/access/transam/xact.c | 33 +++++++++++++++++++++----------
1 file changed, 23 insertions(+), 10 deletions(-)
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 79c9c0d8446..67f817ca4b8 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -907,6 +907,7 @@ RecordTransactionCommit(void)
int nmsgs = 0;
SharedInvalidationMessage *invalMessages = NULL;
bool RelcacheInitFileInval = false;
+ bool wrote_xlog;
/* Get data needed for commit record */
nrels = smgrGetPendingDeletes(true, &rels);
@@ -914,6 +915,7 @@ RecordTransactionCommit(void)
if (XLogStandbyInfoActive())
nmsgs = xactGetCommittedInvalidationMessages(&invalMessages,
&RelcacheInitFileInval);
+ wrote_xlog = (XactLastRecEnd.xrecoff != 0);
/*
* If we haven't been assigned an XID yet, we neither can, nor do we want
@@ -940,7 +942,7 @@ RecordTransactionCommit(void)
* assigned is a sequence advance record due to nextval() --- we want
* to flush that to disk before reporting commit.)
*/
- if (XactLastRecEnd.xrecoff == 0)
+ if (!wrote_xlog)
goto cleanup;
}
else
@@ -1028,16 +1030,27 @@ RecordTransactionCommit(void)
}
/*
- * Check if we want to commit asynchronously. If the user has set
- * synchronous_commit = off, and we're not doing cleanup of any non-temp
- * rels nor committing any command that wanted to force sync commit, then
- * we can defer flushing XLOG. (We must not allow asynchronous commit if
- * there are any non-temp tables to be deleted, because we might delete
- * the files before the COMMIT record is flushed to disk. We do allow
- * asynchronous commit if all to-be-deleted tables are temporary though,
- * since they are lost anyway if we crash.)
+ * Check if we want to commit asynchronously. We can allow the XLOG flush
+ * to happen asynchronously if synchronous_commit=off, or if the current
+ * transaction has not performed any WAL-logged operation. The latter case
+ * can arise if the current transaction wrote only to temporary tables.
+ * In case of a crash, the loss of such a transaction will be irrelevant
+ * since temp tables will be lost anyway. (Given the foregoing, you might
+ * think that it would be unnecessary to emit the XLOG record at all in
+ * this case, but we don't currently try to do that. It would certainly
+ * cause problems at least in Hot Standby mode, where the KnownAssignedXids
+ * machinery requires tracking every XID assignment. It might be OK to
+ * skip it only when wal_level < hot_standby, but for now we don't.)
+ *
+ * However, if we're doing cleanup of any non-temp rels or committing any
+ * command that wanted to force sync commit, then we must flush XLOG
+ * immediately. (We must not allow asynchronous commit if there are any
+ * non-temp tables to be deleted, because we might delete the files before
+ * the COMMIT record is flushed to disk. We do allow asynchronous commit
+ * if all to-be-deleted tables are temporary though, since they are lost
+ * anyway if we crash.)
*/
- if (XactSyncCommit || forceSyncCommit || nrels > 0)
+ if ((wrote_xlog && XactSyncCommit) || forceSyncCommit || nrels > 0)
{
/*
* Synchronous commit case:
--
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