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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.
This commit is contained in:
Robert Haas 2010-12-20 12:59:33 -05:00
parent f9e9763b62
commit f6a0863e3c
1 changed files with 23 additions and 10 deletions
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33
src/backend/access/transam/xact.c
View File

@ -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: