Re: On-the-fly index tuple deletion vs. hot_standby

On 29.11.2010 08:10, Noah Misch wrote:
> I have a hot_standby system and use it to bear the load of various reporting
> queries that take 15-60 minutes each. In an effort to avoid long pauses in
> recovery, I set a vacuum_defer_cleanup_age constituting roughly three hours of
> the master's transactions. Even so, I kept seeing recovery pause for the
> duration of a long-running query. In each case, the culprit record was an
> XLOG_BTREE_DELETE arising from on-the-fly deletion of an index tuple. The
> attached test script demonstrates the behavior (on HEAD); the index tuple
> reclamation conflicts with a concurrent "SELECT pg_sleep(600)" on the standby.
>
> Since this inserting transaction aborts, HeapTupleSatisfiesVacuum reports
> HEAPTUPLE_DEAD independent of vacuum_defer_cleanup_age. We go ahead and remove
> the index tuples. On the standby, btree_xlog_delete_get_latestRemovedXid does
> not regard the inserting-transaction outcome, so btree_redo proceeds to conflict
> with snapshots having visibility over that transaction. Could we correctly
> improve this by teaching btree_xlog_delete_get_latestRemovedXid to ignore tuples
> of aborted transactions and tuples inserted and deleted within one transaction?

Seems reasonable. HeapTupleHeaderAdvanceLatestRemovedXid() will need
similar treatment. Actually, btree_xlog_delete_get_latestRemovedXid()
could just call HeapTupleHeaderAdvanceLatestRemoveXid().

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com