Re: Concurrency question

2009/7/7 Mark Steben <msteben@autorevenue.com>:
> Any help here appreciated.
>
> I ran a vacuum verbose analyze on a database over the weekend.  It ran fine
> until it tried to vacuum a table less than 2000 pages.  It successfully
> acquired a ShareUpdateExclusiveLock as I would expect.
> There was an idle thread that had an AccessSharelock on the same table.
> Compatible locks I would think. But the vacuum hung until the
> AccessSharelock thread was cancelled - 11 hours in all.
> This table normally vacuums in less than 15 seconds.   This AccessSharelock
> came from a query that formerly was part of a transaction sent from a remote
> server.

Not sure what you mean by formerly was part of a transaction.  If the
transaction has rolled back, then the vacuum can proceed.  If the
transaction is till open, then it's not formerly a part of it, it IS a
part of it.  Either way, open transactions block vacuum on updated
tables.

>  Could it be that it hung because it was
> A transaction?   Even so I thought those lock types were compatible.

Nope.  If you've got an idle transaction that's updated tuples, the
vacuum waits on it.  Long running / idle transactions are generally a
bad thing.