Re: PATCH to allow concurrent VACUUMs to not lock each

Hannu Krosing wrote:
> *** src/backend/access/transam/xact.c 28 Apr 2005 21:47:10 -0000 1.200
> --- src/backend/access/transam/xact.c 17 May 2005 22:06:34 -0000
> ***************
> *** 1411,1416 ****
> --- 1411,1424 ----
> AfterTriggerBeginXact();
>
> /*
> + * mark the transaction as not VACUUM (vacuum_rel will set isVacuum to true
> + * directly after calling BeginTransactionCommand() )
> + */
> + if (MyProc != NULL)
> + {
> + MyProc->inVacuum = false;
> + }

I'm a little worried about having this set to "true" after a VACUUM is
executed, and only reset to false when the next transaction is begun: it
shouldn't affect correctness right now, but it seems like asking for
trouble. Resetting the flag to "false" after processing a transaction
would probably be worth doing.

> *** src/backend/commands/vacuum.c 6 May 2005 17:24:53 -0000 1.308
> --- src/backend/commands/vacuum.c 17 May 2005 22:06:35 -0000
> ***************
> *** 420,425 ****
> --- 418,428 ----
> if (use_own_xacts)
> {
> StartTransactionCommand();
> + if (MyProc != NULL) /* is this needed here ? */
> + {
> + /* so other vacuums don't look at our xid/xmin in GetOldestXmin() */
> + MyProc->inVacuum = true;
> + }
> /* functions in indexes may want a snapshot set */
> ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
> }

Is it valid to apply this optimization to ANALYZE? Since ANALYZE updates
pg_statistic, ISTM it can affect tuple visibility.

> *** src/backend/storage/ipc/sinval.c 31 Dec 2004 22:00:56 -0000 1.75
> --- src/backend/storage/ipc/sinval.c 17 May 2005 22:06:36 -0000
> ***************
> *** 845,854 ****
> * them as running anyway. We also assume that such xacts
> * can't compute an xmin older than ours, so they needn't be
> * considered in computing globalxmin.
> */
> if (proc == MyProc ||
> !TransactionIdIsNormal(xid) ||
> ! TransactionIdFollowsOrEquals(xid, xmax))
> continue;
>
> if (TransactionIdPrecedes(xid, xmin))
> --- 845,858 ----
> * them as running anyway. We also assume that such xacts
> * can't compute an xmin older than ours, so they needn't be
> * considered in computing globalxmin.
> + *
> + * there is also no need to consider transaxtions runnibg the
> + * vacuum command as it will not affect tuple visibility
> */

Typos.

-Neil