Tom Lane wrote:

> I'm wondering about doing something similar to what
> TransactionIdIsInProgress does, ie, make use of the PGPROC lists
> of live subtransactions.  Suppose that GetSnapshotData copies not
> only top xids but live subxids into the snapshot, and adds a flag
> indicating whether the subxids are complete (ie, none of the subxid
> lists have overflowed).  Then if the flag is set, tqual.c doesn't
> need to do SubTransGetTopmostTransaction() before searching the
> list.

Well, that sounds awfully more expensive than setting
local-to-my-database Xmins as well as global (all databases) Xmins :-)

On the other hand, ISTM as soon as one cache overflows, you have to go
check pg_subtrans which means the entire optimization buys nothing in
that case.  It would be nice if the optimization degraded more
gracefully.  I don't have any concrete suggestion though.  The changes
proposed in the other CS-storm thread by the NTT person may help.

Alvaro Herrera                      
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

---------------------------(end of broadcast)---------------------------
TIP 3: Have you checked our extensive FAQ?


Reply via email to