Re: Deriving Recovery Snapshots

On Wed, 2008-10-22 at 21:47 +0100, Simon Riggs wrote:

> But once you reach 64 transactions, you'll need to write an extra WAL
> record for every subtransaction, which currently I've managed to avoid.

Some further notes/tests on the optimisation you discovered.

Because of the way I have changed tqual.c, we only need to write to
subtrans if a proc's subxid cache overflows. (I think we would need to
change tqual.c in a similar way in any of the ways so far discussed).

Anyway, quick test with a representative test case shows 3-5%
performance gain from skipping the subtrans updates. I only the test
case files here and the test patch. The test is a simple PL/pgSQL
function with one EXCEPTION clause, so fairly real world.

The patch isn't ready to apply standalone because we need to include the
changes to XidInMVCCSnapshot() also, which would take a little while to
extract. Let me know if that is worth producing a standalone patch for.

--
Simon Riggs www.2ndQuadrant.com
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