Re: Not HOT enough

Simon Riggs <simon@2ndquadrant.com>

From: Simon Riggs <simon@2ndQuadrant.com>

To: Tom Lane <tgl@sss.pgh.pa.us>

Cc: Alvaro Herrera <alvherre@commandprompt.com>, Robert Haas <robertmhaas@gmail.com>, Pg Hackers <pgsql-hackers@postgresql.org>

Date: 2011-11-23T19:33:18Z

Lists: pgsql-hackers

Attachments

fix_getsnapshotdata.v3.patch (application/octet-stream) patch v3

On Wed, Nov 23, 2011 at 6:15 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

>> The real question is do we favour HOT cleanup on those small 8 tables,
>> or do we favour HOT cleanup of every other table?
>
> No, the real question is why not think a little harder and see if we can
> come up with a solution that doesn't involve making some cases worse to
> make others better.

Slightly modified patch attached.

When we access a shared relation and the page is prunable we re-derive
the cutoff value using GetOldestXmin.

That code path is rarely taken. In particular, pg_shdepend is only
accessed during object creation/alter/drop, so this isn't a path we
can't spare a small amount little extra on, just like the trade-off
we've taken to make locking faster for DML while making DDL a little
slower.

If this is still unacceptable, then I'll have to look at reducing
impact on pg_shdepend from temp tables - which is still a plan.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
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