On 11/6/08, Jaime Casanova <jcasanov@systemguards.com.ec> wrote:
> On Thu, Nov 6, 2008 at 9:24 AM, Matteo Beccati <php@beccati.com> wrote:
>> Hi,
>>
>>> Attached test shows a regression in analyze command.
>>> Expected rows in an empty table is 2140 even after an ANALYZE is
>>> executed
>>
>> Doesn't seem to be a regression to me, as I've just checked that 8.0 did
>> behave the same. However the question also was raised a few days ago on
>> the italian mailing list and I couldn't find a reasonable explanation
>> for it.
>>
>
this is related to this hack: src/backend/optimizer/util/plancat.c:342
/* * HACK: if the relation has never yet been vacuumed, use a * minimum estimate of 10
pages. This emulates a desirable aspect * of pre-8.0 behavior, which is that we wouldn't assume a newly
* created relation is really small, which saves us from making * really bad plans during initial data
loading. (The plans are * not wrong when they are made, but if they are cached and used * again
afterthe table has grown a lot, they are bad.) It would * be better to force replanning if the table size
haschanged a * lot since the plan was made ... but we don't currently have any * infrastructure
forredoing cached plans at all, so we have to * kluge things here instead. * * We
approximate"never vacuumed" by "has relpages = 0", which * means this will also fire on genuinely empty
relations. Not * great, but fortunately that's a seldom-seen case in the real * world, and it
shouldn'tdegrade the quality of the plan too * much anyway to err in this direction. */
if(curpages < 10 && rel->rd_rel->relpages == 0) curpages = 10;
commenting that two lines make the estimates correct. now that we have
plan invalidation that hack is still needed?
i know that as the comment suggest this has no serious impact but
certainly this is user visible.
--
Atentamente,
Jaime Casanova
Soporte y capacitación de PostgreSQL
Asesoría y desarrollo de sistemas
Guayaquil - Ecuador
Cel. +59387171157