Re: regression in analyze

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