From: | Fernando Schapachnik <fpscha(at)ns1(dot)via-net-works(dot)net(dot)ar> |
---|---|
To: | tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane) |
Cc: | fpscha(at)via-net-works(dot)net(dot)ar, pgsql-hackers(at)postgreSQL(dot)org |
Subject: | Re: [HACKERS] Neverending query on 6.5.2 over Solaris 2.5.1 |
Date: | 1999-10-23 18:29:01 |
Message-ID: | 199910231829.PAA13176@ns1.via-net-works.net.ar |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
En un mensaje anterior, Tom Lane escribió:
> I wrote:
> > Weird. I assume that your 'activa' field is 'bool'? I've been trying
> > to duplicate this misbehavior here, and as near as I can tell the system
> > handles selectivity estimates for boolean fields just fine. Whatever
> > percentage of 't' values was seen by the last VACUUM ANALYZE is exactly
> > what it uses.
>
> On second thought: 6.5.* can get confused if the column contains more
> NULLs than anything else. Dunno if you have a lot of nulls in activa,
> but if so you might try changing them all to explicit 'f' and then
> redoing the VACUUM ANALYZE. Next release will be smarter about keeping
> stats in the presence of many nulls.
>
> It'd be useful to double-check my theory that the system is
> misestimating the selectivity of the WHERE (u.activa) clause.
> You could try this:
> SELECT count(*) FROM usarios WHERE activa;
10571
> EXPLAIN SELECT count(*) FROM usarios WHERE activa;
> and see how far off the row count estimate in the EXPLAIN is
> from reality.
NOTICE: QUERY PLAN:
Aggregate (cost=498.84 rows=1 width=4)
-> Seq Scan on usuarios (cost=498.84 rows=1 width=4)
EXPLAIN
Don't hesitate in asking any other info/test you may consider useful.
Regards!
Fernando P. Schapachnik
Administración de la red
VIA Net Works Argentina SA
Diagonal Roque Sáenz Peña 971, 4º y 5º piso.
1035 - Capital Federal, Argentina.
(54-11) 4323-3333
http://www.via-net-works.net.ar
From | Date | Subject | |
---|---|---|---|
Next Message | The Hermit Hacker | 1999-10-23 19:23:57 | Re: [ADMIN] Re: [HACKERS] RFC: Industrial-strength logging (long message) |
Previous Message | Fernando Schapachnik | 1999-10-23 18:25:25 | Re: [HACKERS] Neverending query on 6.5.2 over Solaris 2.5.1 |