Re: [HACKERS] Neverending query on 6.5.2 over Solaris 2.5.1

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
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