2010/4/28 Robert Haas <robertmhaas@gmail.com>:
> On Mon, Apr 26, 2010 at 5:33 AM, Cédric Villemain
> <cedric.villemain.debian@gmail.com> wrote:
>> In the first query, the planner doesn't use the information of the 2,3,4.
>> It just does a : I'll bet I'll have 2 rows in t1 (I think it should
>> say 3, but it doesn't)
>> So it divide the estimated number of rows in the t2 table by 5
>> (different values) and multiply by 2 (rows) : 40040.
>
> I think it's doing something more complicated. See scalararraysel().
>
>> In the second query the planner use a different behavior : it did
>> expand the value of t1.t to t2.t for each join relation and find a
>> costless plan. (than the one using seqscan on t2)
>
> I think the problem here is one we've discussed before: if the query
> planner knows that something is true of x (like, say, x =
> ANY('{2,3,4}')) and it also knows that x = y, it doesn't infer that
> the same thing holds of y (i.e. y = ANY('{2,3,4}') unless the thing
> that is known to be true of x is that x is equal to some constant.
> Tom doesn't think it would be worth the additional CPU time that it
> would take to make these sorts of deductions. I'm not sure I believe
> that, but I haven't tried to write the code, either.
Relative to this too :
http://archives.postgresql.org/pgsql-general/2010-05/msg00009.php ?
>
> ...Robert
>
--
Cédric Villemain