Re: performance regression in 9.2/9.3 - Mailing list pgsql-hackers

On 05/06/14 19:39, Tom Lane wrote:
> Merlin Moncure <mmoncure@gmail.com> writes:
>> On Thu, Jun 5, 2014 at 9:54 AM, Linos <info@linos.es> wrote:
>>> What I don't understand is why the statistics have this bad information, all my tests are done on a database just
restoredand analyzed. Can I do something to improve the quality of my database statistics and let the planner do better
choices?Maybe increase the statistics target of the columns involved?
 
>> By that I meant row count estimates coming out of the joins are way
>> off.  This is pushing the planner into making bad choices.  The most
>> pervasive problem I see is that the row count estimate boils down to
>> '1' at some juncture causing the server to favor nestloop/index scan
>> when something like a hash join would likely be more appropriate.
> There's some fairly wacko stuff going on in this example, like why
> is the inner HashAggregate costed so much higher by 9.3 than 8.4,
> when the inputs are basically the same?  And why does 9.3 fail to
> suppress the SubqueryScan on "ven", when 8.4 does get rid of it?
> And why is the final output rows estimate so much higher in 9.3?
> That one is actually higher than the product of the two nestloop
> inputs, which looks like possibly a bug.
>
> I think what's happening is that 9.3 is picking what it knows to be a less
> than optimal join method so that it can sort the output by means of the
> ordered scan "Index Scan using referencia_key on modelo mo", and thereby
> avoid an explicit sort of what it thinks would be 42512461 rows.  With a
> closer-to-reality estimate there, it would have gone for a plan more
> similar to 8.4's, ie, hash joins and then an explicit sort.
>
> There is a lot going on in this plan that we haven't been told about; for
> instance at least one of the query's tables seems to actually be a view,
> and some other ones appear to be inheritance trees with partitioning
> constraints, and I'm suspicious that some of the aggregates might be
> user-defined functions with higher than normal costs.
>
> I'd like to see a self-contained test case, by which I mean full details
> about the table/view schemas; it's not clear whether the actual data
> is very important here.
>
>             regards, tom lane

Query 2 doesn't use any view and you can find the schema here:
http://pastebin.com/Nkv7FwRr


Query 1 use 5 views: ticket_cabecera, ticket_linea, reserva_cabecera, reserva_linea and tarifa_proveedor_modelo_precio,
Ihave factored out the four first with the same result as before, you can find the new query and the new plan here:
 

http://pastebin.com/7u2Dkyxp
http://explain.depesz.com/s/2V9d

Actually the execution time is worse than before.

About the last view if I change join from tarifa_proveedor_modelo_precio to tarifa_modelo_precio (a table with nearly
thesame structure as the view) the query is executed much faster, but I get a similar time changing the
(MIN(cab.time_stamp_recepcion)::DATE= ....) to (WHERE cab.time_stamp_recepcion::date = ....) in the "ent" subquery that
neverwas a view.
 

Anyway I included tarifa_modelo_precio to the query1 schema file for reference and you can find the plan using
tarifa_modelo_precioinstead of the view tarifa_proveedor_modelo_precio here:
 

http://explain.depesz.com/s/4gV

query1 schema file:
http://pastebin.com/JpqM87dr


Regards,
Miguel Angel.