Subject Re: [firebird-support] Suboptimal plan
Author Alexandre Benson Smith
Post date 2014-08-01T21:29:07Z
Hi !

There is some tricks to change the way optimizer will choose the order of tables:

example:
select
   *
from
   LongTable L left join
   ShortTable S on (S.FieldA = L.FieldA)

The outer join would force the table on the left to be scaned first.

Another option is
select
   *
from
   LongTable L join
   ShortTable S on (S.FieldA = L.FieldA+0)

the +0 (or || '' if the field is a char/varchar) would prevent the optimizer to use the index on LongTable.FieldA and could lead to change the join order

see you !




Em 1/8/2014 18:09, Aldo Caruso aldo.caruso@... [firebird-support] escreveu:
Sean and Carlos,

  thanks for your answer.

  Although the statistics are up to date the optimizer still chooses the shorter stream as the controller.

  Nevertheless, I found a workaround using a subquery on the shorter table instead of a join.

  This forced the optimizer to traverse the longer table first and boosts the performance even more than using an optimized query with a fixed plan.

    Thank you very much for your help.

Aldo

 
El 01/08/14 a las 16:01, 'Carlos H. Cantu' listas@... [firebird-support] escibiĆ³:
 

Are the indexes statistics up to date?

Also, post the query, query plan, and indexes stats.

Carlos
Firebird Performance in Detail - http://videos.firebirddevelopersday.com
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ACacacfs> Hello,

ACacacfs> In a select statement there are two tables joined by a single
ACacacfs> matching field, with search conditions in both of them. One of them has
ACacacfs> roughly 130000 records, while the other has 600.

ACacacfs> Inspecting the plan generated by the optimizer, I realized that it
ACacacfs> uses the shorter one as the controlling stream instead of the longer one
ACacacfs> (i.e. the shorter table is at the left side).

ACacacfs> Using a PLAN expression with the corresponding indexes to bypass
ACacacfs> the plan deviced by the optimizer in order to switch the order of the
ACacacfs> streams, the performance is boosted (nearly three times faster).

ACacacfs> What can cause the optimizer to pick a shorter table as the
ACacacfs> controlling stream ?

ACacacfs> Is there a workaround to induce the optimizer to select the streams
ACacacfs> in a fixed way or should I have to resort to a manually imposed PLAN ?

ACacacfs> Thanks,
ACacacfs> Aldo