On 05/06/14 16:40, Merlin Moncure wrote:
> On Thu, Jun 5, 2014 at 6:32 AM, Linos <i...@linos.es> wrote:
>> Hello all,
>> This is a continuation of the thread found here:
>> http://www.postgresql.org/message-id/538f2578.9080...@linos.es
>> Considering this seems to be a problem with the planner I thought that maybe 
>> would be a better idea to post this problem here.
>> To summarize the original thread I upgraded a medium (17Gb) database from 
>> PostgreSQL 8.4 to 9.3 and many of the queries my application uses started 
>> performing a lot slower, Merlin advised me to try disabling nestloop, this 
>> helped out for the particular query I was asking about but it is not a 
>> solution that I "can/would like" to use in the general case.
>> I simplified a little bit the original query and I have added another one 
>> with same problem.
> I believe the basic problem (this is just one example; I've
> anecdotally seen this myself) is that changes in the query planner
> (which I don't follow and fully understand) in recent versions seem to
> be such that the planner makes better decisions in the presence of
> good information but in certain cases makes worse choices when dealing
> with bad information.  Statistics errors tend to accumulate and
> magnify in complicated plans, especially when the SQL is not optimally
> written.
> I have no clue what the right solution is.  There's been several
> discussions about 'plan risk' and trying to get the server to pick
> plans with better worse case behavior in cases where statistics are
> demonstrably suspicious.  Maybe that would work but ISTM is a huge
> research item that won't get solved quickly or even necessarily pan
> out in the end.  Nevertheless, user supplied test cases demonstrating
> performance regressions (bonus if it can be scripted out of
> generate_series) are going to be key drivers in finding a solution.
> merlin

What I don't understand is why the statistics have this bad information, all my 
tests are done on a database just restored and 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?

Miguel Angel.

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