From: Hedayat Vatankhah <hedayat.fwd@gmail.com>
Subject: Re: PostgreSQL seems to create inefficient plans in simple
 conditional joins
To: postgres performance list <pgsql-performance@postgresql.org>
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Cc: David Rowley <david.rowley@2ndquadrant.com>
Message-ID: <56B856E9.8070703@gmail.com>
Date: Mon, 8 Feb 2016 12:20:49 +0330
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Hi again,

/*Hedayat Vatankhah*/ wrote on Sun, 31 Jan 2016 01:20:53 +0330:
> Hi,
>
> /*David Rowley*/ wrote on Sun, 31 Jan 2016 04:57:04 +1300:
>> On 31 January 2016 at 01:30, Hedayat Vatankhah
>> <hedayat.fwd@gmail.com> wrote:
>>> Personally, I expect both queries below to perform exactly the same:
>>>
>>> SELECT
>>>      t1.id, *
>>> FROM
>>>      t1
>>> INNER JOIN
>>>      t2 ON t1.id = t2.id
>>>      where t1.id > -9223372036513411363;
>>>
>>> And:
>>>
>>> SELECT
>>>      t1.id, *
>>> FROM
>>>      t1
>>> INNER JOIN
>>>      t2 ON t1.id = t2.id
>>>      where t1.id > -9223372036513411363 and t2.id >
>>> -9223372036513411363;
>>>
>>> Unfortunately, they do not. PostgreSQL creates different plans for
>>> these
>>> queries, which results in very poor performance for the first one
>>> compared
>>> to the second (What I'm testing against is a DB with around 350 million
>>> rows in t1, and slightly less in t2).
>>>
>>> EXPLAIN output:
>>> First query: http://explain.depesz.com/s/uauk
>>> Second query: link: http://explain.depesz.com/s/uQd
>> Yes, unfortunately you've done about the only thing that you can do,
>> and that's just include both conditions in the query. Is there some
>> special reason why you can't just write the t2.id > ... condition in
>> the query too? or is the query generated dynamically by some software
>> that you have no control over?

I just found another issue with using a query like the second one (using 
LEFT JOINs instead of INNER JOINs): referencing id columns of joined 
tables explicitly disables PostgreSQL join removal optimization when you 
only select column(s) from t1! :(
I should forget about creating views on top of JOIN queries, and build 
appropriate JOIN queries with referenced table and appropriate 
conditions manually, so the whole data model should be exposed to the 
application.

If I'm not wrong, PostgreSQL should understand that ANY condition on t2 
doesn't change the LEFT JOIN output when t2 columns are not SELECTed.

Regards,
Hedayat


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