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From: David G Johnston <david.g.johnston@gmail.com>
To: pgsql-performance@postgresql.org
Subject: Re: UNION and bad performance
Date: Sun, 8 Jun 2014 08:53:27 -0700 (PDT)
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pinker wrote
>>> rhaas=# explain select a from generate_series(1,100) a union select a
>>> from generate_series(1,100) a;
>>>                                      QUERY PLAN
>>> --------------------------------------------------------------------------------------
>>> HashAggregate  (cost=45.00..65.00 rows=2000 width=4)
>>>   ->  Append  (cost=0.00..40.00 rows=2000 width=4)
> 
> 
> Why in this case the estimated number of rows is 2000? Is it standard
> planner behavior?

http://www.postgresql.org/docs/9.1/static/sql-createfunction.html

Note the "ROWS" property.

Functions are black-boxes to the planner so it has no means of estimating a
row count.  So a set returning function uses 1,000 and all others use 1.

Determining "COST" is similarly problematic.

David J.







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