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Re: query question

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: Laurette Cisneros <laurette(at)nextbus(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: query question
Date: 2002-12-07 00:38:18
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Lists: pgsql-performance

> This query:
> select distinct x, y 
>   from table1 t 
>   join table2 t2
>  using (col1) 
> order by x;
> is *slower* than this query:
> select disting x, y 
>   from table1 
>  where col1 = (select col1 from table2) 
> Is this because in the latter case the select col1 is cached?

Yes.   For all of the following structures:

where x = (select col from table)
where x IN (select col from table)
where x NOT IN (select col from table)
where x != ANY(select col from table)

... Postgres must process the full subquery, return the results, and compare 
all of the results as individual values against the reference column.  

However, if you re-wrote the query as:

 select distint x, y 
   from table1 
  where EXISTS (select col1 from table2
	where table2.col1 = table1.col1)

... then Postgres would be able to use JOIN optimizations to evaluate the 
subquery and pull a subset of relevant records or even use an index, making 
the query *much* faster.

> Ooo, I would love to have a web page full of these tidbits (along with how 
> to get around the max and min aggregates and why as an example..., etc.)!


Add your own Wiki page!

-Josh Berkus

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