Message-ID: <4D926BCE.8000001@emolecules.com>
Date: Tue, 29 Mar 2011 16:31:26 -0700
From: Craig James <craig_james@emolecules.com>
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To: pgsql-performance@postgresql.org
Subject: Re: multiple table scan performance
References: <AANLkTim2zSEznvRPJNqrDTfTmgRNCBWZs1KpdjTGEOL2@mail.gmail.com>
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On 3/29/11 3:16 PM, Samuel Gendler wrote:
> I've got some functionality that necessarily must scan a relatively large table.  Even worse, the total workload is actually 3 similar, but different queries, each of which requires a table scan.  They all have a resultset that has the same structure, and all get inserted into a temp table.  Is there any performance benefit to revamping the workload such that it issues a single:
>
> insert into (...) select ... UNION select ... UNION select
>
> as opposed to 3 separate "insert into () select ..." statements.
>
> I could figure it out empirically, but the queries are really slow on my dev laptop and I don't have access to the staging system at the moment.  Also, it requires revamping a fair bit of code, so I figured it never hurts to ask.  I don't have a sense of whether postgres is able to parallelize multiple subqueries via a single scan
You don't indicate how complex your queries are.  If it's just a single table and the conditions are relatively simple, could you do something like this?

   insert into (...) select ... where (...) OR (...) OR (...)

Craig