From: Tom Lane <tgl@sss.pgh.pa.us>
To: Patrice Beliveau <pbeliveau@avior.ca>
Cc: pgsql-performance@postgresql.org
Subject: Re: Optimizing queries
Date: Wed, 09 Aug 2006 10:39:06 -0400
Message-ID: <19962.1155134346@sss.pgh.pa.us> (raw)
In-Reply-To: <44D9CF6E.9030900@avior.ca>
References: <44D8CE92.1080407@avior.ca>
<1155062399.20252.50.camel@state.g2switchworks.com>
<44D8F0B8.2030804@avior.ca>
<13030.1155069749@sss.pgh.pa.us>
<44D9CF6E.9030900@avior.ca>
Patrice Beliveau <pbeliveau@avior.ca> writes:
> Tom Lane wrote:
>> PG 8.1 will not reorder WHERE clauses for a single table unless it has
>> some specific reason to do so (and AFAICT no version back to 7.0 or so
>> has done so either...) So there's something you are not telling us that
>> is relevant.
> here is my query, and the query plan that result
> explain select * from (
> select * from sales_order_delivery
> where sales_order_id in (
> select sales_order_id from sales_order
> where closed=false
> )
> ) as a where outstandingorder(sales_order_id, sales_order_item,
> date_due) > 0;
So this isn't a simple query, but a join. PG will generally push
single-table restrictions down to the individual tables in order to
reduce the number of rows that have to be processed at the join.
In this case that's not a win, but the planner doesn't know enough
about the outstandingorder() function to realize that.
I think what you need is an "optimization fence" to prevent the subquery
from being flattened:
explain select * from (
select * from sales_order_delivery
where sales_order_id in (
select sales_order_id from sales_order
where closed=false
)
OFFSET 0
) as a where outstandingorder(sales_order_id, sales_order_item,
date_due) > 0;
Any LIMIT or OFFSET in a subquery prevents WHERE conditions from being
pushed down past it (since that might change the results). OFFSET 0 is
otherwise a no-op, so that's what people usually use.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, pbeliveau@avior.ca
Subject: Re: Optimizing queries
In-Reply-To: <19962.1155134346@sss.pgh.pa.us>
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