From: Gregory Stark <stark@enterprisedb.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: List pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: WIP Join Removal
Date: Tue, 02 Sep 2008 12:05:38 +0100
Message-ID: <87od36u7st.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <1220352898.4371.355.camel@ebony.2ndQuadrant>
References: <1220176372.4371.118.camel@ebony.2ndQuadrant>
<48BC4127.30706@enterprisedb.com>
<1220348487.4371.307.camel@ebony.2ndQuadrant>
<48BD1374.4030504@enterprisedb.com>
<1220352112.4371.349.camel@ebony.2ndQuadrant>
<48BD1852.1070904@enterprisedb.com>
<1220352898.4371.355.camel@ebony.2ndQuadrant>
Simon Riggs <simon@2ndQuadrant.com> writes:
> Same answer, just slower. Removing the join makes the access to a into a
> SeqScan, whereas it was a two-table index plan when both tables present.
> The two table plan is added by the immediately preceding call add_... -
> i.e. that plan is only added during join time not during planning of
> base relations.
Perhaps it would clearer to discuss a non-outer join here:
select invoices.*
from customer join invoices using (company_id,customer_id)
where customer_id = ?
where there's a foreign key relation guaranteeing that every invoice has a
matching <company_id, customer_id>.
If there's an index on customer(customer_id) but not on invoices(customer_id)
then conceivably it would be faster to use that than scan all of the invoices.
I wonder if it would be more worthwhile to remove them and have a subsequent
phase where we look for possible joins to *add*. So even if the user writes
"select * from invoices where customer_id=?" the planner might be able to
discover that it can find those records quicker by scanning customer, finding
the matching <company_id,customer_id> and then using an index to look them up
in invoices.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!
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To: pgsql-patches@postgresql.org
Cc: stark@enterprisedb.com, simon@2ndQuadrant.com, heikki.linnakangas@enterprisedb.com
Subject: Re: WIP Join Removal
In-Reply-To: <87od36u7st.fsf@oxford.xeocode.com>
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