From: Tom Lane <tgl@sss.pgh.pa.us>
To: JmH <jason.height@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Good/Bad query plans based on text criteria
Date: Wed, 14 Apr 2010 22:42:21 -0400
Message-ID: <21178.1271299341@sss.pgh.pa.us> (raw)
In-Reply-To: <q2naba242eb1004141845je43ba462of5fddfaa8b03d426@mail.gmail.com>
References: <q2naba242eb1004141845je43ba462of5fddfaa8b03d426@mail.gmail.com>
JmH <jason.height@gmail.com> writes:
> I am struggling to understand why for certain criteria that i supply for a
> query alters the the query plan. In my "good" case i can see that an index
> is used, in my bad case where i only change the text value of the criteria,
> but not the criteria itslef (ie change/add the conditions) a hbitmap heap
> scan of the table is performed.
I think you're jumping to conclusions. The second plan is processing
about 100 times as many rows, because the WHERE conditions are much less
selective. A change in plan is entirely appropriate.
It might be that you need to change planner parameters (particularly
random_page_cost/seq_page_cost) to more nearly approximate the operating
conditions of your database, but I'd recommend being very cautious about
doing so on the basis of a small number of example queries. In
particular it's easy to fall into the trap of optimizing for
fully-cached scenarios because repeatedly trying the same example
results in touching only already-cached data --- but that might or might
not be reflective of your whole workload.
regards, tom lane
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, jason.height@gmail.com
Subject: Re: Good/Bad query plans based on text criteria
In-Reply-To: <21178.1271299341@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox