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Message-ID: <dcc563d10709040711k42c113b0md21f02c4281ade95@mail.gmail.com>
Date: Tue, 4 Sep 2007 09:11:18 -0500
From: "Scott Marlowe" <scott.marlowe@gmail.com>
To: "Richard Ray" <rray@mstc.state.ms.us>
Subject: Re: How to influence the planner
Cc: pgsql-sql@postgresql.org
In-Reply-To: <Pine.LNX.4.64.0708311856550.7540@rray.drdc.mstc.ms.gov>
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References: <Pine.LNX.4.64.0708311116000.17749@rray.drdc.mstc.ms.gov>
	<373D2F52-E9CD-485A-9BF1-B019349FA5EC@seespotcode.net>
	<Pine.LNX.4.64.0708311557140.17749@rray.drdc.mstc.ms.gov>
	<2005.1188596277@sss.pgh.pa.us>
	<Pine.LNX.4.64.0708311856550.7540@rray.drdc.mstc.ms.gov>

On 8/31/07, Richard Ray <rray@mstc.state.ms.us> wrote:

> Changing to enable_seqscan = on does solve this problem, thanks
> Is there some method of crafting a query that will assert my wishes to the planner
> When is enable_seqscan = off appropriate

enable_xxx = off are troubleshooting tools.  They override the query planner.

For instance, I had a query that was running slow, and using set
enable_nestloop=off allowed the query to run fast.  However, using
explain analyze I could see that the estimated number of rows was off.
 Analyze didn't fix it, so I increased the stats target for the column
I was working with, reanalyzed, and voila, the query ran fine with
nestloop=on.

So, enable_xxx=off is normally only appropriate when troubleshooting
an issue, not as a fix all.  That's doubly true for
enable_seqscan=off.

If you do have a query that nothing else seems to work on it, you can
set one of the enable_xxx settings off for that connection only and
not worry about messing up all the other sessions connecting to your
db.