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Re: [Gnumed-devel] enable_seqscan

From: Syan Tan
Subject: Re: [Gnumed-devel] enable_seqscan
Date: Mon, 13 Mar 2006 15:51:19 +0800

here's the output of 2 fairly similiar queries, except one will use index_scan and one

will use seq_scan.  The seq_scan takes about 800-900 msec, and is called about

40 times when constructing a large emr browse tree , so it takes about 30 seconds.

when the problem sql statement is broken into the union select of it's child tables,

index scan is used by the query planner,  and the statement takes about 1- 2 msec,

making the total  time taken for this statement less than half a second ( if say 10msec,

then 40 x 10 is 400msec ).  I can't see why the query planner is taking much longer

with pk_health_issue than pk_episode, except that pk_episode happens to be a primary

key of clin.episode , one of the base tables of clin.v_pat_items, whereas pk_health_issue

is a foreign key ( and also the index is not unique in pk_health_issue)

On Sun Mar 12 17:00 , Karsten Hilbert sent:

On Sun, Mar 12, 2006 at 02:16:50PM +0800, Syan Tan wrote:

> I did try that , but in postgresql.conf and it didn't seem to work. perhaps if I
> insert set enable_seqscan=off in the session , it will be the same, in which case , you
> do not have to change the sql. I'll verify it saves time as well.
Yes, please do. Note, however, that this is a better fix
(because it is closer to the real solution) but far from the
proper one.

One thing you should definitely try, too, is to run "vacuum
analyze" on the database after insertion of your large test

See, the real reason (unless there is a bug) why the query
planner chooses a seq scan over an index scan is that the
planner thinks the seq scan is going to be faster ! Which is
very true in the initial database state where most tables
are nearly empty. The planner uses table statistics to
decide whether seq or idx scans should be better. These
statistics need to be updated after significant changes to
table data. Else the planner will work with false
assumptions leading it to choose seq scans over index scans.

I do believe updating the statistics will make the planner
use the indexes without any need for changing the sql or
disallowing seq scans entirely...

The test I did with "set enable_seqscan to off" was simply to
prove that the query planner technically *can* use indexes
on child tables.

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