Message-ID: <42D32259.2000705@arbash-meinel.com>
Date: Mon, 11 Jul 2005 20:52:25 -0500
From: John A Meinel <john@arbash-meinel.com>
User-Agent: Mozilla Thunderbird 1.0.2 (Macintosh/20050317)
MIME-Version: 1.0
To: jobapply <jobapply@nextmail.ru>
Cc: pgsql-performance@postgresql.org
Subject: Re: Sorting on longer key is faster ?
References: <20050709183842.2EABF52820@svr1.postgresql.org>
In-Reply-To: <20050709183842.2EABF52820@svr1.postgresql.org>
Content-Type: multipart/signed; micalg=pgp-sha1;
	protocol="application/pgp-signature";
	boundary="------------enigBEFD86D74D4F6EC79AD16D69"

This is an OpenPGP/MIME signed message (RFC 2440 and 3156)
--------------enigBEFD86D74D4F6EC79AD16D69
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

jobapply wrote:
> The 2 queries are almost same, but ORDER BY x||t is FASTER than ORDER BY x..
>
> How can that be possible?
>
> Btw: x and x||t are same ordered
>
> phoeniks=> explain analyze SELECT * FROM test WHERE i<20 ORDER BY x || t;
>                                                         QUERY PLAN

I also thought of another possibility. Are there a lot of similar
entries in X? Meaning that the same value is repeated over and over? It
is possible that the sort code has a weakness when sorting equal values.

For instance, if it was doing a Hash aggregation, you would have the
same hash repeated. (It isn't I'm just mentioning a case where it might
affect something).

If it is creating a tree representation, it might cause some sort of
pathological worst-case behavior, where all entries keep adding to the
same side of the tree, rather than being more balanced.

I don't know the internals of postgresql sorting, but just some ideas.

John
=:->


--------------enigBEFD86D74D4F6EC79AD16D69
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: OpenPGP digital signature
Content-Disposition: attachment; filename="signature.asc"

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.0 (Darwin)
Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org

iD8DBQFC0yJZJdeBCYSNAAMRAqNvAJ9ISh6cVjxogvmOiwRfIl4qATHrEgCfetFN
P5ittkgSVxFUPpcfR+AztqU=
=MbUn
-----END PGP SIGNATURE-----

--------------enigBEFD86D74D4F6EC79AD16D69--