To: biuro@globeinphotos.com
cc: pgsql-performance@postgresql.org
Subject: Re: Curson prbolem 
In-reply-to: <20060620143932.e7u8gr5s53i808ss@gdn.superhost.pl> 
References: <20060620143932.e7u8gr5s53i808ss@gdn.superhost.pl>
Comments: In-reply-to biuro@globeinphotos.com
	message dated "Tue, 20 Jun 2006 14:39:32 +0200"
Date: Tue, 20 Jun 2006 10:28:18 -0400
Message-ID: <27874.1150813698@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

biuro@globeinphotos.com writes:
> [slow:]
>         OPEN cursor1 FOR SELECT * FROM alias WHERE mask>=alias_out
> ORDER BY mask;
> [fast:]
>         OPEN cursor1 FOR SELECT * FROM alias WHERE mask>=alias_out
> ORDER BY mask LIMIT 100;

The difference is that in the first case the planner has to assume you
intend to fetch all the rows with mask>=something (and I'll bet the
something is a plpgsql variable, so the planner can't even see its
value).  In this case a sort-based plan looks like a winner.  In the
second case, since you only need to fetch 100 rows, it's clearly best to
scan the index beginning at mask = alias_out.

> Can somebody clarify what is wrong with my example? I need select
> without LIMIT 100 part.

Why?  You should always tell the SQL engine what it is that you really
want --- leaving it in the dark about your intentions is a good way to
destroy performance, as you are finding out.  If I were you I would get
rid of the row-counting inside the loop entirely, and use the "LIMIT n"
clause to handle that.

			regards, tom lane