To: Svenne Krap <svenne@krap.dk>
Cc: PgSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: multi-layered view join performance oddities 
In-reply-to: <4364FFD4.2020801@krap.dk> 
References: <4364FFD4.2020801@krap.dk>
Comments: In-reply-to Svenne Krap <svenne@krap.dk>
	message dated "Sun, 30 Oct 2005 18:16:04 +0100"
Date: Sun, 30 Oct 2005 13:27:01 -0500
Message-ID: <13993.1130696821@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Svenne Krap <svenne@krap.dk> writes:
> create view ord_institutes_sum as
>  SELECT ord_property_type_all.dataset_id, ord_property_type_all.nb_property_type_id, 0 AS institut, sum(ord_property_type_all.amount) AS amount
>    FROM ord_property_type_all
>  GROUP BY ord_property_type_all.dataset_id, ord_property_type_all.nb_property_type_id;

> create view ord_result_pct as
>  SELECT t1.dataset_id, t1.nb_property_type_id, t1.institut, t1.amount / t2.amount * 100::numeric AS pct
>    FROM ord_property_type_all t1, ord_institutes_sum t2
>   WHERE t1.dataset_id = t2.dataset_id AND t1.nb_property_type_id = t2.nb_property_type_id;

This is really pretty horrid code: you're requesting double evaluation
of the ord_property_type_all view, and then joining the two calculations
to each other.  No, the planner will not detect how silly this is :-(,
nor will it realize that there's guaranteed to be a match for every row
--- I believe the latter is the reason for the serious misestimation
that Steinar noted.  The misestimation doesn't hurt particularly when
evaluating ord_result_pct by itself, because there are no higher-level
decisions to make ... but it hurts a lot when you join ord_result_pct to
some other stuff.

It seems like there must be a way to get the percentage amounts with
only one evaluation of ord_property_type_all, but I'm not seeing it
right offhand.

			regards, tom lane