Cc: "AI Rumman" <rummandba@gmail.com>,"Evgeny Shishkin"
	<itparanoia@gmail.com>, "Andrew Dunstan" <andrew@dunslane.net>, "postgres
	performance list" <pgsql-performance@postgresql.org>
Content-Type: text/plain; charset="utf-8"
Date: Fri, 14 Dec 2012 15:34:03 -0500
From: "Kevin Grittner" <kgrittn@mail.com>
Message-ID: <20121214203403.80070@gmx.com>
MIME-Version: 1.0
Subject: Re: Why does the number of rows are different in actual and
 estimated.
To: "Claudio Freire" <klaussfreire@gmail.com>,"Tom Lane" <tgl@sss.pgh.pa.us>
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Sender: pgsql-performance-owner@postgresql.org

Claudio Freire wrote:

> Selectivity is decided based on the number of distinct values on
> both sides, and the table's name "entity" makes me think it's a
> table that is reused for several things. That could be a problem,
> since that inflates distinct values, feeding misinformation to
> the planner.
> 
> Rather than a generic "entity" table, perhaps it would be best to
> separate them different entities into different tables.

I missed that; good catch. Good advice.

Don't try to build a "database within a database" by having one
table for different types of data, with a code to sort them out.
EAV is a seriously bad approach for every situation where I've seen
someone try to use it. I was about to say it's like trying to drive
a nail with a pipe wrench, then realized it's more like putting a
bunch of hammers in a bag and swinging the bag at the nail.

-Kevin


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