| From: | "Kevin Grittner" <kgrittn(at)mail(dot)com> |
|---|---|
| To: | "Claudio Freire" <klaussfreire(at)gmail(dot)com>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | "AI Rumman" <rummandba(at)gmail(dot)com>,"Evgeny Shishkin" <itparanoia(at)gmail(dot)com>, "Andrew Dunstan" <andrew(at)dunslane(dot)net>, "postgres performance list" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Why does the number of rows are different in actual and estimated. |
| Date: | 2012-12-14 20:34:03 |
| Message-ID: | 20121214203403.80070@gmx.com |
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| Lists: | pgsql-performance |
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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