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Re: Estimation of HashJoin Cost

From: "Kevin Grittner" <kgrittn(at)mail(dot)com>
To: "Qi Huang" <huangqiyx(at)outlook(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Estimation of HashJoin Cost
Date: 2012-11-01 13:45:18
Message-ID: 20121101134518.61240@gmx.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Qi Huang wrote:

> I need to estimate the hashjoin cost in my research.

> I looked at the code of final_cost_hashjoin() . It is not clear
> what factor it is considering. So, except the I/O to and from disk,
> what other factors are affecting the cost of hahsjoin?

http://www.postgresql.org/docs/9.2/interactive/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS

If you don't find it practical to read the code, you could run
ANALYZE of a query which uses a hashjoin with different cost factors
(these can be changed for your current connection with the SET
command) and observe the cost numbers in the output.

> Also, is there any way to force postgres abide on the estimation of
> Hashjoin cost as 3(R+S), which also means, to make hashjoin cost
> mainly spend on I/O?

How useful would that be for workloads where data is fully cached?

-Kevin


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