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Re: 7.4 vs 7.3 ( hash join issue )

From: Gaetano Mendola <mendola(at)bigfoot(dot)com>
To: Dennis Bjorklund <db(at)zigo(dot)dhs(dot)org>
Subject: Re: 7.4 vs 7.3 ( hash join issue )
Date: 2004-09-22 10:28:04
Message-ID: 415153B4.5040902@bigfoot.com (view raw or flat)
Thread:
Lists: pgsql-patchespgsql-performance
Dennis Bjorklund wrote:
 > On Wed, 22 Sep 2004, Gaetano Mendola wrote:
 >
 >
 >>  Limit  (cost=10.21..10.21 rows=1 width=24) (actual time=0.885..0.885 rows=0 loops=1)
 >>  Limit  (cost=10.28..10.28 rows=1 width=24) (actual time=0.429..0.429 rows=0 loops=1)
 >
 >
 > These estimated costs are almost the same, but the runtime differs a bit.
 > This means that maybe you need to alter settings like random_page_cost,
 > effective_cache and maybe some others to make the cost reflect the runtime
 > better.
 >
 > Since the costs are so close to each other very small changes can make it
 > choose the other plan. It's also very hard to make an estimate that is
 > correct in all situations. That's why it's called an estimate after all.

Is not feseable.

That values are obtained with random_page_cost = 2, effective_cache_size = 20000,
cpu_tuple_cost = 0.01
increasing or decreasing random_page_cost this means increase or decrease both
costs:


random_page_cost = 1.5
	hashjoin on  =>  8.47
         hashjoin off =>  8.53


random_page_cost = 3
	hashjoin on  =>  13.70
         hashjoin off =>  13.76


so is choosen the hasjoin method in both cases.

In the other side the effective_cache_size doesn't affect this costs.

Decreasing the cpu_tuple_cost have the same effect

cpu_tuple_cost = 0.005
	hashjoin on  =>  10.11
         hashjoin off =>  10.17

cpu_tuple_cost = 0.001
	hashjoin on  =>  10.03
         hashjoin off =>  10.03

cpu_tuple_cost = 0.0005
	hashjoin on  =>  10.01
         hashjoin off =>  10.01
	
	And when the two costs are the same the hashjoin path is choosen.

I think cpu_tuple_cost less then 0.001 is not a good idea

I think the only way is set the hashjoin = off.  Any other suggestion ?

Regards
Gaetano Mendola





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