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Re: Extra cost of "lossy mode" Bitmap Scan plan

From: higepon <higepon(at)gmail(dot)com>
To: Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Extra cost of "lossy mode" Bitmap Scan plan
Date: 2009-04-28 06:45:27
Message-ID: f07386410904272345i10f6052fied886610f95af214@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi.

> Good point. I saw the bad behavior on DBT-3 (TPC-H) benchmark before.
> Loss-less bitmap scan was faster than seq Scan,
> but lossy bitmap scan was slower than seq Scan:

Thank you.

> Sure, we need it! Also, I hope some methods to determine whether the
> bitmap scan was lossy or not, and how amount of work_mem is required to do
> loss-less bitmap scan. For example, a new GUC variable trace_bitmapscan to
> print the information of bitmap scan, like trace_sort for sorting.

I think it is not hard to implement these methods and trace function.

The problem that remains to be solved is
"How much extra cost should we add for lossy mode?".

Any ideas?

Best regards,

-----
MINOWA Taro (Higepon)

Cybozu Labs, Inc.

http://www.monaos.org/
http://code.google.com/p/mosh-scheme/

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Subject: Re: Extra cost of "lossy mode" Bitmap Scan plan
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