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From: Andy Fan <zhihuifan1213@163.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Maxim Boguk <maxim.boguk@gmail.com>
Cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18588: Cannot force/let database use parallel execution in simple case.
Date: Fri, 23 Aug 2024 09:47:45 +0800
Message-ID: <87h6bcrovy.fsf@163.com> (raw)
In-Reply-To: <CAApHDvo_nOEKOYLasBKnuw6eYFcObhhpgxq=kEw_G0N0-vx9jA@mail.gmail.com>
References: <18588-204730f9394441a8@postgresql.org>
	<CAApHDvqGgO7c8N0kq5H4jZ0NZe=NGZDo4VVF5Fi5CVdJNbvStg@mail.gmail.com>
	<CAK-MWwTqDRerxrpaN8Xb25DqDGBw1UT+YSkfp4QN-MgOH97c+Q@mail.gmail.com>
	<CAK-MWwS_ZAsaGrz0byqCO38NNrNqPkNt1ww1dorHz_wiaFbWvw@mail.gmail.com>
	<2059550.1724353316@sss.pgh.pa.us>
	<2073208.1724359323@sss.pgh.pa.us>
	<CAApHDvqVvFz1L-Yxyts+Q4+rbyuOmsJwFK9=pJwQS=cukb+z0g@mail.gmail.com>
	<2088498.1724367160@sss.pgh.pa.us>
	<CAApHDvo_nOEKOYLasBKnuw6eYFcObhhpgxq=kEw_G0N0-vx9jA@mail.gmail.com>



> .. I think Maxim might need to try tweaking effective_cache_size. ..

I'm right now doing some troubleshooting for Bitmap Index Scan vs Index
Scan and effective_cache_size caught my attention. I want to take this
chance to confirm my understanding about this parameter.

1. By design, effective_cache_size should not less than shared_buffer
since file-system cache should be considered as well.
2. Mackert and Lohman's algorithm in index_pages_fetched looks doesn't
consider the number of concurrency? so if we have N sessions to access N
different small relations, but sum_of_n_relations is larger than
effective_cache_size, would the index_pages_fetched figure out a much
smaller value than the fact?

-- 
Best Regards
Andy Fan






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