From: Tom Lane <tgl@sss.pgh.pa.us>
To: kimaidou <kimaidou@gmail.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Simple JOIN on heavy table not using expected index
Date: Fri, 09 Feb 2024 10:12:55 -0500
Message-ID: <1394633.1707491575@sss.pgh.pa.us> (raw)
In-Reply-To: <CAMKXKO4tUH=nTZ-g09=wng+wSxtkj7K8yePMkfTiOpW4zT4Jdg@mail.gmail.com>
References: <CAMKXKO4tUH=nTZ-g09=wng+wSxtkj7K8yePMkfTiOpW4zT4Jdg@mail.gmail.com>
kimaidou <kimaidou@gmail.com> writes:
> It seems PostgreSQL does not use the index parcelles_dep_idx on "dep" (text
> field), even if the corresponding number of lines for this WHERE clause is
> a smal subset of the entire data:
> approx 6M against 80M in total
6M out of 80M rows is not a "small subset". Typically I'd expect
the planner to use an index-based scan for up to 1 or 2 percent of
the table. Beyond that, you're going to be touching most pages
of the table anyway.
You can try reducing random_page_cost to favor indexscans, but
you might not find that the query gets any faster.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, kimaidou@gmail.com, pgsql-performance@lists.postgresql.org
Subject: Re: Simple JOIN on heavy table not using expected index
In-Reply-To: <1394633.1707491575@sss.pgh.pa.us>
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