Index-only scan and random_page_cost

From: Konstantin Knizhnik <knizhnik(at)garret(dot)ru>
To: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Index-only scan and random_page_cost
Date: 2023-02-03 17:55:37
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Hi hackers,

Right now cost of index-only scan is using `random_page_cost`.
Certainly for point selects we really have random access pattern, but
queries like "select count(*) from hits"  access pattern is more or less
we are iterating through subsequent leaf B-Tree pages.  As far as
default value of `random_page_cost`  is 4 times larger than `seq_page_cost`
it may force Postgres optimizer to choose sequential scan, while
index-only scan is usually much faster in this case.
Can we do something here to provide more accurate cost estimation?

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