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From: Tom Lane <[email protected]>
To: Jehan-Guillaume de Rorthais <[email protected]>
Cc: Frédéric Yhuel <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Christophe Courtois <[email protected]>
Cc: Laurenz Albe <[email protected]>
Subject: Re: Indexes on expressions with multiple columns and operators
Date: Thu, 18 Sep 2025 12:59:11 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <20250918172628.44301672@karst>
References: <[email protected]>
	<[email protected]>
	<20250918172628.44301672@karst>

Jehan-Guillaume de Rorthais <[email protected]> writes:
> On a fresh instance from HEAD with its default configuration, it shows:

>   Index Scan using foo_s_idx on foo  (cost=0.29..8.39 rows=33333 width=13)
>     Index Cond: (s(crit, ackid) = true)

> It seems statistics shown in "pg_stats" view for function "s()" are good. The
> query itself even have the same costs than the query using the syntax tips you
> provide before.

> However, the estimated row number seems wrong in regard with the costs shown
> and statistics.

Yeah.  The problem is that clause_selectivity_ext fails to consider
use of statistics if the clause looks like "bool_valued_function(...)".
If it looks like "bool_valued_function(...) = true", that goes down
a different code path that does the right thing.

Additional factors:

* If you just write "WHERE bool_valued_function(...) = true", that
gets stripped down to "WHERE bool_valued_function(...)" in the name
of making equivalent expressions look equivalent.  (IS TRUE doesn't
get stripped, which is why you have to use that wording to avoid
that.)

* Index condition building puts back the "= true" in order to
construct something that satisfies the index AM API.  And then it
uses that form to get a selectivity estimate for costing purposes
--- so the right number goes into the indexscan cost estimate.

* But the rowcount estimate is made on the form without "= true".
That's the number shown in EXPLAIN and used when considering
joins.

			regards, tom lane

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