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From: David G. Johnston <[email protected]>
To: Ron Johnson <[email protected]>
Cc: PostgreSQL mailing lists <[email protected]>
Subject: Re: BTREE index: field ordering
Date: Fri, 28 Mar 2025 10:02:29 -0700
Message-ID: <CAKFQuwZKD=6PQyuC-_Krw8qOESYNhOC+ZNDQYYgqC5402kY0nA@mail.gmail.com> (raw)
In-Reply-To: <CANzqJaA17=QxTjNHMmy3xBUcaT5o5ZwxDu74d9tatb8Lwka=2w@mail.gmail.com>
References: <[email protected]>
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On Fri, Mar 28, 2025 at 9:24 AM Ron Johnson <[email protected]> wrote:

>
> "at least for WHERE conditions that are selective" confuses me.  Aren't
> _all_ WHERE clauses selective?
>
>
From earlier in the email, selectivity is a scale, the wording here implies
"has a meaningful selectivity".

"Fundamentally yes, but you also have to consider how selective the
conditions are." - Laurenz

where true - selectivity 0
where not(is_deleted) - selectivity < .01 ish in many cases

Selectivity is evaluating cardinality with an eye on the frequency of
the values you are actually going to be filtering on.  So low cardinality
booleans can be highly selective in usage if you are looking for the rare
false in a sea of trues but low selectivity if looking through those trues.

David J.

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