| From: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
|---|---|
| To: | Ron Johnson <ronljohnsonjr(at)gmail(dot)com> |
| Cc: | PostgreSQL mailing lists <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: BTREE index: field ordering |
| Date: | 2025-03-28 17:04:24 |
| Message-ID: | CAKFQuwZwwoODiq5ZW7EvCotrfk7jK-wKVnzx=s_NCT7TgM7DmA@mail.gmail.com |
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| Lists: | pgsql-general |
On Fri, Mar 28, 2025 at 10:02 AM David G. Johnston <
david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> 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.
>
And to accommodate the highly-selective case a partial index is much better
than indexing the low cardinality boolean column values.
David J.
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