Date: Wed, 23 Jun 2021 12:08:56 -0400
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Zach Aysan <zachaysan@gmail.com>,
	Pg Docs <pgsql-docs@lists.postgresql.org>
Subject: Re: Reverse btree indexes
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On 2021-Jun-22, David G. Johnston wrote:

> On Tue, Jun 22, 2021 at 2:28 PM Zach Aysan <zachaysan@gmail.com> wrote:

> >> for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar',
> >> which would require a reversed index on the field. To use the reversed
> >> index, query with reverse(col) like reverse('%bar').
> >
> This type of commentary isn't usually something we include in the
> documentation...and I'm not too keen on "reversed index" as a phrase
> regardless.

Maybe we can add it as a parenthical comment.  Something like

: The optimizer can also use a B-tree index for queries involving the
: pattern matching operators LIKE and ~ if the pattern is a constant and
: is anchored to the beginning of the string — for example, col LIKE
: 'foo%' or col ~ '^foo', but not col LIKE '%bar' (it is possible to
: use an expressional index to support queries such as the latter; see
: Section 11.7).  However, [...]

Then add an example in 11.7 Indexes on Expressions below the two
existing examples.  I think this is a noteworthy use of expressional
indexes; I've had to explain the reverse() idea a couple of times.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W
"The eagle never lost so much time, as
when he submitted to learn of the crow." (William Blake)