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From: Tom Lane <tgl@sss.pgh.pa.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
cc: "lucvanlind.en@gmail.com" <lucvanlind.en@gmail.com>,
        "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #18239: select position ('' in 'A') returns 1
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Comments: In-reply-to "David G. Johnston" <david.g.johnston@gmail.com>
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"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Sunday, December 10, 2023, PG Bug reporting form <noreply@postgresql.org>
> wrote:
>> select position ('' in 'A') returns 1
>> Thus, case a field has an empty string while, checking positions in another
>> string, it seems to always fit the position 1.

> Just as an empty set is a subset of all sets the empty string is a
> substring within all strings.

Yes.  This behavior is explicitly required by the SQL standard:

         2) If <string position expression> is specified, then

            Case:

            a) If the first <string value expression> has a length of 0
              (zero), then the result is 1 (one).

            b) [ otherwise, ... ]

> I do not believe we document or refer to this external rule anywhere though.

No, our documentation for position() is just one sentence.  With the
new function-table layout it'd be possible to add a second example
to exhibit this behavior, but I'm not sure it's worth the trouble.
Personally I don't find this surprising.

			regards, tom lane