Re: BUG #18239: select position ('' in 'A') returns 1

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
Cc: "lucvanlind(dot)en(at)gmail(dot)com" <lucvanlind(dot)en(at)gmail(dot)com>, "pgsql-bugs(at)lists(dot)postgresql(dot)org" <pgsql-bugs(at)lists(dot)postgresql(dot)org>
Subject: Re: BUG #18239: select position ('' in 'A') returns 1
Date: 2023-12-10 16:13:00
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"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Sunday, December 10, 2023, PG Bug reporting form <noreply(at)postgresql(dot)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


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

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