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

On Sunday, December 10, 2023, PG Bug reporting form <noreply(at)postgresql(dot)org>
wrote:

> The following bug has been logged on the website:
>
> Bug reference: 18239
> Logged by: Luc Van Linden
> Email address: lucvanlind(dot)en(at)gmail(dot)com
> PostgreSQL version: 15.3
> Operating system: windows 10 64 bit
> Description:
>
> We simplified the bug, or maybe undocumented behavior to its simplest
> testcase.
>
> 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.
>
> Not sure if this is intended or not, at least we experience this as a bug.
>
>
Just as an empty set is a subset of all sets the empty string is a
substring within all strings.

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

David J.