Re: strpos behavior change around empty substring in PG12

On 28/10/2019 17:57, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Mon, Oct 28, 2019 at 11:02 AM Shay Rojansky <roji(at)roji(dot)org> wrote:
>>> Before PG12, select strpos('test', '') returns 1 (empty substring found at first position of the string), whereas starting with PG12 it returns 0 (empty substring not found).
>
>> It looks to me like this got broken here:
>
>> commit 9556aa01c69a26ca726d8dda8e395acc7c1e30fc
>> Author: Heikki Linnakangas <heikki(dot)linnakangas(at)iki(dot)fi>
>> Date: Fri Jan 25 16:25:05 2019 +0200
>> Use single-byte Boyer-Moore-Horspool search even with multibyte encodings.
>
>> Not sure what happened exactly.
>
> I think the problem is lack of clarity about the edge cases.
> The patch added this short-circuit right at the top of text_position():
>
> + if (VARSIZE_ANY_EXHDR(t1) < 1 || VARSIZE_ANY_EXHDR(t2) < 1)
> + return 0;
>
> and as this example shows, that's the Wrong Thing. Fortunately,
> it also seems easily fixed.

Tom fixed this in commit bd1ef5799b; thanks!

To be sure, I also checked the SQL standard for what POSITION('' IN
'test') is supposed to return. It agrees that 1 is correct:

> If CHAR_LENGTH(CVE1) is 0 (zero), then the result is 1 (one).

- Heikki