Re: Row pattern recognition

From: Vik Fearing <vik(at)postgresfriends(dot)org>
To: Jacob Champion <jchampion(at)timescale(dot)com>, Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>
Cc: er(at)xs4all(dot)nl, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Row pattern recognition
Date: 2023-09-08 21:43:10
Message-ID: 1e2a49a7-e74e-4a85-9540-16250a3b8214@postgresfriends.org
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On 9/8/23 21:27, Jacob Champion wrote:
> On 9/7/23 20:54, Tatsuo Ishii wrote:

>>> But it's easy
>>> to come up with a pattern where that's the wrong order, like
>>>
>>> PATTERN ( A+ (B|A)+ )
>>>
>>> Now "aaa" will be considered before "aab", which isn't correct.
>>
>> Can you explain a little bit more? I think 'aaa' matches a regular
>> expression 'a+(b|a)+' and should be no problem before "aab" is
>> considered.
>
> Assuming I've understood the rules correctly, we're not allowed to
> classify the last row as 'A' if it also matches 'B'. Lexicographic
> ordering takes precedence, so we have to try "aab" first. Otherwise our
> query could return different results compared to another implementation.

Your understanding is correct.
--
Vik Fearing

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