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Re: WIP: index support for regexp search

From: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
To: Erik Rijkers <er(at)xs4all(dot)nl>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WIP: index support for regexp search
Date: 2012-01-21 05:26:36
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Lists: pgsql-hackers

Thank you for your feedback!

On Fri, Jan 20, 2012 at 3:33 AM, Erik Rijkers <er(at)xs4all(dot)nl> wrote:

> The patch yields spectacular speedups with small, simple-enough regexen.
>  But it does not do a
> good enough job when guessing where to use the index and where fall back
> to Seq Scan.  This can
> lead to (also spectacular) slow-downs, compared to Seq Scan.
Could you give some examples of regexes where index scan becomes slower
than seq scan?

> I guessed that MAX_COLOR_CHARS limits the character class size (to 4, in
> your patch), is that
> true?   I can understand you want that value to be low to limit the above
> risk, but now it reduces
> the usability of the feature a bit: one has to split up larger
> char-classes into several smaller
> ones to make a statement use the index: i.e.:
Yes, MAX_COLOR_CHARS is number of maximum character in automata color when
that color is divided to a separated characters. And it's likely there
could be better solution than just have this hard limit.

> Btw, it seems impossible to Ctrl-C out of a search once it is submitted; I
> suppose this is
> normally necessary for perfomance reasons, but it would be useful te be
> able to compile a test
> version that allows it.  I don't know how hard that would be.
I seems that Ctrl-C was impossible because procedure of trigrams
exctraction becomes so long while it is not breakable. It's not difficult
to make this procedure breakable, but actually it just shouldn't take so

> There is also a minor bug, I think, when running with  'set
> enable_seqscan=off'  in combination
> with a too-large regex:
Thanks for pointing. Will be fixed.

With best regards,
Alexander Korotkov.

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