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Re: [HACKERS] like/ilike improvements

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Zeugswetter Andreas ADI SD <ZeugswetterA(at)spardat(dot)at>, andrew(at)supernews(dot)com, "Patches (PostgreSQL)" <pgsql-patches(at)postgresql(dot)org>
Subject: Re: [HACKERS] like/ilike improvements
Date: 2007-06-01 03:39:30
Message-ID: 465F94F2.1070707@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches

Tom Lane wrote:
> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>   
>>   do { (t)++; (tlen)--}  while ((*(t) & 0xC0) == 0x80 && tlen > 0)
>>     
>
> The while *must* test those two conditions in the other order.
> (Don't laugh --- we've had reproducible bugs before in which the backend
> dumped core because of running off the end of memory due to this type
> of mistake.)
>
>   
>> In fact, I'm wondering if that might make the other UTF8 stuff redundant 
>> - the whole point of what we're doing is to avoid expensive calls to 
>> NextChar;
>>     
>
> +1 I think.  This test will be approximately the same expense as what
> the outer loop would otherwise be (tlen > 0 and *t != firstpat), and
> doing it this way removes an entire layer of intellectual complexity.
> Even though the code is hardly different, we are no longer dealing in
> misaligned pointers anywhere in the match algorithm.
>
>
>   

OK, here is a patch that I think incorporates all the ideas discussed 
(including part of Mark Mielke's suggestion about optimising %_). There 
is now no special treatment of UTF8 other than its use of a faster 
NextChar macro.

cheers

andrew

Attachment: like.patch
Description: text/x-patch (26.7 KB)

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