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Re: ilike multi-byte pattern cache

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Patches (PostgreSQL)" <pgsql-patches(at)postgresql(dot)org>
Subject: Re: ilike multi-byte pattern cache
Date: 2007-09-22 16:35:48
Message-ID: 46F54464.1020100@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-patches

Tom Lane wrote:
> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>   
>> The attached patch implements a one-value pattern cache for the 
>> multi-byte encoding case for ILIKE. This reduces calls to lower() by 
>> (50% -1) in the common case where the pattern is a constant.  My own 
>> testing and Guillaume Smet's show that this cuts roughly in half the 
>> performance penalty we inflicted by using lower() in that case.
>>     
>
>   
>> Is this sufficiently low risk to sneak into 8.3?
>>     
>
> This seems awfully ugly ... and considering that you don't get to
> avoid lower() on the data side, it seems pretty dubious that it
> buys very much percentagewise.  It would also be a net loss for
> non-constant patterns, which are by no means unheard of --- or even
> two constant patterns used in the same query.
>   

The cost of using lower() is demonstrably high. Even on a very small 
pattern the speedup is easily measurable.

The cost of the patch is effectively 1 call to strcmp() per call and 2 
calls to memcpy() per cache miss, which should be quite cheap.

> We've lived with this in 8.2 without much complaint.  I think we can
> let it go until we think of a better solution.  To my mind this is
> all tied up in the problem of handling locales in a better fashion
> --- a lot of the inefficiency of lower() is due to having a poor
> impedance match to the libc locale-related functions, and might be
> eliminated if we had locale support with better APIs.
>
> 			
>   

Well, we have a complaint now :-( This was aimed at being some temporary 
relief rather than a long term fix. I guess Guillaume can use this as a 
patch if he wants.

I agree that we need better locale APIs.

cheers

andrew

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