Re: ilike multi-byte pattern cache

Tom Lane wrote:
> Andrew Dunstan <andrew@dunslane.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