Dennis Bjorklund wrote:
Tom Lane skrev:
You could imagine trying to do
% a byte at a time (and indeed that's what I'd been thinking it did)
but that gets you out of sync which breaks the _ case.

It is only when you have a pattern like '%_' when this is a problem and we could detect this and do byte by byte when it's not. Now we check (*p == '\\') || (*p == '_') in each iteration when we scan over characters for '%', and we could do it once and have different loops for the two cases.

Other than this part that I think can be optimized I don't see anything wrong with the idea behind the patch. To make the '%' case fast might be an important optimization for a lot of use cases. It's not uncommon that '%' matches a bigger part of the string than the rest of the pattern.

Are you sure? The big remaining char-matching bottleneck will surely be in the code that scans for a place to start matching a %. But that's exactly where we can't use byte matching for cases where the charset might include AB and BA as characters - the pattern might contain %BA and the string AB. However, this isn't a danger for UTF8, which leads me to think that we do indeed need a special case for UTF8, but for a different improvement from that proposed in the original patch. I'll post an updated patch shortly.



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