On Thu, May 24, 2007 at 11:20:51PM -0400, Tom Lane wrote:
> I wrote:
> > Andrew Dunstan <andrew@dunslane.net> writes:
> >> Yes, I agree completely. However it looks to me like IsFirstByte will in
> >> fact always be true when we get to call NextChar for matching "_" for UTF8.
> > If that's true, the patch is failing to achieve its goal of treating %
> > bytewise ...
> OK, I studied it a bit more and now see what you're driving at: in this
> form of the patch, we treat % bytewise unless it is followed by _, in
> which case we treat it char-wise. That seems a good tradeoff,
> considering that such a pattern is probably pretty uncommon --- we
> should be willing to handle it a bit slower to simplify other cases.
Is it worth the effort to pre-process the pattern?
For example:
%% -> % %_ -> _%
If applied recursively, this would automatically cover:
%_% -> _% _%_ -> __%
The 'benefit' would be that the pattern matching code would not
need an inner if statement?
Also - I didn't see a response to my query with regard treating UTF-8
as a two pass match. First pass treating it as bytes. If the first pass
matches, the second pass doing a full analysis. In the case of low
selectivity, this will be a win, as the primary filter would be the
full speed byte-based matching.
I had also asked why the focus would be on high selectivity. Why would
the primary filter criteria for a properly designed select statement by
a like with high selectivity? The only time I have ever used like is
when I expect low selectivity. Is there a reasonable case I am missing?
Cheers,
mark
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