Re: [PERFORM] Estimation problem with a LIKE clause containing a /

"Tom Lane" <tgl@sss.pgh.pa.us> writes:

> This rule works for all the locales I have installed ... but I don't
> have any Far Eastern locales installed.  Also, my test cases are only
> covering ASCII characters, and I believe many locales have some non-ASCII
> letters that sort after 'Z'.  I'm not sure how hard we need to try to
> cover those corner cases, though.  It is ultimately only an estimate...

If I understand correctly what we're talking about it's generating estimates
for LIKE 'foo%' using the algorithm which makes sense for C locale which means
generating the next range of values which start with 'foo%'.

It seems to me the problematic situations is when the most-frequent-values
come into play. Being off slightly in the histogram isn't going to generate
very inaccurate estimates but including or not a most-frequent-value could
throw off the estimate severely.

Could we not use the bogus range to calculate the histogram estimate but apply
the LIKE pattern directly to the most-frequent-values instead of applying the
bogus range? Or would that be too much code re-organization for now?

--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!