|From:||Peter Eisentraut <peter_e(at)gmx(dot)net>|
|Subject:||Selectivity estimation for equality and range queries|
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I have been observing a case where the row count estimation for LIKE 'foo' is
(much) higher than for LIKE 'foo%', the rest of the query being the same.
This is a special case of the estimation for equality being higher than for a
range query that includes the value used in the equality.
I haven't been able to get a copy of the data from the client yet, but
considering the nature of the data and the description of the selectivity
this behavior appears to be mathematically plausible. I have been wondering
whether in general the eqsel should try to compare its result with the
estimation of (x >= 'foo' AND x <= 'foo') and use that as a ceiling or
Has anyone else observed something similar?
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