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postgres-lambda-diff

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    Tom Lane authored
    I made multiple errors in commit 97532f7c,
    stemming mostly from failure to think about the available frequency data
    as being element frequencies not value frequencies (so that occurrences of
    different elements are not mutually exclusive).  This led to sillinesses
    such as estimating that "word" would match more rows than "word:*".
    
    The choice to clamp to a minimum estimate of DEFAULT_TS_MATCH_SEL also
    seems pretty ill-considered in hindsight, as it would frequently result in
    an estimate much larger than the available data suggests.  We do need some
    sort of clamp, since a pattern not matching any of the MCELEMs probably
    still needs a selectivity estimate of more than zero.  I chose instead to
    clamp to at least what a non-MCELEM word would be estimated as, preserving
    the property that "word:*" doesn't get an estimate less than plain "word",
    whether or not the word appears in MCELEM.
    
    Per investigation of a gripe from Bill Martin, though I suspect that his
    example case actually isn't even reaching the erroneous code.
    
    Back-patch to 9.1 where this code was introduced.
    1faf866a
    History
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