Jeff Janes wrote:

> The attached patch forces there to be at least one element in MCE, keeping
> the one element with the highest predicted frequency if the MCE would
> otherwise be empty.  Then any other element queried for is assumed to be no
> more common than this most common element.

Hmm, what happens if a common-but-not-an-MCE element is pruned out of
the array when a bucket is filled?  I imagine it's going to mis-estimate
the selectivity (though I imagine the effect is going to be pretty
benign anyway, I mean it's still going to be better than stock 0.5%.)

> I'd also briefly considered just having the part of the code that pulls the
> stats out of pg_stats interpret a MCE array as meaning that nothing is more
> frequent than the threshold, but that would mean that that part of the code
> needs to know about how the threshold is chosen, which just seems wrong.

I wonder if we shouldn't add a separate stats STATISTIC_KIND for this,
instead ot trying to transfer knowledge.

Given how simple this patch is, I am tempted to apply it anyway.  It
needs a few additional comment to explain what is going on, though.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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