Do not use virtual (and hence redundant) WHERE-clause terms to restrict the
content of a automatic partial index.  Show when an automatic partial index
is used in the EXPLAIN QUERY PLAN output.
Increase the resolution of the second parameter to the likelihood() SQL
function (the probability value) so that it can handle probabilities
as small as 0.00000001.  Formerly, it ran out of precision at 0.001.