pgsql: Modify prefix_selectivity() so that it will never estimate the

Log Message:
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Modify prefix_selectivity() so that it will never estimate the selectivity
of the generated range condition var >= 'foo' AND var < 'fop' as being less
than what eqsel() would estimate for var = 'foo'. This is intuitively
reasonable and it gets rid of the need for some entirely ad-hoc coding we
formerly used to reject bogus estimates. The basic problem here is that
if the prefix is more than a few characters long, the two boundary values
are too close together to be distinguishable by comparison to the column
histogram, resulting in a selectivity estimate of zero, which is often
not very sane. Change motivated by an example from Peter Eisentraut.

Arguably this is a bug fix, but I'll refrain from back-patching it
for the moment.

Modified Files:
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pgsql/src/backend/utils/adt:
selfuncs.c (r1.243 -> r1.244)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c?r1=1.243&r2=1.244)