BUG #8274: Wildly insane boolean selectivity estimates

The following bug has been logged on the website:

Bug reference:      8274
Logged by:          Andrew Gierth
Email address:      andrew@tao11.riddles.org.uk
PostgreSQL version: 9.1.9
Operating system:   any
Description:

The guy on IRC who ran into this one was using 9.1.9, but it seems to still
exist in master.


The handling of freq_null for estimating IS NOT TRUE/FALSE for boolean
columns which are all null or almost all null (enough that analyze doesn't
find any non-null values to put in MCV) is completely off in the weeds.


Suggested fix at:


https://gist.github.com/RhodiumToad/5901567