pgsql: Improve comments for estimate_multivariate_ndistinct()

Improve comments for estimate_multivariate_ndistinct()

estimate_multivariate_ndistinct() is coded to assume the caller handles
passing it a list of GroupVarInfos with unique 'var' fields over the
entire list. 6bb6a62f3 added code which didn't ensure this and that
could result in estimate_multivariate_ndistinct() erroring out with:

ERROR: corrupt MVNDistinct entry

This occurred because estimate_multivariate_ndistinct() first searches
for a set of stats that match to at least two of the given GroupVarInfos
and then later assumes that the MVNDistinctItem.items array of the
best matching stats will have an entry for those two columns. If the
GroupVarInfos List contained a duplicate entry then the same column could
be matched to twice and that could trick the code into thinking we have
>= 2 columns matched in cases where only a single distinct column has been
matched. This could result in a failure to find the correct
MVNDistinctItem in the stats as the array containing those never
contains an item for single columns.

Here we make it more clear that the function needs a distinct set of
GroupVarInfos and also tidy up a few other comments to make things a bit
easier to follow.

Author: David Rowley <drowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAApHDvocZCUhM9W9mJ39d6oQz7ePKoqFnao_347mvC-A7QatcQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f3281f9f9395099724b55ddc52991a92795c6068

Modified Files
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src/backend/utils/adt/selfuncs.c | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)