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f3281f9f93
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@gmail.com> Discussion: https://postgr.es/m/CAApHDvocZCUhM9W9mJ39d6oQz7ePKoqFnao_347mvC-A7QatcQ@mail.gmail.com