Actually, I think you don't particularly need stats for that in most
cases --- if the planner simply took note that the FK relationship
exists, it would know that each row of the FK side joins to exactly
one row of the PK side, which in typical cases is sufficient.

Is it? What about the other direction? Currently, doesn't the planner assume that the rowcount relationship is 1 to ( child total rows / parent total rows) ? That's ok for tables with relatively even distribution, but not for skewed ones.

--Josh Berkus

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TIP 6: explain analyze is your friend

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