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84f9a35e39
When adjusting the estimate for the number of distinct values from a rel in a grouped query to take into account the selectivity of the rel's restrictions, use a formula that is less likely to produce under-estimates. The old formula simply multiplied the number of distinct values in the rel by the restriction selectivity, which would be correct if the restrictions were fully correlated with the grouping expressions, but can produce significant under-estimates in cases where they are not well correlated. The new formula is based on the random selection probability, and so assumes that the restrictions are not correlated with the grouping expressions. This is guaranteed to produce larger estimates, and of course risks over-estimating in cases where the restrictions are correlated, but that has less severe consequences than under-estimating, which might lead to a HashAgg that consumes an excessive amount of memory. This could possibly be improved upon in the future by identifying correlated restrictions and using a hybrid of the old and new formulae. Author: Tomas Vondra, with some hacking be me Reviewed-by: Mark Dilger, Alexander Korotkov, Dean Rasheed and Tom Lane Discussion: http://www.postgresql.org/message-id/flat/56CD0381.5060502@2ndquadrant.com |
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