[HACKERS] Strange heuristic in analyze.c
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From: Greg Stark on 29 Jan 2010 11:38 So I never realized the consequences of this little heuristic in analyze.c in the handling of very low cardinality columns where we want to just capture the complete list of values in the mcv and throw away the histogram: else if (toowide_cnt == 0 && nmultiple == ndistinct) { /* * Every value in the sample appeared more than once. Assume the * column has just these values. */ stats->stadistinct = ndistinct; } The problem with this heuristic is that if the table is small enough you might expect you can set the statistics target high and "sample" the entire table and get a very accurate mcv covering all the values. However if any of the values in the table appears only once this heuristic will defeat you. The following code will then throw out of the mcv any value which isn't 25% more common than "average". Leaving you with a histogram for those values which often does very poorly if the values don't fit any pattern and are just discrete arbitrary values. -- greg -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers |