<marin@xxxxxxxx> writes: > Yesterday I was trying to calculate by hand the row number estimates using > the examples from > http://www.postgresql.org/docs/9.3/static/row-estimation-examples.html > It is usually correct, except when you try to get an estimate near the > first (or last) histogram bound. Let me demonstrate with an example: > ... > As no statistics are collected for the index (as stated in the > pg_statistic documentation "No entry is made for an ordinary non-expression > index column, however, since it would be redundant with the entry for the > underlying table column.") it is mystery to me how these estimates are > calculated. I tried to look at the code (my guess was that I could find it > in btcostestimate) but I wasn't able to figure it out. scalarineqsel() is where to look. > Can somebody explain how are the estimates calculated when there is an > index involved? If the comparison value is outside the range recorded in the histogram, and there's a suitable index available, the planner uses the index to find out the actual column min or max rather than believing the histogram completely. See get_actual_variable_range(). Possibly this behavior ought to be mentioned in the docs ... regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
