I found that results of previous tests with USNOA2 were not fully correct. Tables for tests with various parameters contained tuples in different order. I assumed that query
CREATE TABLE tab2 AS (SELECT * FROM tab1);
creates tab2 as exact copy of tab1, i.e. tab2 contain tuples in same order as tab1. But actually, it isn't always so. In aggregate with only few used test cases it can cause significant error.
I'm going to make use some more thought-out testing method. Probably, some more precise index quality measure exists (even for R-tree).
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With best regards,
Alexander Korotkov.