Count estimate

gistfile1.sql

CREATE LANGUAGE plpgsql;
CREATE FUNCTION count_estimate(query text) RETURNS integer AS $$
DECLARE
    rec   record;
    rows  integer;
BEGIN
    FOR rec IN EXECUTE 'EXPLAIN ' || query LOOP
        rows := substring(rec."QUERY PLAN" FROM ' rows=([[:digit:]]+)');
        EXIT WHEN rows IS NOT NULL;
    END LOOP;
 
    RETURN rows;
END;
$$ LANGUAGE plpgsql VOLATILE STRICT;

Тестируем:

CREATE TABLE foo (r double precision);
INSERT INTO foo SELECT random() FROM generate_series(1, 1000);
ANALYZE foo;

# SELECT count(*) FROM foo WHERE r < 0.1;
 count 
-------
    92
(1 row)

# SELECT count_estimate('SELECT * FROM foo WHERE r < 0.1');
 count_estimate 
----------------
             94
(1 row)