Guillaume Cottenceau <gc@xxxxxx> writes: > It seems to me that if the correlation is 0.99[1], and you're > looking for less than 1% of rows, the expected rows may be at the > beginning or at the end of the heap? Right, but if you know the value being searched for, you could then estimate where it is in the table by consulting the histogram. Actually, an even easier hack (which would have the nice property of not needing to know the exact value being searched for), would simply use the existing cost estimates if the WHERE variables have low correlation (meaning the random-locations assumption is probably good), but apply some sort of penalty factor if the correlation is high. This would amount to assuming that the universe is perverse and high correlation will always mean that the rows we want are at the wrong end of the table not the right one. But any DBA will tell you that the universe is indeed perverse ;-) OTOH, since indexscans get a cost estimate reduction in the presence of high correlation, we're already biasing the choice in the direction of indexscans for high correlation. We may not need to do it twice. I don't recall whether the OP ever showed us his statistics for the table in question --- did it even appear to have high correlation? regards, tom lane
