On 5/11/19 2:33 AM, bb ddd wrote:
Thanks, Adrian, but i was looking for something that goes into more depth.
For example there is one case described there where we have a Nested Loop with
rows=33, and its 2 child nodes have each rows=10.
But first of all this is a very exotic join condition (t1.hundred < t2.hundred)
and second of all i cannot find any explanation how this number 33 is derived.
They literally spend less than 2 sentences on this case.
In my case i as well have Nested Loop's rows value different than the product
of its 2 children's rows values, but with a normal join condition on a foreign
key.
My guesses (again) are the same way it keeps some statistics (very curious what exactly) about what is the probability 2 random rows from each table satisfy that condition
https://www.postgresql.org/docs/11/planner-stats-details.html
(t1.hundred < t2.hundred), it also keeps statistics what is the
probability 2 random rows from each table satisfy the regular normal
join condition like the one i have in my case (basically
t1.t2_id=t2.id). And in both cases it just applies that probability to
the product of the rows values of the 2 child nodes, to calculate the
expected rows value of the result of the Nested Loop. But i am just
guessing.
--
Adrian Klaver
adrian.kla...@aklaver.com
