> > On Fri, Oct 28, 2016 at 10:29 AM, Kim Rose Carlsen <k...@hiper.dk> wrote:
> > Hi > > > > I was wondering if there is a way to hint that two columns in two different > > tables IS NOT DISTINCT FROM each other. So that the optimizer may assume if > > table_a.key = 'test' THEN table_b.key = 'test' . > > > > The equals operator already does this but it does not handle NULLS very well > > (or not at all). And it seems IS NOT DISTINCT FROM is not indexable and > > doesn't establish the same inference rules as equals. > > The whole idea behing Postgres' query planner is that you don't have > to use any hints. Late model versions of postgres handle nulls fine, > but nulls are never "equal" to anything else. I.e. where xxx is null > works with indexes. Where x=y does not, since null <> null. > > Suggestion for getting help, put a large-ish aka production sized > amount of data into your db, run your queries with explain analyze and > feed them to https://explain.depesz.com/ and post the links here along > with the slow queries. A lot of times the fix is non-obvious if you're > coming from another db with a different set of troubleshooting skills > for slow queries. The problem is how to reduce the problem into its core, without introducing all the unnecessary. Maybe simplifying the problem, also makes it impossible to say where I go wrong. It might be that I try to push too much logic into the SQL layer and Im adding too many layers of abstraction to accomplish what I want. So let me try and elaborate a little more. I have couple a tables describing resources (circuits) and allocation of resources to customers and products. First layer is a view called view_circuit. This view (left) join any table the circuit table reference through a foreign key (it gives exactly the same rows and columns as circuit table + some extra information like customer_id). Second layer is 2 views 1) a view describing if the circuit is active or inactive, lets call it view_circuit_product_main 2) a view describing line_speed about the circuit, lets call it view_circuit_product These views use aggregations (both GROUP BY and SELECT DISTINCT ON (...)) if this has any relevance. Third layer Next step is to add a view that tells both (joins the two views together on circuit_id). lets call the new view view_circuit_with_status This view is defined as CREATE VIEW view_circuit_with_status AS ( SELECT r.*, s.circuit_status, s.customer_id AS s_customer_id, p.line_speed, p.customer_id AS p_customer_id FROM view_circuit r JOIN view_circuit_product_main s ON r.circuit_id = s.circuit_id AND r.customer_id IS NOT DISTINCT FROM s.customer_id JOIN view_circuit_product p ON r.circuit_id = p.circuit_id AND r.customer_id IS NOT DISTINCT FROM s.customer_id ); SELECT * FROM view_circuit_with_status WHERE customer_id = 1; Since customer_id is exposed through view_circuit the planner assumes view_circuit.customer_id = 1 and from there attempts to join view_circuit_product_main and view_circuit_product using circuit_id. This is not running optimal. However if we change our query to allow the inference rule to take place, the query is executed very fast. SELECT * FROM view_circuit_with_status WHERE customer_id = 1 AND s_customer_id = 1 AND p_customer_id = 1; If a circuit is not assigned to any customers customer_id is set to NULL. This is the reason I can't use = operator. If I do use = then I can't find circuit which are unassigned, but the query do run effective. I can see this still ends up being quite abstract, but the point is it would be quite beneficial if IS NOT DISTINCT used the same rules as = operator. I have attached the 2 query plans Bad plan: https://explain.depesz.com/s/SZN Good plan: https://explain.depesz.com/s/61Ro - Kim Carlsen Do you use potatoes for long posts here?
