Re: How to hint 2 coulms IS NOT DISTINCT FROM each other

> > On Fri, Oct 28, 2016 at 10:29 AM, Kim Rose Carlsen <krc(at)hiper(dot)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?