Slow query on “UNION ALL” view

Question

I have a DB view which basically consists of two SELECT queries with UNION ALL, like this:

CREATE VIEW v AS
SELECT time, etc. FROM t1 /         


        

Answer 1

I think i don't have much points to post it as comments so i am posting it as an answer

I don't know how PostgreSQL works behind the scene, i think you may get a clue if it would have been Oracle, so it is here how Oracle would work

Your UNION ALL view is slower because, behind the scene, records from both SELECT #1 and #2 are combined in a temporary table first, which is created on the fly, and then your SELECT ... FROM v WHERE time >= ... AND time < ... is executed on this temporary table. Since both #1 and #2 are indexed so they are working faster individually as expected, but this temporary table is not indexed (of course) and the final records are being selected from this temporary table so resulting in a slower response.

Now, at least, i don't see any way to have it faster + view + non-materialized

One way, other than running SELECT #1 and #2 and UNION them explicitly, to make it faster would be to use a stored procedure or a function in your application programming language (if it is the case), and in this procedure you make separate calls to each indexed table and then combine results, which is not as simple as SELECT ... FROM v WHERE time >= ... AND time < ... :(