Generally speaking, when executing UNION ; a DISTINCT is run afterward on the resultset.
So, if you're sure that each part of UNION cannot return a line returned by another one, you may use UNION ALL, you'll cut the cost of the final implicit DISTINCT.
I was wondering if someone could help me understands what a union all actually does.
For my thesis I am using Apache Calcite to rewrite queries into using materialized views which I then give to a Postgres database. For some queries, this means that they will be rewritten in a UNION ALL style query between an expression and a table scan of a materialized view. However, contrary to what I expected, the UNION ALL query is actually a lot slower.
As an example, say I have 2 tables: actor and movie. Furthermore, there is also a foreign key index on movie to actor. I also have a materialized view with the join of these 2 tables for all movies <= 2015 called A. Now, if I want to query all entries in the join between actor and movie, I would assume that a UNION ALL between the join of actor and movie for movies >2015 and A is faster than executing the original query.. If I look at the explain analyze part, I can certainly see a reduction in cost up until the UNION ALL part, which carries a respective cost more than negating the cost reduction up to a point where I might as well not use the existing materialized view.
I have some trouble understanding this phenomenon. One thought which came to my mind was that perhaps UNION ALL might create a temporary table containing both result sets, and then do a table scan and return that result.
this would greatly increase IO cost which could attribute to the problem. However, I am really not sure what UNION ALL actually does to append both result sets so I was wondering if someone would be able to help me out with this.
Mark
066ce286@free.fr: Please, avoid top-posting. It makes harder to follow the discussion.
