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

Encountered same scenario on 11g:

Scenario 1:

CREATE VIEW v AS
  SELECT time, etc. FROM t1 // #1...

The following query runs fast, plan looks okay:

SELECT ... FROM v WHERE time >= ... AND time < ...

Scenario 2:

CREATE VIEW v AS
  SELECT time, etc. FROM t2 // #2...

The following query runs fast, plan looks okay:

SELECT ... FROM v WHERE time >= ... AND time < ...

Scenario 3, with UNION ALL:

CREATE VIEW v AS
  SELECT time, etc. FROM t1 // #1...
  UNION ALL
  SELECT time, etc. FROM t2 // #2...

The following runs slow. Plan breaks apart t1 and t2 (which were also views) and assembles them as a big series of unions. The time filters are being applied properly on the individual components, but it is still very slow:

SELECT ... FROM v WHERE time >= ... AND time < ...

I would have been happy to just get a time in the ballpark of t1 plus t2, but it was more than double. Adding the parallel hint did the trick for me in this case. It re-arranged everything into a better plan:

SELECT /*+ parallel */ ... FROM v WHERE time >= ... AND time < ...