+materialize causes I/O ?

Question

We use a query like this on Oracle 11.2g (in a PL/SQL stored procedure):
WITH x AS ( SELECT /*+materialize*/ FK1ID, FK2Id, FK3Id  FROM SOME_VIEW  WHERE some_ID = p_some_parameter) -- an IN parameter of the stored procedure
  SELECT  foo, bar, baz FROM t1, t2, ...
  WHERE exists (
    select 1 from  x where
        x.FK1ID= 42
      or
        x.FK1ID = 34 and
        x.FK2ID = t2.xyz)
    AND some other conditionsThis is much faster (50 times) than without the +materialize hint or if we put that (sub)query in line.
But apparently it causes extra I/O (writing the subquery result to temporary table and reading it back).
Is that true? If yes, what else to use instead then?
Regards,
David
PS: The subquery would have 10-20 rows, while the main query would return tens of thousands of rows in our test case.

Answer 1

But apparently it causes extra I/O (writing the subquery result to temporary table and reading it back).
Is that true? Yes, it's true that writing the result to a memory table causes IO
If yes, what else to use instead then?Sounds like you already have an answer:
This is much faster (50 times) than without the +materialize hint or if we put that (sub)query in line.The materialize hint forces the materialization of the subquery.
Without hinting it, this is something that will happen automatically usually when you reference it twice in subsequent selects.
As ever, there's a balance.
Why would we want to force a materialisation of a subquery referenced once?
In general, it seems like something that should be avoided and treated with a no_merge or a no_unnest or whatever is appropriate for the issue that the materialize hint was trying to solve.
As Centinul suggests, execution plans would be a useful addition to the post.
I would double check those predicates in the subquery as well.
You have :
        x.FK1ID= 42
      or
        x.FK1ID = 34 and
        x.FK2ID = t2.xyzDo you not want this?
       (x.FK1ID= 42
      or
        x.FK1ID = 34) and
        x.FK2ID = t2.xyzor even
       (x.FK1ID IN (42,34) and
        x.FK2ID = t2.xyzAt the moment, you have something more akin to:
WITH x AS ( SELECT /*+materialize*/ FK1ID, FK2Id, FK3Id  FROM SOME_VIEW  WHERE some_ID = p_some_parameter) -- an IN parameter of the stored procedure
  SELECT  foo, bar, baz FROM t1, t2, ...
  WHERE exists (
    select 1 from  x where
        x.FK1ID= 42
   union all
    select 1 from  x where
        x.FK1ID = 34 and
        x.FK2ID = t2.xyz)Even if the code is correct, the latter wording is at least perhaps clearer about it.