Oracle: why doesn't use parallel execution?

Question

Look at the following query: If I comment the subquery it uses parallel execution otherwise it doesn\'t.

After the query has been

SELECT  /*+ parall         


        

Answer 1

You could have found the answer in the documentation:

A SELECT statement can be parallelized only if the following conditions are satisfied:

• The query includes a parallel hint specification (PARALLEL or PARALLEL_INDEX) or the schema objects referred to in the query have a PARALLEL declaration associated with them.

• At least one of the tables specified in the query requires one of the following:

  • A full table scan

  • An index range scan spanning multiple partitions

• No scalar subqueries are in the SELECT list.

Your query falls at the final hurdle: it has a scalar subquery in its projection. If you want to parallelize the query you need to find another way to write it.