Unnesting of Nested Subqueries

Subqueries are "nested" when they appear in the WHERE clause of the parent statement. When Oracle evaluates a statement with a nested subquery, it must evaluate the subquery portion multiple times and may overlook some efficient access paths or joins.

Subquery unnesting unnests and merges the body of the subquery into the body of the statement that contains it, allowing the optimizer to consider them together when evaluating access paths and joins. The optimizer can unnest most subqueries, with some exceptions. Those exceptions include hierarchical subqueries and subqueries that contain a ROWNUM pseudocolumn, one of the set operators, a nested aggregate function, or a correlated reference to a query block that is not the subquery's immediate outer query block.

Assuming no restrictions exist, the optimizer automatically unnests some (but not all) of the following nested subqueries:

• Uncorrelated IN subqueries
• IN and EXISTS correlated subqueries, as long as they do not contain aggregate functions or a GROUP BY clause

You can enable extended subquery unnesting by instructing the optimizer to unnest additional types of subqueries:

• You can unnest an uncorrelated NOT IN subquery by specifying the HASH_AJ or MERGE_AJ hint in the subquery.
• You can unnest other subqueries by specifying the UNNEST hint in the subquery.

  See Also: Chapter 2, "Basic Elements of Oracle SQL" for information on hints