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Don Burleson Blog 







Tuning Correlated Subqueries for Fast Speed 

Oracle Database Tips by Donald BurlesonJune 23, 2015

Question:  I have a complex SQL statements with two correlated subqueries that runs very slow:

select <columns>
inner join
    table_a.o_id = table_c.o_id
 . .
    (table_b.date_dt =
         table_b.o_id = table_c.o_id)
     table_b.date_ts =
          table_b.e_nb = table_c.e_nb)
    ) order by <columns>;

The correlated subqueries are making this SQL very slow to execute.  How do I tune a correlated subquery to make it run faster?

Answer:  Correlated subqueries are usually used for EXISTS Booleans, and scalar subqueries (e.g. subqueries in the SELECT clause).

Correlated subqueries and slow because the sub-query is executed ONCE for each row returned by the outer query.  

Start by comparing the number of rows returned to the number of consistent gets using autotrace.  

There are several ways to tune a correlated subquery:

  •  Query rewrite:  Inspect the correlated subquery execution plan for the subqueries, and see if the explain plan is re-writing the correlated subquery internally (set query_rewrite_enabled = true) into a more efficient form, a standard join.
  •  Check indexes:   If you MUST use a correlated subquery, MAKE SURE that each of the referenced columns in the subquery has an index!  Without indexes on the correlated subquery columns, the correlated subquery might be forced to do an expensive full-table scan, executed over and over, once for each outer row returned.
  •  Replace correlated query with a standard join:  See if you can replicate the output without the subqueries, using vanilla joins.
  •  Materialize the correlated subqueries with global temporary tables of the WITH clause:  See if you can create temporary tables to represent the contents of the subquery. If you break down the query into its component pieces and use the WITH clause, it may run faster. 

See the book Advanced Oracle SQL Tuning for complete details on tuning correlated subqueries.  

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