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 merge join hint Tips

Oracle Database Tips by Donald BurlesonMarch 14, 2015

Question:  I want to replace a nested loops join with a hash join, and I need the syntax for a hash join hint.  How do I add a hash join hint to my SQL?

Answer:  The use_merge hint forces a sort merge operation. The sort merge operation is often used in conjunction with parallel query because a sort merge join always performs full-table scans against the tables.

Sort merge joins are generally best for queries that produce very large result sets such as daily reports and table detail summary queries, or tables that do not possess indexes on the join keys. Here we see a simple query that has been formed to perform a sort merge using parallel query against both tables.

select /*+ use_merge(e,b) parallel(e, 4) parallel(b, 4) */
   e.ename,
   hiredate,
   b.comm
from
   emp   e,
   bonus b
where
   e.ename = b.ename

;

Here is the output of the execution plan for this query. Note the full-table scans and the sort merge operation:

OPERATION
----------------------------------------------------------------------
OPTIONS OBJECT_NAME POSITION
------------------------------ ---------------------------- ----------
SELECT STATEMENT
5
MERGE JOIN
1
PARALLEL_TO_SERIAL
SORT
JOIN 1
PARALLEL_COMBINED_WITH_PARENT
TABLE ACCESS
FULL EMP 1
PARALLEL_TO_PARALLEL

SORT
JOIN 2
PARALLEL_COMBINED_WITH_PARENT
TABLE ACCESS
FULL BONUS 1
PARALLEL_TO_PARALLEL

It is important to note that a sort merge join hint (use_merge) does not use indexes to join the tables. In most cases, index access is faster, but a sort merge join may be appropriate for a large tale join without a where clause, or in queries that do not have available indexes to join the tables.

   
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