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Values for the other_tag Column in plan_table

Oracle Tips by Burleson Consulting

To fully understand a parallel execution plan, you must review the possible values for the other_tag column in the plan_table. This column is used by Oracle to provide additional detail about the type of parallel operation that is being performed during the execution of the query. Here are the possible values for other_tag:

  • Parallel_to_serial   This is usually the first operation in the execution plan, and it is where a parallel full-table scan is passed to the query coordinator for merging.

  • Parallel_from_serial   This is a condition where the parallel processes wait for a serial operation to complete. In many cases, this is a warning that your query may not be optimized, since the parallel processes are not allowed to begin immediately upon execution of the query.

  • Parallel_to_parallel   This tag is seen in cases such as a sort merge join where a parallel full-table scans is immediately followed by a parallel disk sort in the TEMP tablespace.

  • Parallel_combined_with_parent   This is a parallel full-table scan that is combined with an index lookup, as is the case with a nested loop join.

  • Parallel_combined_with_child   This is a case where a parallel full-table scan is combined with a child operation.

  • Serial   This is a linear operation such as an index scan.

WARNING: Always investigate an other_tag value of parallel_from_serial. This is because your query may have a bottleneck whereby the parallel query slaves are waiting unnecessarily for a serial operation, such as an index range scan.

Now that you see the basics of parallel execution plans, let’s explore some of the typical implementations of parallel join operations.

Nested Loop Joins with Parallel Query

In a nested loop join, Oracle normally uses indexes to join the tables. However, you can create an execution plan that will invoke a nested loop join that performs a parallel full-table scan against one of the tables in the join. In general, you should perform the parallel full-table scans on only one of the tables that are being joined, and this is normally the driving table for the query. As you will recall, in the CBO the driving table is the first table in the from clause.

Here is an example of a query execution plan that is forced to perform a nested loop join with a parallel query on the emp table. Note that we have invoked the ordered hint to direct the CBO to evaluate the tables in the order they are presented in the where clause.

select /*+ ordered use_nl(bonus) parallel(e, 4) */
   emp e,
   bonus b
   e.ename = b.ename

Here is the execution plan for this query. There we see the parallel operations on the emp table.

OPTIONS                        OBJECT_NAME                    POSITION
------------------------------ ---------------------------- ----------
FULL                           EMP                                   1
BY INDEX ROWID                 BONUS                                 2
RANGE SCAN                     ENAME_BONUS_IDX                       1

Depending on the query, a parallel nested loop join will often provide excellent performance. However, depending upon the characteristics of the data in your tables, you may find that a parallel sort merge join or hash join will offer faster response time.

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.

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