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Demystifying OUTLINE DATA in an Execution Plan …

Posted by FatDBA on March 7, 2022

‘Outline Data’ section displays the list of hints that would be needed to replicate the execution plan, even if the statistics change., but in case of a complex execution plans it comes with lot of strange looking and obscure terms used. As there isn’t any published explanation of stored outline hints, so today’s post is to decipher few of the terms that you see in an outline data of an execution plan.

If you want to read more about outlines, what are they, their purpose, please read my previous post on the same subject at –> https://fatdba.com/2017/11/30/how-to-fix-sql-plan-issues-using-outline-data/

Okay, coming back to the post, let me quickly generate the outline data for one of the test SQL and will try to explain about each of the hints, query blocks, aliases and other representations used.

SQL> explain plan for 
SELECT d.department_name,e.employee_name
FROM departments d
LEFT OUTER JOIN employees e ON d.department_id = e.department_id
WHERE d.department_id >= 30
ORDER BY d.department_name, e.employee_name;  


SQL> select * from table(dbms_xplan.display('PLAN_TABLE',NULL,'+alias +outline'));

Plan hash value: 3871261979
| Id  | Operation           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |             |     4 |   168 |     7  (15)| 00:00:01 |
|   1 |  SORT ORDER BY      |             |     4 |   168 |     7  (15)| 00:00:01 |
|*  2 |   HASH JOIN OUTER   |             |     4 |   168 |     6   (0)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL| DEPARTMENTS |     2 |    44 |     3   (0)| 00:00:01 |
|*  4 |    TABLE ACCESS FULL| EMPLOYEES   |     6 |   120 |     3   (0)| 00:00:01 |

Query Block Name / Object Alias (identified by operation id):
   1 - SEL$2BFA4EE4
   3 - SEL$2BFA4EE4 / D@SEL$1
   4 - SEL$2BFA4EE4 / E@SEL$1

Outline Data
      USE_HASH(@"SEL$2BFA4EE4" "E"@"SEL$1")
      LEADING(@"SEL$2BFA4EE4" "D"@"SEL$1" "E"@"SEL$1")
      FULL(@"SEL$2BFA4EE4" "E"@"SEL$1")
      FULL(@"SEL$2BFA4EE4" "D"@"SEL$1")
      MERGE(@"SEL$8812AA4E" >"SEL$948754D7")

Predicate Information (identified by operation id):

   2 - access("D"."DEPARTMENT_ID"="E"."DEPARTMENT_ID"(+))
   3 - filter("D"."DEPARTMENT_ID">=30)
   4 - filter("E"."DEPARTMENT_ID"(+)>=30)

As per the above outline data captured for the execution plan, the very first entry is USE_HASH(@”SEL$2BFA4EE4″ “E”@”SEL$1”) This represents the use of HASH join methods used in the query. Other join method outline options you might see are USE_NL and USE_MERGE. Here in our example we’ve used the LEFT OUTER JOIN which will return all valid rows from the table on the left side of the JOIN keyword, that’s table EMPLOYEE (alias ‘E’) along with the values from the table on the right side.

i.e LEFT OUTER JOIN employees e ON d.department_id = e.department_id

LEADING(@”SEL$2BFA4EE4″ “D”@”SEL$1” “E”@”SEL$1”) The LEADING hints specifies the exact join order for the SQL that is followed by the optimizer in the plan. It displays the join order as an ordered list of table aliases and query block names. The aliases appears in the ordering in which we access those tables in the query.

The first part of the hint @”SEL$2BFA4EE4″ represents the main SELECT block and if you closely see the full hint definition you will see “D”@”SEL$1”, where “SEL$1” is the query block name for table alias ‘D’ for Department table, which is followed by “E”@”SEL$1”, that represents query block name “SEL$1” on second table joined ‘E’ for Employee table.

FULL(@”SEL$2BFA4EE4″ “E”@”SEL$1”) & FULL(@”SEL$2BFA4EE4″ “D”@”SEL$1”) are what you see next and these two hints for FULL TABLE SCANS on table alias “E”, that is EMPLOYEE table, followed by FTS on table alias “D”, that is DEPARTMENT table. Same you can see in the execution plan too.

OUTLINE(@”SEL$1″) & OUTLINE(@”SEL$2″) These two OUTLINE hints correspond to initial and the intermediate query blocks.

ANSI_REARCH(@”SEL$1″) This is the hint that instructs the optimizer to re-architecture of ANSI left, right, and full outer joins. In our case this was to re-arch the left outer join.

MERGE(@”SEL$8812AA4E” >”SEL$948754D7″) is the MERGE query block hint. SEL$8812AA4E and SEL$948754D7 are the transformed query blocks.

OUTLINE_LEAF(@”SEL$2BFA4EE4″) This hint builds an outline leaf for the specified query block. In our example it represents that the query block SEL$2BFA4EE4 is a “final” query block that has actually been subject to independent optimization. Outline leaf cannot be transformed. You can see multiple outline_leaf hints for the query blocks.

IGNORE_OPTIM_EMBEDDED_HINTS is a special hint instructs the CBO to ignore most of all the other supplied hints.

ALL_ROWS This hint instructs to optimize the query/statement block for best throughput with lowest resource utilization.

DB_VERSION(‘19.1.0’) & OPTIMIZER_FEATURES_ENABLE(‘19.1.0’) these two hints allows for the CBO to process the SQL on the said version of the database, 19.1.0 in our case.

Few others that you might see in OUTLINE DATA sections and are easy to decipher are INDEX_RS_ASC (Index Range Scan in ascending order) and it happens when the INDEX RANGE SCAN is used as an access path/method in SQL execution plan. Few others that you can see in case of Nested Loop Joins are NLJ_BATCHING which happens when Oracle batches multiple physical I/O requests and process them using a vector I/O (array) instead of processing them one at a time, and batching improves performance because it uses asynchronous reads..
Few others that you can see in case of NL’s are USE_NL, NLJ_PREFETCH etc.

INDEX_FFS in case of Index Fast Full Scans and the list is long …

Hope It Helped!
Prashant Dixit

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