Execution order of conditions in SQL 'where' clause

Question

I have a set of conditions in my where clause like

WHERE 
d.attribute3 = 'abcd*'  
AND x.STATUS != 'P' 
AND x.STATUS != 'J' 
AND x.STATUS != 'X' 
AND x.STATUS != 'S' 
AND x.STATUS != 'D' 
AND CURRENT_TIMESTAMP - 1 < x.CREATION_TIMESTAMP

Which of these conditions will be executed first? I am using oracle.

Will I get these details in my execution plan? (I do not have the authority to do that in the db here, else I would have tried)

Answer 1

Are you sure you "don't have the authority" to see an execution plan? What about using AUTOTRACE?

SQL> set autotrace on
SQL> select * from emp
  2  join dept on dept.deptno = emp.deptno
  3  where emp.ename like 'K%'
  4  and dept.loc like 'l%'
  5  /

no rows selected


Execution Plan
----------------------------------------------------------

----------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)|
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    62 |     4   (0)|
|   1 |  NESTED LOOPS                |              |     1 |    62 |     4   (0)|
|*  2 |   TABLE ACCESS FULL          | EMP          |     1 |    42 |     3   (0)|
|*  3 |   TABLE ACCESS BY INDEX ROWID| DEPT         |     1 |    20 |     1   (0)|
|*  4 |    INDEX UNIQUE SCAN         | SYS_C0042912 |     1 |       |     0   (0)|
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("EMP"."ENAME" LIKE 'K%' AND "EMP"."DEPTNO" IS NOT NULL)
   3 - filter("DEPT"."LOC" LIKE 'l%')
   4 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")

As you can see, that gives quite a lot of detail about how the query will be executed. It tells me that:

• the condition "emp.ename like 'K%'" will be applied first, on the full scan of EMP
• then the matching DEPT records will be selected via the index on dept.deptno (via the NESTED LOOPS method)
• finally the filter "dept.loc like 'l%' will be applied.

This order of application has nothing to do with the way the predicates are ordered in the WHERE clause, as we can show with this re-ordered query:

SQL> select * from emp
  2  join dept on dept.deptno = emp.deptno
  3  where dept.loc like 'l%'
  4  and emp.ename like 'K%';

no rows selected


Execution Plan
----------------------------------------------------------

----------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)|
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    62 |     4   (0)|
|   1 |  NESTED LOOPS                |              |     1 |    62 |     4   (0)|
|*  2 |   TABLE ACCESS FULL          | EMP          |     1 |    42 |     3   (0)|
|*  3 |   TABLE ACCESS BY INDEX ROWID| DEPT         |     1 |    20 |     1   (0)|
|*  4 |    INDEX UNIQUE SCAN         | SYS_C0042912 |     1 |       |     0   (0)|
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("EMP"."ENAME" LIKE 'K%' AND "EMP"."DEPTNO" IS NOT NULL)
   3 - filter("DEPT"."LOC" LIKE 'l%')
   4 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")

Answer 2

The database will decide what order to execute the conditions in.

Normally (but not always) it will use an index first where possible.

Answer 3

As has been said, looking at the execution plan will give you some information. However, unless you use the plan stability feature, you can't rely on the execution plan always remaining the same.

In the case of the query you posted, it doesn't look like the order of evaluation will change the logic in any way, so I guess what you are thinking about is efficiency. It's fairly likely that the Oracle optimizer will choose a plan that is efficient.

There are tricks you can do to encourage a particular ordering if you want to compare the performance with base query. Say for instance that you wanted the timestamp condition to be executed first. You could do this:

WITH subset AS
  ( SELECT /*+ materialize */
      FROM my_table
      WHERE CURRENT_TIMESTAMP - 1 < x.CREATION_TIMESTAMP
  )
SELECT *
  FROM subset
  WHERE 
  d.attribute3 = 'abcd*'  
  AND x.STATUS != 'P' 
  AND x.STATUS != 'J' 
  AND x.STATUS != 'X' 
  AND x.STATUS != 'S' 
  AND x.STATUS != 'D'

The "materialize" hint should cause the optimizer to execute the inline query first, then scan that result set for the other conditions.

I'm not advising you do this as a general habit. In most cases just writing the simple query will lead to the best execution plans.

Answer 4

To add to the other comments on execution plans, under the cpu-based costing model introduced in 9i and used by default in 10g+ Oracle will also make an assessment of which predicate evaluation order will result in lower computational cost even if that does not affect the table access order and method. If executing one predicate before another results in fewer predicates calculations being executed then that optimisaton can be applied.

See this article for more details: http://www.oracle.com/technology/pub/articles/lewis_cbo.html

Furthermore, Oracle doesn't even have to execute predicates where comparison with a check constraint or partition definitions indicates that no rows would be returned anyway.

Complex stuff.

Answer 5

Finally, relational database theory says that you can never depend on the order of execution of the query clauses, so best not to try. As others have said, the cost-based optimizer tries to choose what it thinks is best, but even viewing explain plan won't guarantee the actual order that's used. Explain plan just tells you what the CBO recommends, but that's still not 100%.

Maybe if you explain why you're trying to do this, some could suggest a plan?

Answer 6

Tricky question. Just faced the same dilemma. I need to mention a function within a query. The function itself makes another query, so you understand how it affects performance in general. But in most cases we have, the function wouldn't be called so often if the rest of conditions executed first.

Well, thought it would be useful to post here another article for topic.

The following quote is copied from Donald Burleson's site (http://www.dba-oracle.com/t_where_clause.htm) .

The ordered_predicates hint is specified in the Oracle WHERE clause of a query and is used to specify the order in which Boolean predicates should be evaluated.

In the absence of ordered_predicates, Oracle uses the following steps to evaluate the order of SQL predicates:

• Subqueries are evaluated before the outer Boolean conditions in the WHERE clause.

• All Boolean conditions without built-in functions or subqueries are evaluated in reverse from the order they are found in the WHERE clause, with the last predicate being evaluated first.

• Boolean predicates with built-in functions of each predicate are evaluated in increasing order of their estimated evaluation costs.