Subquery Factoring

Here’s an interesting little conundrum about subquery factoring that hasn’t changed in the recent (11.2.0.3) patch for subquery factoring. It came to me from Jared Still (a fellow member of Oak Table Network) shortly after I’d made some comments about the patch. It’s an example based on the scott/tiger schema – which I’ve extracted from the script $ORACLE_HOME/rdbms/admin/utlsampl.sql (though the relevant scripts may be demobld.sql or scott.sql, depending on version).

As usual I’ve used an 8KB block size, LMT with uniform 1MB extents, and no ASSM to hold the data. I won’t reproduce the code to generate the schema, just the versions of the query with, then without, subquery factoring:

with e as (
	select
		/*+ gather_plan_statistics */
		deptno
	from	emp
	order by
		deptno
)
select
	deptno, count(*)
from
	e
group by deptno
;

select   deptno, count(*)
from (
	select
		/*+ gather_plan_statistics */
		deptno
	from	emp
	order by
		deptno
) e
group by deptno
;

-- reference script: with_subq_anomaly.sql

I have to say that I’m not entirely sure what Oracle should do with these queries – but I do think it should end up with the same result in both cases, but here are the two sets of results and their execution plans (in the same order that I listed the queries  above):

    DEPTNO   COUNT(*)
---------- ----------
        10          3
        20          5
        30          6

3 rows selected.

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID  fk4xrmz1wpcb9, child number 0
-------------------------------------
with e as (  select   /*+ gather_plan_statistics */   deptno  from emp
order by   deptno ) select  deptno, count(*) from  e group by deptno

Plan hash value: 1536940522

-------------------------------------
| Id  | Operation            | Name |
-------------------------------------
|   0 | SELECT STATEMENT     |      |
|   1 |  SORT GROUP BY NOSORT|      |
|   2 |   VIEW               |      |
|   3 |    SORT ORDER BY     |      |
|   4 |     TABLE ACCESS FULL| EMP  |
-------------------------------------

    DEPTNO   COUNT(*)
---------- ----------
        30          6
        20          5
        10          3

3 rows selected.

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID  17m8h7wgmxvr0, child number 0
-------------------------------------
select   deptno, count(*) from (  select   /*+ gather_plan_statistics
*/   deptno  from emp  order by   deptno ) e group by deptno

Plan hash value: 4067220884

-----------------------------------
| Id  | Operation          | Name |
-----------------------------------
|   0 | SELECT STATEMENT   |      |
|   1 |  HASH GROUP BY     |      |
|   2 |   TABLE ACCESS FULL| EMP  |
-----------------------------------

As you can see, with the inline view Oracle has managed to eliminate the order by clause; but when we use subquery factoring Oracle inlines the subquery as a non-mergeable view and preserves the order by, then recognises that it can achieve the aggregation through a “sort group by” on data that doesn’t need any more sorting.

Personally I think that the original authors of this SQL have got lucky – so far. Neither version of the statement, as written, can guarantee any order to the data because the final operator is not an order by. It is simply a limitation in the current optimizer code that the order by clause hasn’t been eliminated from the factored subquery. At the moment, then, this limitation means that this third party application is getting the expected results by accident – and the next upgrade might make the accident stop happening.

There is a fairly simple workaround to this type of problem – if you write the query with the inline view but add the hint /*+ no_eliminate_oby */ to the inline view, you can capture the baseline for the view and attach it to the version of the query with the inline view but without the hint. (See Fake Baselines for one possible method of doing this capture.)

[Further reading on "subquery factoring"]

Update 7th Aug 2013

This difference between the inline approach and the subquery factoring that chooses to move the subquery inline is still present in 12c (12.1.0.1). This suggests that you may find other cases in 12c where rewriting a query with factored subqueries to reduce the apparent complexity may still produce a change of execution plan.