Quiz Night

Here’s an execution plan from a recent thread on the Oracle developer forum:

 
------------------------------------------------------------------------------------------------
| Id  | Operation                 | Name               | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT          |                    |     1 |   231 |   160   (6)| 00:00:02 |
|   1 |  UPDATE                   | GS_TABLE           |       |       |            |          |
|*  2 |   HASH JOIN SEMI          |                    |     1 |   231 |   130   (0)| 00:00:02 |
|*  3 |    TABLE ACCESS FULL      | GS_TABLE           |     5 |   895 |   123   (0)| 00:00:02 |
|*  4 |    TABLE ACCESS FULL      | UPDATEDPROGRAMCODE |   850 | 44200 |     7   (0)| 00:00:01 |
|*  5 |   VIEW                    |                    |    11 |  2024 |     8  (13)| 00:00:01 |
|*  6 |    WINDOW SORT PUSHED RANK|                    |    11 |   440 |     8  (13)| 00:00:01 |
|*  7 |     FILTER                |                    |       |       |            |          |
|*  8 |      TABLE ACCESS FULL    | UPDATEDPROGRAMCODE |    11 |   440 |     7   (0)| 00:00:01 |
|   9 |   VIEW                    |                    |   850 |  1138K|     9  (23)| 00:00:01 |
|  10 |    SORT ORDER BY          |                    |   850 |   685K|     9  (23)| 00:00:01 |
|* 11 |     VIEW                  |                    |   850 |   685K|     8  (13)| 00:00:01 |
|  12 |      WINDOW SORT          |                    |   850 | 47600 |     8  (13)| 00:00:01 |
|* 13 |       FILTER              |                    |       |       |            |          |
|* 14 |        TABLE ACCESS FULL  | UPDATEDPROGRAMCODE |   850 | 47600 |     7   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Unfortunately the originator of this plan declined to show us the query or answer any questions about where the work was going, but did confirm a speculative comment I had made that the instance was 12c. So the quiz question tonight is this: can you spot what it was that made me think that the plan came from 12c?

I have to say, by the way, that there may be ways to get this plan from 11g, it was just that my first impression was that it was probably 12c and I didn’t attempt to come up with a way of getting a similar plan from 11g. (And, as far as the general shape of the plan is concerned, I can think of two different types of query that could produce it.)

Footnote

You are allowed to prove me wrong.

Answer

(Which might be me showing ignorance rather than inspiration)

The basic shape of the plan suggests to me that the query is of the form:

update gs_table 
set     col1 = (select from updatedprogramcode),
        col2 = (select from updatedprogramcode)
where   exists (select from updatedprogramcode)         -- possibly "where IN (subquery)"
;

There are a couple of variations in how the “set” subquery content might vary, and I’ll write up a short blog about that later.

Having noted this basic shape, I then noted that the subqueries involved analytic functions – as indicated by the WINDOW SORT operations; moreover one of them used a PUSHED RANK option and the other was embedded in a non-mergeable VIEW (operation 11). Updates with subqueries generally involve correlated columns – and prior to 12c there are some restrictions on how far up the tree the correlation can go. Here’s a sample query (using two tables that I’ve cloned from all_objects) to demonstrate:

update t1 set
        data_object_id = (
                select  objno
                from    (
                        select
                                object_id objno,
                                row_number() over (order by  object_id desc) rn
                        from
                                t2
                        where
                                t2.object_type = t1.object_type
                        )
                where rn = 1
        )
/

We need to embed the inner select statement in an inline view because we want to use the result of the row_number() analytic function in a filter predicate, but in Oracle 11g the reference to t1.object_id can’t correlate back to the outer t1 table, while in 12c this isn’t a problem. Here’s the 12c plan, followed by the 11g error:

12c Plan (autotrace)
----------------------------------------------------------------------------------
| Id  | Operation                 | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT          |      |  5000 | 45000 | 70012  (15)| 00:04:34 |
|   1 |  UPDATE                   | T1   |       |       |            |          |
|   2 |   TABLE ACCESS FULL       | T1   |  5000 | 45000 |    12   (0)| 00:00:01 |
|*  3 |   VIEW                    |      |     1 |    26 |    13   (8)| 00:00:01 |
|*  4 |    WINDOW SORT PUSHED RANK|      |   556 |  6116 |    13   (8)| 00:00:01 |
|*  5 |     TABLE ACCESS FULL     | T2   |   556 |  6116 |    12   (0)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   3 - filter("RN"=1)
   4 - filter(ROW_NUMBER() OVER ( ORDER BY INTERNAL_FUNCTION("OBJECT_ID")
              DESC )<=1)
   5 - filter("T2"."OBJECT_TYPE"=:B1)


11g Error
---------
                                t2.object_type = t1.object_type
                                                 *
ERROR at line 11:
ORA-00904: "T1"."OBJECT_TYPE": invalid identifier

Notice, by the way that my predicate “rn = 1” has resulted in the WINDOW SORT PUSHED RANK that appeared in the original plan.

In case I haven’t said it enough times: this is a just a rapid inference I drew from looking briefly at the plan and I haven’t tried hard to work out whether there is a way to get a plan like this in 11g. It was nice being proved right by the follow-up post from the OP, but my guess may have been right by accident – I’d rather be proved wrong than carry on thinking I’d got it right when I hadn’t … so feel free to supply an alternative in the comments.