2016-12-01

Plan Shaping and Cardinality Miss Estimate on row_number over partition by

Recalling talks with Tim Hall I wrote in my last post. He mentionend Jonathan Lewis telling about plan shaping. Every now and then there comes a need to tell the optimizer where the query execution should be starting. Write your query from part in order and use ordered hint or most often leading hint will be enough. Hitting the need for such plan shaping comes when the optimizer sees inline views in a query returning only one row and actually there are more in execution time. If there are several missleading one liners and a cartesian join between those a quite simple query can consume significant query time. 12.1.0.2 database has a anoying bug involving top-n queries with analytic partition by part. Luckily a patch for the 21971099 bug is available for some environments. Here is a simplified test getting cardinality one. Imagine having a couple of such in your bigger query.
create table s as (
select round(level/2) n 
     , level n2 
 from dual connect by level < 10e4
)
;

select /*+gather_plan_statistics*/ n
     , n2
  from (
   select n
        , n2
        , row_number()over(partition by n order by n2 desc) rn
     from s)
 where rn <= 1
;

select * 
  from table(dbms_xplan.display_cursor(format=>'iostats last'))
;

Plan hash value: 2407482549
 
-------------------------------------------------------------------------------------------
| Id  | Operation                | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |      |      1 |        |  50000 |00:00:00.12 |     208 |
|*  1 |  VIEW                    |      |      1 |      1 |  50000 |00:00:00.12 |     208 |
|*  2 |   WINDOW SORT PUSHED RANK|      |      1 |  99999 |  50000 |00:00:00.11 |     208 |
|   3 |    TABLE ACCESS FULL     | S    |      1 |  99999 |  99999 |00:00:00.02 |     208 |
-------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - filter("RN"<=1)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "N" ORDER BY 
              INTERNAL_FUNCTION("N2") DESC )<=1)

While waiting for the patching to happen, an alternative to bypass the problem is not to use row_number analytic function. By using rank and order by part that won't return competing winners, we get another cardinality estimate. It is as much wrong as the earlier one, but most likely will help with the cartesian join problem.
select /*+gather_plan_statistics*/ n
     , n2
  from (
   select n
        , n2
        , rank()over(partition by n order by n2 desc, rowid) rn
     from s)
 where rn <= 1
;

select * 
  from table(dbms_xplan.display_cursor(format=>'iostats last'))
;

Plan hash value: 2407482549
 
-------------------------------------------------------------------------------------------
| Id  | Operation                | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |      |      1 |        |  50000 |00:00:00.13 |     208 |
|*  1 |  VIEW                    |      |      1 |  99999 |  50000 |00:00:00.13 |     208 |
|*  2 |   WINDOW SORT PUSHED RANK|      |      1 |  99999 |  50000 |00:00:00.11 |     208 |
|   3 |    TABLE ACCESS FULL     | S    |      1 |  99999 |  99999 |00:00:00.02 |     208 |
-------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - filter("RN"<=1)
   2 - filter(RANK() OVER ( PARTITION BY "N" ORDER BY INTERNAL_FUNCTION("N2") DESC 
              ,ROWID)<=1)

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I am Timo Raitalaakso. I have been working since 2001 at Solita Oy as a Senior Database Specialist. I have received Oracle ACE nomination. My main focus is on projects involving Oracle database. In this Rafu on db blog I write some interesting issues that evolves from my interaction with databases. Mainly Oracle.