Where / Having

There’s a very old mantra about the use of the “having” clause that tells us that if it’s valid (i.e. will always give the same results) then any predicate that could be moved from the having clause to the where clause should be moved. In recent versions of Oracle the optimizer will do this for itself in some cases but (for reasons that I’m not going to mention) I came across a silly example recently where a little manual editing produced a massive performance improvement.

Here’s a quick demo:

rem
rem     Script:         where_having.sql
rem     Author:         Jonathan Lewis
rem     Dated:          Oct 2018
rem     Purpose:
rem
rem     Last tested
rem             18.3.0.0
rem             12.2.0.1
rem             11.2.0.4
rem

reate table t1
as
select * 
from all_objects 
where rownum <= 50000   -- > comment to avoid WordPress format issue
;

spool where_having.lst

set serveroutput off

select /*+ gather_plan_statistics */ 
        object_type, count(*) 
from    t1 
group by 
        object_type 
having  count(*) > 0 
and     1 = 2
;

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'))
;

The big question is: will Oracle do a full tablescan of t1, or will it apply a “null is not null” filter early to bypass that part of the plan. Here’s the plan pulled from memory, with run-time statistics (all versions from 11g to 18c):

--------------------------------------------------------------------------------------------------------------------------
| Id  | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |  OMem |  1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |      1 |        |      0 |00:00:00.02 |     957 |    955 |       |       |          |
|*  1 |  FILTER             |      |      1 |        |      0 |00:00:00.02 |     957 |    955 |       |       |          |
|   2 |   HASH GROUP BY     |      |      1 |      1 |     27 |00:00:00.02 |     957 |    955 |  1186K|  1186K| 1397K (0)|
|   3 |    TABLE ACCESS FULL| T1   |      1 |  50000 |  50000 |00:00:00.01 |     957 |    955 |       |       |          |
--------------------------------------------------------------------------------------------------------------------------

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

   1 - filter((COUNT(*)>0 AND 1=2))

As you can see, the filter at operation 1 includes the contradiction “1=2”, but Oracle tests this only after doing the full tablescan and aggregation. If you move the “1=2” into the where clause the tablescan doesn’t happen.

Interestingly, if you write the query with an in-line view and trailing where clause:

select /*+ gather_plan_statistics */
        *
from    (
        select
                object_type, count(*)
        from    t1
        group by
                object_type
        having  count(*) > 0
        )
where
        1 = 2
;

The optimizer is clever enough to push the final predicate inside the view (where you might expect it to become part of the having clause) and push it all the way down into a where clause on the base table.

-----------------------------------------------------------------------------
| Id  | Operation            | Name | Starts | E-Rows | A-Rows |   A-Time   |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |      1 |        |      0 |00:00:00.01 |
|*  1 |  FILTER              |      |      1 |        |      0 |00:00:00.01 |
|   2 |   HASH GROUP BY      |      |      1 |      1 |      0 |00:00:00.01 |
|*  3 |    FILTER            |      |      1 |        |      0 |00:00:00.01 |
|   4 |     TABLE ACCESS FULL| T1   |      0 |  50000 |      0 |00:00:00.01 |
-----------------------------------------------------------------------------

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

   1 - filter(COUNT(*)>0)
   3 - filter(NULL IS NOT NULL)

A quirky case of the optimizer handling the (apparently) more complex query than it does the simpler query.