Oracle Scratchpad

January 9, 2015

count(*) – again !

Filed under: bitmaps,humour,Indexing,Oracle,Troubleshooting,Tuning — Jonathan Lewis @ 12:56 pm BST Jan 9,2015

Because you can never have enough of a good thing.

Here’s a thought – The optimizer doesn’t treat all constants equally.  No explanations, just read the code – execution plans at the end:


SQL> drop table t1 purge;
SQL> create table t1 nologging as select * from all_objects;
SQL> create bitmap index t1_b1 on t1(owner);

SQL> alter session set statistics_level = all;

SQL> set serveroutput off
SQL> select count(*) from t1;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

SQL> select count(1) from t1;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

SQL> select count(-1) from t1;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

SQL> alter session set cursor_sharing = force;
SQL> alter system flush shared_pool;

SQL> select count(1) from t1;
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

So, are you expecting to see the same results and performance from every single one of those queries ?


select count(*) from t1
----------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |       |      1 |        |      1 |00:00:00.01 |       9 |      5 |
|   1 |  SORT AGGREGATE               |       |      1 |      1 |      1 |00:00:00.01 |       9 |      5 |
|   2 |   BITMAP CONVERSION COUNT     |       |      1 |  84499 |     31 |00:00:00.01 |       9 |      5 |
|   3 |    BITMAP INDEX FAST FULL SCAN| T1_B1 |      1 |        |     31 |00:00:00.01 |       9 |      5 |
----------------------------------------------------------------------------------------------------------

select count(1) from t1
-------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |       |      1 |        |      1 |00:00:00.01 |       9 |
|   1 |  SORT AGGREGATE               |       |      1 |      1 |      1 |00:00:00.01 |       9 |
|   2 |   BITMAP CONVERSION COUNT     |       |      1 |  84499 |     31 |00:00:00.01 |       9 |
|   3 |    BITMAP INDEX FAST FULL SCAN| T1_B1 |      1 |        |     31 |00:00:00.01 |       9 |
-------------------------------------------------------------------------------------------------

select count(-1) from t1
-------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |       |      1 |        |      1 |00:00:00.43 |       9 |
|   1 |  SORT AGGREGATE               |       |      1 |      1 |      1 |00:00:00.43 |       9 |
|   2 |   BITMAP CONVERSION TO ROWIDS |       |      1 |  84499 |  84499 |00:00:00.22 |       9 |
|   3 |    BITMAP INDEX FAST FULL SCAN| T1_B1 |      1 |        |     31 |00:00:00.01 |       9 |
-------------------------------------------------------------------------------------------------

SQL> alter session set cursor_sharing = force;
SQL> alter system flush shared_pool;

select count(1) from t1
select count(:"SYS_B_0") from t1    -- effect of cursor-sharing
-------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |       |      1 |        |      1 |00:00:00.46 |       9 |
|   1 |  SORT AGGREGATE               |       |      1 |      1 |      1 |00:00:00.46 |       9 |
|   2 |   BITMAP CONVERSION TO ROWIDS |       |      1 |  84499 |  84499 |00:00:00.23 |       9 |
|   3 |    BITMAP INDEX FAST FULL SCAN| T1_B1 |      1 |        |     31 |00:00:00.01 |       9 |
-------------------------------------------------------------------------------------------------

Check operation 2 in each plan – with the bitmap index in place there are two possible ways to count the rows referenced in the index – and one of them converts to rowids and does a lot more work.

The only “real” threat in this set of examples, of course, is the bind variable one – there are times when count(*) WILL be faster than count(1). Having said that, there is a case where a redundant “conversion to rowids” IS a threat – and I’ll write that up some time in the near future.

Trick question: when is 1+1 != 2 ?
Silly answer: compare the plan for: “select count (2) from t1” with the plan for “select count(1+1) from t1”

Note: All tests above run on 12.1.0.2

3 Comments »

  1. Hmmm….

    1+1 != 2 when we want to use indexes.
    Because indexing is verbotten for expressions or functions, unless one creates FBIs for each.

    Butbutbut…
    count(n) is a function, so it should not be using an index:

    https://docs.oracle.com/cd/E11882_01/server.112/e41573/data_acc.htm#PFGRF94785

    But it is: you just proven it above!

    Enter the illogical nature of the CBO rules…

    ;)

    Comment by Noons — January 12, 2015 @ 4:01 am BST Jan 12,2015 | Reply

  2. […] an earlier (not very serious) post about count(*) I pointed out how the optimizer sometimes does a redundant “bitmap conversion […]

    Pingback by Bitmap Counts | Oracle Scratchpad — January 20, 2015 @ 9:06 pm BST Jan 20,2015 | Reply

  3. […] Anomaly 1 […]

    Pingback by Why Bother | Oracle Scratchpad — September 20, 2016 @ 11:17 am BST Sep 20,2016 | Reply


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