Index Competition in #Oracle 12c

Uwe Hesse's picture

Suppose you want to find out which type of index is best for performance with your workload. Why not set up a competition and let the optimizer decide? The playground:

ADAM@pdb1 > select max(amount_sold) from sales where channel_id=9;

MAX(AMOUNT_SOLD)
----------------
            5000

ADAM@pdb1 > @lastplan

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------
SQL_ID  3hrvrf1r6kn8s, child number 0
-------------------------------------
select max(amount_sold) from sales where channel_id=9

Plan hash value: 3593230073

----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |       |       |       |     4 (100)|          |
|   1 |  SORT AGGREGATE                      |       |     1 |     6 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| SALES |     1 |     6 |     4   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN                  | BSTAR |     1 |       |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   3 - access("CHANNEL_ID"=9)


20 rows selected.

There is a standard B*tree index on the column CHANNEL_ID that speeds up the SELECT above. I think a bitmap index would be better:

ADAM@pdb1 > create bitmap index bmap on sales(channel_id) invisible nologging;

Index created.

ADAM@pdb1 > alter index bstar invisible;

Index altered.

ADAM@pdb1 > alter index bmap visible;

Index altered.

ADAM@pdb1 > select max(amount_sold) from sales where channel_id=9;

MAX(AMOUNT_SOLD)
----------------
            5000

ADAM@pdb1 > @lastplan

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
select max(amount_sold) from sales where channel_id=9

Plan hash value: 2178022915

----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |       |       |       |     3 (100)|          |
|   1 |  SORT AGGREGATE                      |       |     1 |     6 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| SALES |     1 |     6 |     3   (0)| 00:00:01 |
|   3 |    BITMAP CONVERSION TO ROWIDS       |       |       |       |            |          |
|*  4 |     BITMAP INDEX SINGLE VALUE        | BMAP  |       |       |            |          |
----------------------------------------------------------------------------------------------

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

   4 - access("CHANNEL_ID"=9)


21 rows selected.

With this 12c New Feature (two indexes on the same column), I got a smooth transition to the new index type. But this left no choice to the optimizer. What about this?

ADAM@pdb1 > alter index bmap invisible;

Index altered.

ADAM@pdb1 > alter session set optimizer_use_invisible_indexes=true;

Now both indexes are invisible and the optimizer may choose any of them. Turns out that it likes the bitmap index better here. Instead of watching the execution plans, V$SEGMENT_STATISTICS can also be used to find out:

ADAM@pdb1 > select object_name,statistic_name,value
            from v$segment_statistics
            where object_name in ('BSTAR','BMAP')
            and statistic_name in ('physical reads','logical reads');

OBJECT STATISTIC_NAME                      VALUE
------ ------------------------------ ----------
BSTAR  logical reads                       22800
BSTAR  physical reads                       6212
BMAP   logical reads                        1696
BMAP   physical reads                          0

The numbers of BSTAR remain static while BMAP numbers increase. You may also monitor that with DBA_HIST_SEG_STAT across AWR snapshots. Now isn’t that cool?:-)
Couple of things to be aware of here:
Watch out for more than just physical/logical reads – bitmap indexes may cause a locking problem in an OLTP environment.
Don’t keep the two indexes invisible forever – after you saw which one performs better, drop the other one. Invisible indexes need to be maintained upon DML and therefore slow it down.

Tagged: 12c New Features, Performance Tuning

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