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Storage Indexes vs Database Indexes Part I MIN/MAX (Maxwell’s Silver Hammer) December 18, 2012

Posted by Richard Foote in Exadata, Oracle Indexes, Storage Indexes.
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It’s often stated that in Exadata, you don’t need conventional database indexes anymore as everything runs so damn fast that indexes are simply a waste of time and space. Simply drop all database indexes and things will run just as fast.

Well, not quite …

There are many many scenarios where database indexes are still critical for optimal performance and scalability in an Exadata environment.

Prompted by a question in an earlier post (thanks Paul), I thought I might start looking at some scenarios where dropping a database index in Exadata would not be the best of ideas.

The first example is the scenario where one wants either the minimum/maximum of a column value. As database index entries are always ordered, this can very efficiently be found by traversing down to either the first or last index leaf block within an appropriate index. There’s an execution the INDEX FULL SCAN MIN/MAX execution path by which the CBO can decide to access an index in this manner.

For example, if I create an index on the ID column of my large DWH_BOWIE table and select the MIN(ID):

SQL> create index dwh_bowie_id_i on dwh_bowie(id);
Index created.

SQL> select min(id) from dwh_bowie;

MIN(ID)
 ----------
 1

Elapsed: 00:00:00.01

Execution Plan
 ---------------------------------------------------------------------------------------------
 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
 ---------------------------------------------------------------------------------------------
 | 0 | SELECT STATEMENT | | 1 | 6 | 4 (0)| 00:00:01 |
 | 1 | SORT AGGREGATE | | 1 | 6 | | |
 | 2 | INDEX FULL SCAN (MIN/MAX)| DWH_BOWIE_ID_I | 1 | 6 | 4 (0)| 00:00:01 |
 ---------------------------------------------------------------------------------------------

Statistics
 ----------------------------------------------------------
 1 recursive calls
 0 db block gets
 4 consistent gets
 3 physical reads
 0 redo size
 525 bytes sent via SQL*Net to client
 524 bytes received via SQL*Net from client
 2 SQL*Net roundtrips to/from client
 0 sorts (memory)
 0 sorts (disk)
 1 rows processed

With only a handful of consistent gets, we have managed to get the minimum ID value extremely quickly and efficiently.

However, if we drop this index and re-run the same query in Exadata numerous times:

SQL> select min(id) from dwh_bowie;
MIN(ID)
 ----------
 1

Elapsed: 00:00:41.31

Execution Plan
 ----------------------------------------------------------------------------------------
 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
 ----------------------------------------------------------------------------------------
 | 0 | SELECT STATEMENT | | 1 | 6 | 2345K (1)| 07:49:12 |
 | 1 | SORT AGGREGATE | | 1 | 6 | | |
 | 2 | TABLE ACCESS STORAGE FULL| DWH_BOWIE | 640M| 3662M| 2345K (1)| 07:49:12 |
 ----------------------------------------------------------------------------------------

Statist5ics
 ----------------------------------------------------------
 0 recursive calls
 0 db block gets
 8626963 consistent gets
 8625479 physical reads
 0 redo size
 525 bytes sent via SQL*Net to client
 524 bytes received via SQL*Net from client
 2 SQL*Net roundtrips to/from client
 0 sorts (memory)
 0 sorts (disk)
 1 rows processed

NAME MB
 ---------------------------------------------------------------- ----------
 cell physical IO bytes saved by storage index 0
 cell physical IO interconnect bytes returned by smart scan 7644.38194

We notice the query takes considerably longer and that the Storage Indexes have been unable to be of any help with not a single byte saved.

This is because all aggregation type operations are performed at the database level, not within the storage servers. Even though in theory Storage Indexes could be seen as perhaps being of value here, they are totally ignored by Oracle when retrieving either the minimum/maximum of a column value. Remember also that a key difference between a Storage Index and a Database Index is that a Storage Index does not necessarily have to exist fully for all regions of a given table.

Drop database indexes used in this manner at your peril …

Many more examples to come :)

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