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03 May, 2016

Compression -- 6 : Advanced Index Compression

Earlier, I had covered Index (Key) Compression which is included in the Enterprise Edition.

In Key Compression, the DBA must specify the Prefix (i.e. number of leading columns in a composite index) that must be used as the compression key.

12.1.0.2 Advanced Index Compression does not require the DBA to manually identify the prefix key length.  Advanced Index Compression auto(magically) identifies the optimal prefix columns.
(Note : Advanced Index Compression requires the Advanced Compression licence Option and is available only in 12.1.0.2 and higher)

SQL> create table target_data as select * from source_data where 1=2;

Table created.

SQL> insert /*+ APPEND */ into target_data select * from source_data;

367156 rows created.

SQL> commit;

Commit complete.

SQL> insert /*+ APPEND */ into target_data select * from source_data;

367156 rows created.

SQL> commit;

Commit complete.

SQL> insert /*+ APPEND */ into target_data select * from source_data;

367156 rows created.

SQL> commit;

Commit complete.

SQL> 


Creating a Key Compression Index by specifying the Prefix size (the leading 2 columns) :

SQL> create index target_data_ndx_1_comp on
  2  target_data (owner, object_type, object_name)  compress 2;

Index created.

SQL> exec dbms_stats.gather_index_stats('','TARGET_DATA_NDX_1_COMP');

PL/SQL procedure successfully completed.

SQL> select leaf_blocks
  2  from user_indexes
  3  where index_name = 'TARGET_DATA_NDX_1_COMP'
  4  /

LEAF_BLOCKS
-----------
       5508

SQL> 


Note how I specified "2" as the Prefix size as I want to compress on repeated values of (OWNER, OBJECT_NAME).

Using Advanced Index Compression by specifying "COMPRESS ADVANCED LOW" and letting Oracle decide on the compression strategy in each leaf block :

SQL> drop index target_data_ndx_1_comp;

Index dropped.

SQL> create index target_data_ndx_2_advcomp on
  2  target_data (owner, object_type, object_name)
  3  compress advanced low;

Index created.

SQL> exec dbms_stats.gather_index_stats('','TARGET_DATA_NDX_2_ADVCOMP');

PL/SQL procedure successfully completed.

SQL> select leaf_blocks
  2  from user_indexes
  3  where index_name = 'TARGET_DATA_NDX_2_ADVCOMP'
  4  /

LEAF_BLOCKS
-----------
       2277

SQL> 


Wow, that's significantly smaller.  What's more, I did not have to spend time analyzing the data and the index definition to identify the "correct" Prefix size.

However, it is now possible to specify the entire composite key as the Prefix, although that is not what I would have done in earlier versions.  Identifying the Prefix size requires analyzing the data.

SQL> create index target_data_ndx_1_comp on
  2  target_data (owner, object_type, object_name)  compress 3
  3  /

Index created.

SQL> exec dbms_stats.gather_index_stats('','TARGET_DATA_NDX_1_COMP');

PL/SQL procedure successfully completed.

SQL> select leaf_blocks
  2  from user_indexes
  3  where index_name = 'TARGET_DATA_NDX_1_COMP'
  4  /

LEAF_BLOCKS
-----------
       2277

SQL> 


So, the Advanced option allows me to let Oracle automatically decide the appropriate mechanism to compress the Index keys.

UPDATE : Also see the subsequent test with a reversal of the columns in the composite index.

Just for comparison, here is a regular index :

SQL> drop index target_data_ndx_2_advcomp;

Index dropped.

SQL> create index target_data_ndx_3_nocomp on
  2  target_data (owner, object_type, object_name)
  3  /

Index created.

SQL> exec dbms_stats.gather_index_stats('','TARGET_DATA_NDX_3_NOCOMP');

PL/SQL procedure successfully completed.

SQL> select leaf_blocks
  2  from user_indexes
  3  where index_name = 'TARGET_DATA_NDX_3_NOCOMP'
  4  /

LEAF_BLOCKS
-----------
       7289

SQL> 


That is a much larger regular index !
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