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12.2 Index Advanced Compression “High” Part III (I Can’t Give Everything Away) January 25, 2017

Posted by Richard Foote in 12c Rel 2, 12c Release 2 New Features, Advanced Index Compression, Oracle Indexes.
1 comment so far

If you like the look of the new Index Advanced Compression setting of “High” available with Oracle Database 12c Release 2 as I’ve discussed previously in Part I and Part II, well there’s a way to make this the default index compression method in your 12.2 database.

Let’s begin by creating a table and explicitly creating a NOCOMPRESS index in the BOWIE tablespace:


SQL> create table bowie (id number, code number, name varchar2(42));

Table created.

SQL> insert into bowie select rownum, mod(rownum,10), 'DAVID BOWIE' from dual connect by level <= 1000000;

1000000 rows created.

SQL> commit;

Commit complete.

SQL> create index bowie_code_idx on bowie(code) tablespace bowie nocompress;

Index created.

If we look at the size and compression type of this index:


SQL> select index_name, tablespace_name, leaf_blocks, compression
from dba_indexes where index_name='BOWIE_CODE_IDX';

INDEX_NAME      TABLESPACE_NAME      LEAF_BLOCKS COMPRESSION
--------------- -------------------- ----------- -------------
BOWIE_CODE_IDX  BOWIE                       1939 DISABLED

We notice the index has 1939 leaf blocks and that index compression is indeed disabled as expected.

Let’s now drop the index and recreate again it in the BOWIE tablespace, but this time without explicitly stating any compression option:


SQL> drop index bowie_code_idx;

Index dropped.

SQL> create index bowie_code_idx on bowie(code) tablespace bowie;

Index created.

SQL> exec dbms_stats.gather_index_stats(ownname=>null, indname=>'BOWIE_CODE_IDX');

PL/SQL procedure successfully completed.

If we look at the index now:


SQL> select index_name, tablespace_name, leaf_blocks, compression from dba_indexes
where index_name='BOWIE_CODE_IDX';

INDEX_NAME      TABLESPACE_NAME      LEAF_BLOCKS COMPRESSION
--------------- -------------------- ----------- -------------
BOWIE_CODE_IDX  BOWIE                        355 ADVANCED HIGH

We notice the index now only has 355 leaf blocks (down from 1939 leaf blocks) and that it has automatically used the new index advanced compression option of “HIGH”.

The secret lies with the following new settings.

Firstly, with the new db_index_compression_inheritance parameter, you can specify how during index creation the index inherits its index compression attributes (tablespace or table or not at all):


SQL> alter system set db_index_compression_inheritance=tablespace scope=both;

System altered.

Secondly, you can give a tablespace an index compression attribute on how indexes are compressed by default within the tablespace:


SQL> alter tablespace bowie default index compress advanced high;

Tablespace altered.

SQL> select tablespace_name, def_index_compression, index_compress_for
from dba_tablespaces where tablespace_name = 'BOWIE';

TABLESPACE_NAME      DEF_INDE INDEX_COMPRES
-------------------- -------- -------------
BOWIE                ENABLED  ADVANCED HIGH

So in this database, all indexes created within the BOWIE tablespace are automatically created with index advanced compression set to HIGH.

There are however some disadvantages with high index advanced compression that need to be considered.

More to come.

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