Search My Oracle Blog

Custom Search

21 February, 2016

Compression -- 1 : BASIC Table Compression

Now, starting a new series of blog posts on Compression.

Beginning with  BASIC Table Compression.

Basic Table Compression was introduced in 9i Release 2 (9.2.0).  It is free with the Enterprise Edition.  Basic Compression works only with Direct Path (Bulk Load) INSERTs.  It does not apply to "normal" INSERT and UPDATE operations.
Basic Compression is actually "de-duplication".  It identifies values that are repeated within the same database block and replaces them with tokens mapped to the list of values.   Note the section that I have underlined.  De-duplication does not span database blocks.

Here is a first demo of Basic Compression using INSERT /*+ APPEND */  (for Direct Path Insert).

I start with a table that is a multiplied copy of DBA_OBJECTS, named as SOURCE_DATA.

PDB1@ORCL> show user
USER is "HEMANT"
PDB1@ORCL> select count(*) from source_data;

  COUNT(*)
----------
    364496

PDB1@ORCL> 
PDB1@ORCL> select tablespace_name, segment_type, bytes/1048576
  2  from user_segments
  3  where segment_name = 'SOURCE_DATA';

TABLESPACE_NAME         SEGMENT_TYPE   BYTES/1048576
------------------------------ ------------------ -------------
HEMANT          TABLE        49

PDB1@ORCL> 


I then create table to hold compressed data.  Note (let me repeat again : Basic Compression works only with DIRECT PATH INSERT).

PDB1@ORCL> create table compressed_1 tablespace hemant compress as select * from source_data where 1=2;

Table created.

PDB1@ORCL> insert /*+ APPEND */ into compressed_1
  2  select * from source_data;

364496 rows created.

PDB1@ORCL> commit;

Commit complete.

PDB1@ORCL>
PDB1@ORCL> col segment_name format a30
PDB1@ORCL> select segment_name, segment_type, bytes/1048576
  2  from user_Segments
  3  where tablespace_name = 'HEMANT';

SEGMENT_NAME         SEGMENT_TYPE   BYTES/1048576
------------------------------ ------------------ -------------
COMPRESSED_1         TABLE        12
SOURCE_DATA         TABLE        49

PDB1@ORCL> 


So, a 49MB table is compressed down to 12MB.  What if I UPDATE these rows ?

PDB1@ORCL> update compressed_1
  2  set owner = owner;

364496 rows updated.

PDB1@ORCL> commit;

Commit complete.

PDB1@ORCL> select segment_name, segment_type, bytes/1048576
  2  from user_segments
  3  where tablespace_name = 'HEMANT';

SEGMENT_NAME         SEGMENT_TYPE   BYTES/1048576
------------------------------ ------------------ -------------
COMPRESSED_1         TABLE        22
SOURCE_DATA         TABLE        49

PDB1@ORCL> 


Updating only a single column without changing the length of the data in that column has increased the size of the table.

Let me continue the UPDATE experiment further, without increasing the length of data in any columns.

PDB1@ORCL> update compressed_1
  2  set owner = owner, object_name = object_name, subobject_name = subobject_name, object_id = object_id,
  3  data_object_id = data_object_id, object_type = object_type, created = created
  4  /  

364496 rows updated.

PDB1@ORCL> commit;

Commit complete.

PDB1@ORCL> select segment_name, segment_type, bytes/1048576
  2  from user_segments
  3  where tablespace_name = 'HEMANT';

SEGMENT_NAME         SEGMENT_TYPE   BYTES/1048576
------------------------------ ------------------ -------------
COMPRESSED_1         TABLE        22
SOURCE_DATA         TABLE        49

PDB1@ORCL> 


No further degradation ? (unless my UPDATE expands the size of data in one or more column(s)).

Note a difference between the two tables :

PDB1@ORCL> select table_name, compression, compress_for, pct_free
  2  from user_tables;

TABLE_NAME         COMPRESS COMPRESS_FOR    PCT_FREE
------------------------------ -------- ------------------------------ ----------
COMPRESSED_1         ENABLED BASIC     0
SOURCE_DATA         DISABLED            10

PDB1@ORCL> 


The Compressed table is created with PCT_FREE=10.  PCT_FREE=0  (Note that this can also have an impact when you issue UPDATEs subsequently, UPDATEs that increase the size of data in one or more columns).

UPDATE : In 12c the syntax for BASIC Compression is "ROW STORE COMPRESS BASIC". Simply specifying "COMPRESS" or "COMPRESS BASIC" is supported for backward compatibility.

.
.
.

2 comments:

Filipe Martins said...

hello, a very usual side effect when you have several threads doing updates on compressed tables is the occurrence of dead locks, because of itl contention. Normally you solved this itl problems increasing the initrans parameter of the table , but with a compressed table the solution is uncompress the table.

vau said...

Typo:
The Compressed table is created with PCT_FREE=0

Aggregated by orafaq.com

Aggregated by orafaq.com
This blog is being aggregated by orafaq.com

Top 50 Oracle SQL Blogs 2016

Top 50 Oracle SQL Blogs 2016
Top 50 Oracle SQL Blogs 2016