Improved Table Compression in 11g

Oracle 11g extends the use of compressed tables to be used for all DML operations.  Previous releases of Oracle only supported compression for direct-path inserts.  Using compression saves disk space, reduces memory use in the buffer cache and increases the speed of read operations.  Compressed tables typically consume two to three times less disk space than uncompressed copies of the same data. The concept of data compression fits well with new technology trends such as larger and faster memory and the use of solid-state disks.  While data compression has many benefits, note that it does incur a CPU overhead when writing data. 

Oracle 11g uses a table compression algorithm that minimizes the performance overhead of using compression for OLTP tables.  The algorithm works by compressing a block in batch mode instead of each time a write operation takes place.  A newly initialized block becomes compressed only after reaching an internally controlled threshold, at which time all data in the block is compressed.  For an OLTP table, this guarantees a high level of compression and limits the performance overhead to only transactions that trigger the block compression.  The frequency of triggering block compression will be affected by the block size and row length.  In most cases, the overwhelming majority of transactions will have the same performance when writing to compressed blocks as uncompressed blocks.  Once the blocks have been compressed, Oracle can read the compressed blocks directly without requiring the block to first be uncompressed. 

This new enhancement for table compression allows data compression to be utilized in OLTP environments.  By default, enabling compression by using ?compress? in a table's DDL will be the same as specifying compress for direct_load operations.  The syntax to enable compression for all DML operations is ?compress for all operations? in the compression portion of the table's DDL:

 create table
   table_name ( ... )
compress
   for all operations;

To enable compression for all DML on an uncompressed table, the alter table syntax is:

alter table
   table_name
compress
   for all operations;

To disable compression for all DML operations and only use it for direct-path inserts, the syntax is:

alter table
   table_name
compress
   for direct_load operations;

To completely disable compression from a table, the syntax is:

alter table
   table_name
nocompress;

For partitioned tables, compression can be controlled at the partition level. This allows the same table to have partitions that are compressed differently.  Defining compression at the partition level overrides the compression settings at the table level, allowing for a finer level of control for compressing large tables based on specific business requirements.