在oracle 9i版本中，表的压缩技术就已经引入.
oracle可以做到，直接读取压缩的数据，不用解压再读取，大大提高了读写效率.
压缩后，存储空间减少，占用的块也会减少，读取时I/O必然也会减少，自然而然地性能就提高了.关键是在读取的时候不需要解压，即可进行读取，如此更加大大提高读取的速率.
Table Compression:Overview
Reducing storage costs by compressing all data:
- Basic compression for direct-path insert operations:10x
- OLTP compression for all DML operations:2-4x
{COMPRESS [ BASIC | FOR { OLTP } ] | NOCOMPRESS}
| Compression Method | Compression Ratio | CPU Overhead | CREATE and ALTER TABLE Syntax | Typical Applications |
| Basic table compression | High | Minimal | COMPRESS [BASIC] | DSS |
| OLTP table compression | High | Minimal | COMPRESS FOR OLTP | OLTP,DSS |
Compression for Direct-Path Insert Operations
- Is enabled with CREATE TABLE ...COMPRESS BASIC ...;
- Is recommended for bulk loading data warehouses
- Replaces deprecated COMPRESS FOR DIRECT_LOAD_OPERATIONS
- Maximizes contiguous free space in blocks
压缩原理：用户在执行DML语句的时候，不是实时压缩的，而是当数据块中快满的时候，会触发一个压缩功能的触发器将现有数据进行压缩，压缩完之后会继续进行DML操作，直至块满；
OLTP Compression for DML Operations
- Is enabled with CREATE TABLE ... COMPRESS FOR OLTP ...;
- Is recommended for active OLTP enviroments
- Resplaces deprecated COMPRESS FOR ALL OPERATIONS
Using the Compression Advisor
The compression advisor:
- Analyzes objects to given an estimate of space savings for different compression methods
- Helps in deciding the correct compression level for an application.
- Recommends various strategies for compression
- Picks the right compression algorithm for a particular data set.
- Sorts on a particular column for increasing the compression ratio.
- Presents tradeoffs between different compression algorithms
- Works for OLTP compression(via EM);
Using the DBMS_COMPRESSION Package
To determine optimal compression ratios:
