36 DBMS_COMPRESSION

The DBMS_COMPRESSION package provides an interface to facilitate choosing the correct compression level for an application.

Using DBMS_COMPRESSION

Overview
Security Model
Constants

Overview

The DBMS_COMPRESSION package gathers compression-related information within a database environment. This includes tools for estimating compressibility of a table for both partitioned and non-partitioned tables, and gathering row-level compression information on previously compressed tables. This gives the user with adequate information to make compression-related decision.

Security Model

The DBMS_COMPRESSSION package is defined with AUTHID CURRENT USER, so it executes with the privileges of the current user.

Constants

The DBMS_COMPRESSION package uses the constants shown in Table 36-1, "DBMS_COMPRESSION Constants - Compression Types"e:

Table 36-1 DBMS_COMPRESSION Constants - Compression Types

Constant	Type	Value	Description
`COMP_NOCOMPRESS`	`NUMBER`	`1`	No compression
`COMP_ADVANCED`	`NUMBER`	`2`	Advanced compression level
`COMP_QUERY_HIGH`	`NUMBER`	`4`	High compression level for query operations
`COMP_QUERY_LOW`	`NUMBER`	`8`	Low compression level for query operations
`COMP_ARCHIVE_HIGH`	`NUMBER`	`16`	High compression level for archive operations
`COMP_ARCHIVE_LOW`	`NUMBER`	`32`	Low compression level for archive operations
`COMP_BLOCK`	`NUMBER`	`64`	Compressed row
`COMP_LOB_HIGH`	`NUMBER`	`128`	High compression level for LOB operations
`COMP_LOB_MEDIUM`	`NUMBER`	`256`	Medium compression level for LOB operations
`COMP_LOB_LOW`	`NUMBER`	`512`	Low compression level for LOB operations
`COMP_INDEX_ADVANCED_HIGH`	`NUMBER`	`1024`	High compression level for indexes
`COMP_INDEX_ADVANCED_LOW`	`NUMBER`	`2048`	Low compression level for indexes
`COMP_RATIO_LOB_MINROWS`	`NUMBER`	`1000`	Minimum required number of LOBs in the object for which LOB compression ratio is to be estimated
`COMP_BASIC`	`NUMBER`	`4096`	Basic compression level
`COMP_RATIO_LOB_MAXROWS`	`NUMBER`	`5000`	Maximum number of LOBs used to compute the LOB compression ratio
`COMP_INMEMORY_NOCOMPRESS`	`NUMBER`	`8192`	In-Memory with no compression
`COMP_INMEMORY_DML`	`NUMBER`	`16384`	In-Memory compression level for DML
`COMP_INMEMORY_QUERY_LOW`	`NUMBER`	`32768`	In-Memory compression level optimized for query performance
`COMP_INMEMORY_QUERY_HIGH`	`NUMBER`	`65536`	In-Memory compression level optimized on query performance as well as space saving
`COMP_INMEMORY_CAPACITY_LOW`	`NUMBER`	`32768`	In-Memory low compression level optimizing for capacity
`COMP_INMEMORY_CAPACITY_HIGH`	`NUMBER`	`65536`	In-Memory high compression level optimizing for capacity
`COMP_RATIO_MINROWS`	`NUMBER`	`1000000`	Minimum required number of rows in the object for which HCC ratio is to be estimated
`COMP_RATIO_ALLROWS`	`NUMBER`	`-1`	To indicate the use of all the rows in the object to estimate HCC ratio
`OBJTYPE_TABLE`	`PLS_INTEGER`	`1`	Identifies the object whose compression ratio is estimated as of type table
`OBJTYPE_INDEX`	`PLS_INTEGER`	2	Identifies the object whose compression ratio is estimated as of type index

Note:

Hybrid columnar compression is a feature of certain Oracle storage systems. See Oracle Database Concepts for more information.

Data Structures

The DBMS_COMPRESSION package defines a RECORD type and a TABLE type.

RECORD TYPES

COMPREC Record Type

TABLE TYPES

COMPRECLIST Table Type

COMPREC Record Type

Record for calculating an individual index compression ratio on a table.

Syntax

TYPE COMPREC IS RECORD(
  ownname           varchar2(255),
  objname           varchar2(255),
  blkcnt_cmp        PLS_INTEGER,
  blkcnt_uncmp      PLS_INTEGER,
  row_cmp           PLS_INTEGER,
  row_uncmp         PLS_INTEGER,
  cmp_ratio         NUMBER,
  objtype           PLS_INTEGER);

Fields

Table 36-2 COMPREC Attributes

Field	Description
`ownname`	Schema of the object owner
`objname`	Name of the object
`blkcnt_cmp`	Number of blocks used by the compressed sample of the object
`blkcnt_uncmp`	Number of blocks used by the uncompressed sample of the object
`row_cmp`	Number of rows in a block in compressed sample of the object
`row_uncmp`	Number of rows in a block in uncompressed sample of the object
`cmp_ratio`	Compression ratio, `blkcnt_uncmp` divided by `blkcnt_cmp`
`objtype`	Type of the object

COMPRECLIST Table Type

A table type of COMPREC Record Type.

Syntax

TYPE compreclist IS TABLE OF comprec;

Summary of DBMS_COMPRESSION Subprograms

Table 36-3 DBMS_COMPRESSION Package Subprograms

Subprogram	Description
GET_COMPRESSION_RATIO Procedure	Analyzes the compression ratio of a table, and gives information about compressibility of a table
GET_COMPRESSION_TYPE Function	Returns the compression type for a specified row

GET_COMPRESSION_RATIO Procedure

This procedure analyzes the compression ratio of a table, and gives information about compressibility of a table. Various parameters can be provided by the user to selectively analyze different compression types.

Syntax

Get compression ratio for an object (table or index, default is table):

DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
   scratchtbsname        IN     VARCHAR2, 
   ownname               IN     VARCHAR2, 
   objname               IN     VARCHAR2,
   subobjname            IN     VARCHAR2,
   comptype              IN     NUMBER,
   blkcnt_cmp            OUT    PLS_INTEGER,
   blkcnt_uncmp          OUT    PLS_INTEGER,
   row_cmp               OUT    PLS_INTEGER,
   row_uncmp             OUT    PLS_INTEGER,
   cmp_ratio             OUT    NUMBER,
   comptype_str          OUT    VARCHAR2,
   subset_numrows        IN     NUMBER  DEFAULT COMP_RATIO_MINROWS,
   objtype               IN     PLS_INTEGER DEFAULT OBJTYPE_TABLE);

Get compression ratio for LOBs:

DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
   scratchtbsname        IN     VARCHAR2,
   tabowner              IN     VARCHAR2,
   tabname               IN     VARCHAR2,
   lobname               IN     VARCHAR2,
   partname              IN     VARCHAR2,
   comptype              IN     NUMBER,
   blkcnt_cmp            OUT    PLS_INTEGER,
   blkcnt_uncmp          OUT    PLS_INTEGER,
   lobcnt                OUT    PLS_INTEGER,
   cmp_ratio             OUT    NUMBER,
   comptype_str          OUT    VARCHAR2,
   subset_numrows        IN     number DEFAULT COMP_RATIO_LOB_MAXROWS);

Get compression ratio for all indexes on a table. The compression ratios are returned as a collection.

DBMS_COMPRESSION.GET_COMPRESSION_RATIO (
  scratchtbsname        IN     VARCHAR2,
  ownname               IN     VARCHAR2,
  tabname               IN     VARCHAR2,
  comptype              IN     NUMBER,
  index_cr              OUT    compRecList,
  comptype_str          OUT    VARCHAR2,
  subset_numrows        IN     NUMBER DEFAULT COMP_RATIO_INDEX_MINROWS);

Parameters

Table 36-4 GET_COMPRESSION_RATIO Procedure Parameters

Parameter	Description
`scratchtbsname`	Temporary scratch tablespace that can be used for analysis
`ownname` / `tabowner`	Schema of the table to analyze
`tabname`	Name of the table to analyze
`objname`	Name of the object
`subobjname`	Name of the partition or sub-partition of the object
`comptype`	Compression types for which analysis should be performed
`blkcnt_cmp`	Number of blocks used by compressed sample of the table
`blkcnt_uncmp`	Number of blocks used by uncompressed sample of the table
`row_cmp`	Number of rows in a block in compressed sample of the table
`row_uncmp`	Number of rows in a block in uncompressed sample of the table
`cmp_ratio`	Compression ratio, `blkcnt_uncmp` divided by `blkcnt_cmp`
`comptype_str`	String describing the compression type
`subset_numrows`	Number of rows sampled to estimate compression ratio.
`objtype`	Type of the object. Should be a constant to indicate object types defined in this package.
`lobname`	Name of the LOB column
`partname`	In case of partitioned tables, the related partition name
`lobcnt`	Number of lobs actually sampled to estimate compression ratio
`index_cr`	List of indexes and their estimated compression ratios

Usage Notes

The procedure creates different tables in the scratch tablespace and runs analysis on these objects. It does not modify anything in the user-specified tables.

GET_COMPRESSION_TYPE Function

This function returns the compression type for a specified row. If the row is chained, the function returns the compression type of the head piece only, and does not examine the intermediate or the tail piece since head pieces can be differently compressed.

Syntax

DBMS_COMPRESSION.GET_COMPRESSION_TYPE (
   ownname      IN    VARCHAR2, 
   tabname      IN    VARCHAR2, 
   row_id       IN    ROWID,
   subobjname   IN    VARCHAR2 DEFAULT NULL))
  RETURN NUMBER;

Parameters

Table 36-5 GET_COMPRESSION_TYPE Function Parameters

Parameter	Description
`ownname`	Schema name of the table
`tabname`	Name of table
`rowid`	Rowid of the row
`subobjname`	Name of the table partition or subpartition

Return Values

Flag to indicate the compression type (see DBMS_COMPRESSION Constants - Compression Types).