Pooled and Cluster Tables

概念性问题, 转自SAP Help: http://help.sap.com/saphelp_nw04/helpdata/en/cf/21f083446011d189700000e8322d00/content.htm

Pooled and Cluster Tables

Table pools (pools) and table clusters (clusters) are special table types in the ABAP Dictionary. The data from several different tables can be stored together in a table pool or table cluster. Tables assigned to a table pool or table cluster are referred to as pooled tables or cluster tables.

A table pool or table cluster should be used exclusively for storing internal control information (screen sequences, program parameters, temporary data, continuous texts such as documentation). Data of commercial relevance is usually stored in transparent tables.

Table Pools

A table in the database in which all records from the pooled tables assigned to the table pool are stored corresponds to a table pool.

The definition of a pool consists essentially of two key fields (Tabname and Varkey) and a long argument field (Vardata). A pool has the following structure:

Field	Data type	Meaning
Tabname	CHAR(10)	Name of pooled table
Varkey	CHAR (n)	Contains the entries from all key fields of the pooled table record as a string, max. length for n is 110
Dataln	INT2(5)	Length of the string in Vardata
Vardata	RAW (n)	Contains the entries from all data fields of the pooled table record as a string, max. length n depends on the database system used

If a pooled table record is saved, it is stored in the table pool assigned. The name of the pooled table is written to the field Tabname. The contents of all key fields of the pooled table are written as a string to field Varkey and the contents of all data fields as a string to field Vardata. The length of the string stored in Vardata is entered in field Dataln by the database interface.

Due to the structure of a table pool, there are certain restrictions on the pooled tables assigned to it. The name of a pooled table may not exceed 10 characters. Since Varkey is a character field, all key fields of a pooled table must have character data types (for example, CHAR, NUMC, CLNT). The total length of all key fields or all data fields of a pooled table must not exceed the length of the Varkey or Vardata field of the assigned pool.

Table Clusters

Several logical data records from different cluster tables can be stored together in one physical record in a table cluster.

A cluster key consists of a series of freely definable key fields and a field (Pageno) for distinguishing continuation records. A cluster also contains a long field (Vardata) that contains the contents of the data fields of the cluster tables for this key. If the data does not fit into the long field, continuation records are created. Control information on the structure of the data string is still written at the beginning of theVardata field. A table cluster has the following structure:

Field	Data type	Meaning
CLKEY1	*	First key field
CLKEY2	*	Second key field
...	...	...
CLKEYn	*	nth key field
Pageno	INT2(5)	Number of the continuation record
Timestamp	CHAR(14)	Time stamps
Pagelg	INT2(5)	Length of the string in Vardata
Vardata	RAW (n)	Contains the entries from the data fields of the assigned cluster tables as a string, max. length n depends on the database system used

The records of all cluster tables with the same key are stored under one key in the assigned table cluster. The values of the key fields are stored in the corresponding key fields of the table cluster. The values of all data fields of the assigned cluster tables are written as a string to the Vardata field of the table cluster. Besides the actual data values, the data string contains information on the structure of the data and which table it comes from. If the string exceeds the maximum length of the Vardata field, a continuation record is written with the same key values. The continuation records for a key are distinguished by their value in field Pageno. The actual length of the string in the Vardata field is stored in the Pagelg field by the database interface.

You need the structural information stored in the ABAP Dictionary to read the data from a pooled table or cluster table correctly. These tables can therefore only be processed using Open SQL with the cluster interface, and not with Native SQL directly in the database.

See also:

Creating Table Pools/Table Clusters

Deleting Table Pools/Table Clusters

Creating Pooled Tables/Cluster Tables

Changing Pooled Tables/Cluster Tables