Clone tables allow for two exact structural duplicate instances of a specific table to exist in a database. One instance will be the “active” instance, and the second instance will be the clone. They will be structurally identical in almost every way (columns, indexes, check constraints, tablespace, etc.), with the exception that the clone instance of the table will have a unique name. The only significant difference would be in the data they contain.
The intent of clone tables is to allow for a fast replacement of the data loaded in a table. The active instance of the table will contain the active data, while the clone table will initially be empty when it is first created. The clone can then be populated with data using normal DB2 processing, such as SQL INSERT statements or a DB2 LOAD utility. This allows for a new copy of the data to be staged in the clone instance of the table without impacting the active instance of the table.
When it is time to replace the active instance of the table with the new data that resides in the clone instance of the table, the EXCHANGE DATA statement will be issued. This will cause the active and clone instances of the table to be swapped such that the active table now contains the new data and the clone table now contains the old data. Any references to the active table now will have access to the new data. The data in the clone instance of the table can now be prepared for the next refresh.
The main advantage of clone tables is that they allow for very quick refreshes of data with nearly zero outage time for the table in question. The EXCHANGE DATA statement just requires a momentary outage in order to perform the physical swap of the underlying VSAM datasets that make up the active table tablespace and associated indexes.
Prior to the availability of clone tables, the most practical way to do a full refresh of data in a table was via a LOAD REPLACE utility operation. Depending on the amount of data being loaded, this could result in an outage of anywhere from seconds to minutes to hours. The utilization of clone table greatly enhances the availability of DB2 tables that need to be periodically refreshed with new data.
