MDC introduced in DB2 V8

This article will introduce the MDC table introduced in DB2 V8. In the MDC table, block ing block map will track all the extended data blocks belonging to this table, it also indicates which blocks or extended data blocks contain data and which blocks or extended data blocks do not contain data.

MDC is introduced in DB2 V8, which physically aggregates rows with similar values on multiple dimensions on a disk. This aggregation can provide efficient I/O for common analytical queries. For example, for all the rows with Product = car, Region = East, and SaleMonthYear = Jan09, they can be stored in the same storage location, that is, the so-called block ). The size of a block is equal to the extended data block extent of the tablespace. The extended data block is a set of consecutive pages on the disk, therefore, rows with similar values are physically stored on consecutive data pages. For more information about MDC, see the "multi-dimensional cluster Table" in the DB2 9.7 Information Center ".

In the MDC table, block ing block map will track all the extended data blocks in the table, it also indicates which blocks or extended data blocks contain data and which blocks or extended data blocks do not contain data. The block containing data is marked as "in use "). When a block entry is deleted or transferred out, it is released for reuse in the MDC table instead of being marked as "in use. However, other objects in the tablespace cannot use these extended data blocks.

You can reorganize the MDC table to release the available data expansion blocks from the MDC table. Before DB2 9.7, data can only be reorganized completely offline. in DB2 9.7, data blocks of extended data can be released by reorganizing MDC online. The reclaim extents only option is added to the reorg table command in DB2 9.7. You can use this option to release extended data blocks dedicated to the mdc table and make the space available for other database objects in the tablespace. This option also allows you to control concurrent access to MDC tables during the release of extended data blocks. The write access permission is the default access permission, you can also select read and no access permissions to control parallel access.

In addition to using the reorg table command to release extended data blocks, you can also use the db2Reorg API to recycle extended data blocks.

In addition, releasing extended data blocks from the MDC table only supports the DMS tablespace.