prefetching

Prefetching reads the data needed by a query before it is referenced by the query, so that the query does not have to wait for I/O to be performed.

The number of pages that the database manager will prefetch can be defined for each table space using the PREFETCHSIZE clause with either the CREATE TABLESPACE or ALTER TABLESPACE statements. The value specified is maintained in the PREFETCHSIZE column of the SYSCAT.TABLESPACES system catalog table.

It is a good practice to explicitly set the PREFETCHSIZE value as a multiple of the EXTENTSIZE value for your table space and the number of table space containers. (The extent size is the number of pages that the database manager writes to a container before using a different container.) For example, if the extent size is 16 pages and the table space has two containers, you could choose to set the prefetch quantity to 32 pages.

For more information about prefetching, see the Administration Guide .