Prefetch facility

Table data on disk normally is read from a global buffer (or a local buffer) one page at a time. However, this data can also be read in batches of multiple pages, rather than one page at a time. This capability is provided by the prefetch facility. By using the prefetch facility, you can reduce the number of I/O operations between the table data on disk and the global buffer (or local buffer).

Organization of this subsection: (1) Application criteria for the prefetch facility; (2) Specifying the prefetch facility

(1) Application criteria for the prefetch facility

The prefetch facility is available when the following conditions are satisfied:

The accessed data is in a HiRDB file system area for which the raw I/O facility is used.
A large data set is being manipulated.
One of the following SQL statements is being executed:

A SELECT, UPDATE, or DELETE statement that does not use an index
A SELECT, UPDATE, or DELETE statement that performs an ascending search^* using an index or cluster key (except for = and IN conditions)

^* For multi-column indexes, searching is performed in the order specified by the index definitions.

(2) Specifying the prefetch facility

For a global buffer: The prefetch facility is enabled by specifying a value of at least 1 for the -m option of the pdbuffer operand. The specification in the -p option of the pdbuffer operand is of the number of pages to be fetched at a time. For details about the pdbuffer operand, see the HiRDB Version 8 System Definition.

For a local buffer: The prefetch facility is enabled by specifying the number of pages to be fetched at a time in the -p option of the pdlbuffer operand. For details about the pdbuffer operand, see the HiRDB Version 8 System Definition.

6.8.2 Prefetch facility

(1) Application criteria for the prefetch facility

(2) Specifying the prefetch facility