IBM i Series, Руководство пользователя

Страница: 492 / 908

Operating System/400 (5722-SS1)
462
iSeries Handbook
• Explain function
Examines and reports the access method used by individual SQL queries. The output can
be examined to determine whether the access method generated for the query could be
improved by query and/or database changes.
Multiple isolation levels. Four record locking options are available to fine tune application
performance:
– Read Stability (*ALL): Locks all rows read during transaction and maintains locks until
the transaction ends.
– Repeatable Read (*RR): Locks all rows read during remote unit of work and maintains
locks until the unit of work ends.
– Cursor Stability (*CS): Maintains locks on the current and previously changed rows
only, which can offer increased performance for multiple applications contending for
the same rows.
– Uncommitted Read (*CHG): Offers higher performance by not acquiring or examining
database locks for read-only operations.
– No Commit (*NONE): Provides the highest performance of the four isolation levels, by
not examining locks and disabling transaction management: the ability to either
COMMIT or ROLLBACK a transaction.
• Block INSERT and FETCH
Provides applications with the ability to store and retrieve arrays of data directly, instead
of one row at a time.
• Automatic record blocking
Improves client/server performance by returning rows to the client in blocks rather than
individually. Subsequent record access of the current block can then be performed locally
at the client without accessing the server. This ability is now provided for all isolation
levels.
• Parallel data access
Queries returning or requiring DB2 UDB for iSeries to process large amounts of data
require significant I/O activity. Due to the iSeries' single-level store architecture, this data
is often spread across many physical devices. The parallel data access feature now
allows multiple internal DB2 UDB for iSeries tasks to be activated for each physical
device, allowing DB2 UDB for iSeries to transfer data from disk to memory faster than
with the previous single task I/O architecture.
• Query Governor
Long-running queries can have negative performance effects for other users of a
database. The governor facility allows a time limit to be set for a query, so as to avoid a
single query from consuming unusually large amounts of resources, negatively affecting