Read e-book online DB2 developer's guide PDF

By Craig Mullins

ISBN-10: 0768656885

ISBN-13: 9780768656886

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Introduction to the Optimizer The DB2 optimizer is integral to the operation of SQL statements. The optimizer, as its name implies, determines the optimal method of satisfying a SQL request. DEPT WHERE DEPTNO = WORKDEPT; This statement, whether embedded statically in an application program or executed dynamically, must be passed through the DB2 optimizer before execution. The optimizer parses the statement and determines the following: ƒ Which tables must be accessed ƒ Whether or not the tables are in partitioned tablespaces ƒ Which columns from those tables need to be returned ƒ Which columns participate in the SQL statement's predicates ƒ Whether or not there are any indexes for this combination of tables and columns ƒ What statistics are available in the DB2 Catalog Based on this information (and system information), the optimizer analyzes the possible access paths and chooses the best one for the given query.

Plus, to make matters worse, a new version or release of DB2 can cause completely different results. The release level of DB2 is not the only factor that can cause performance problems. Changes to the OS/390 operating system, the DB2 database environment, the application code, or the application database can cause performance fluctuations. The following is a sample list of system changes that can affect DB2 query performance: - 43 - ƒ Enterprise-wide changes Distributing data Moving data from site to site Replicating and propagating data Downsizing, upsizing, and rightsizing Integrating legacy applications to the web Changing to a new hardware environment ƒ MVS system-level changes Modifying DB2 dispatching priorities Modifying CICS, IMS/TM, or TSO dispatching priorities Implementing Workload Manager Installing a new release of OS/390, CICS, IMS/TM, or TSO Implementing parallel sysplex Modifying TSO parameters Adding or removing memory Installing additional hardware that consumes memory Increasing system throughput ƒ DB2 system-level changes Installing a new DB2 version or release Applying maintenance to the DB2 software Changing DSNZPARMs Modifying IRLM parameters Incurring DB2 growth, causing the DB2 Catalog to grow without resizing or reorganizing Ensuring proper placement of the active log data sets Implementing data sharing ƒ Application-level changes Increasing the application workload Adding rows to a table Deleting rows from a table Increasing the volume of inserts, causing unclustered data or data set extents Increasing the volume of updates to indexed columns Updating variable character columns or compressed rows, possibly causing storage space to expand and additional I/O to be incurred Changing the distribution of data values in the table - 44 - Updating RUNSTATS information (see Chapters 1 and 32 for more information on RUNSTATS) Rebinding application packages and plans Implementing or changing stored procedures or user-defined functions Enabling parallel processing ƒ Database-level changes Adding or removing indexes Changing the clustering index Altering a table to add a column Adding or removing triggers from a table Reorganizing tablespaces and indexes Compressing data Moving physical data sets for tablespaces or indexes to different volumes Luckily, you can prepare yourself to deal with performance problems by understanding the dynamic nature of DB2 performance features and keeping abreast of SQL tricks of the trade.

Partition Independence Using resource serialization, DB2 has the ability to process a single partition while permitting concurrent access to independent partitions of the same tablespace by utilities and SQL. This partition independence enhances overall data availability by enabling users concurrent access to data in separate partitions. Limited Partition Scanning When processing against a partitioned tablespace, DB2 can enhance the performance of tablespace scans by limiting the partitions that are read.

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DB2 developer's guide by Craig Mullins

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