DB2 developer's guide by Craig Mullins

By Craig Mullins

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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.

DB2 might be able to use an index-only access path that is unavailable for SELECT *. - 47 - Singleton SELECT Versus the Cursor To return a single row, an application program can use a cursor or a singleton SELECT. INTO. Usually, the singleton SELECT outperforms the cursor. When the selected row must be updated after it is retrieved, however, using a cursor with the FOR UPDATE OF clause is recommended over a singleton SELECT. The FOR UPDATE OF clause ensures the integrity of the data in the row because it causes DB2 to hold an X lock on the page containing the row to be updated.

Also, DB2 supports the ability to create user-defined data types. Each column of a DB2 table must be assigned to a data type. Appropriately enough, the data type defines the type of data that can be stored in the column. DB2 supports the following native data types: CHAR fixed length alphanumeric data VARCHAR variable length alphanumeric data GRAPHIC fixed length graphical data VARGRAPHIC variable length graphical data SMALLINT small integer numbers INTEGER larger integer numbers DECIMAL(p,s) numeric data FLOAT(n) or FLOAT single precision floating point (if n>21) FLOAT(n) or REAL double precision floating point (if n<21) DATE calendar date data TIME time data TIMESTAMP combination date and time data ROWID unique row identifier (internally generated by DB2) BLOB binary large object CLOB character large object DBCLOB double byte character large object The last three data types, BLOB, CLOB, and DBCLOB, are used to store object/relational data.

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