Administration and Programming Guide for OS/2, Windows NT, and UNIX

SQL Language Environment

In the sections that follow, some performance techniques about the database and SQL language environment are described. To learn about DB2 performance considerations, visit the web at: http://review.software.ibm.com/data/db2/performance

Database Techniques

The following summary outlines some of the most simple database techniques that can improve database access:

• Activate the database. By issuing the command db2 activate database, connections to the database are made in significantly less time. See DB2 Administration Guide. When running Net.Data using CGI, this performance technique is a good alternative to using Live Connection, which is required for FastCGI or Web APIs.

• Avoid numeric conversion. When a column value and a literal value are being compared, try to specify the same data types and attributes. DB2 does not use an index for the named column if the literal value has a greater precision than the precision of the column. If the two items being compared have different data types, DB2 will have to convert one or the other of the values, which can result in inaccuracies (because of limited machine precision).

  For example, EDUCLVL is a halfword integer value (SMALLINT). Specify:

   ... WHERE EDUCLVL < 11 AND EDUCLVL >= 2
  

  Instead of:

  ... WHERE EDUCLVL < 1.1E1 AND EDUCLVL > 1.3
  

• Avoid character string padding. Try to use the same data length when comparing a fixed-length character string column value to a literal value. DB2 does not use an index if the literal value is longer than the column length.

  For example, EMPNO is CHAR(6) and DEPTNO is CHAR(3). Specify:

  ... WHERE EMPNO > '000300' AND  DEPTNO < 'E20'
  

  Instead of:

  ... WHERE EMPNO > '000300 ' AND DEPTNO < 'E20 ' 
  

• Avoid the use of LIKE patterns beginning with % or _. The percent sign (%), and the underline (_), when used in the pattern of a LIKE predicate, specify a character string that is similar to the column value of rows you want to select. When used to denote characters in the middle or at the end of a character string, LIKE patterns can take advantage of indexes. For example:

  ... WHERE LASTNAME LIKE 'J%SON%'
  

  However, when used at the beginning of a character string, LIKE patterns can prevent DB2 from using any indexes that might be defined on the LASTNAME column to limit the number of rows scanned. For example:

  ... WHERE LASTNAME LIKE '%SON' 
  

  Avoid using these symbols at the beginning of character strings, especially if you are accessing a particularly large table.

SQL Language Environment Techniques

Use the following SQL language environment techniques to improve performance.

• Use the START_ROW_NUM and RPT_MAX_ROWS Net.Data variables to reduce the size of returned tables. If a result set contains a large number of rows, you can specify a subset of the result set that is returned to the browser by using START_ROW_NUM and RPT_MAX_ROWS. START_ROW_NUM specifies the row number of the first row to return, and RPT_MAX_ROWS specifies the number of rows to return.

  Important:

  Net.Data reissues the query for every request because cursor position is not maintained across requests.

• Consider calling a stored procedure that uses static SQL. Dynamic SQL is prepared at run time, while static SQL is prepared at the precompile stage. The SQL language environment uses dynamic SQL, which allows it to run SQL statements at program run time. Because preparing statements requires additional processing time, static SQL might be more efficient.