Query performance and query optimization in the Database topic.
Chapter 4 of the Performance Capabilities Reference. Links to several editions of the Performance Capabilities Reference are listed in the Performance Management Resource Library. 
For tuning information for DB2 UDB for iSeries, see these resources:
Query performance and query optimization in the Database topic.
Chapter 4 of the Performance Capabilities Reference. Links to several editions of the Performance Capabilities Reference are listed in the Performance Management Resource Library.
You might also want to adjust the QSQSRVR prestart job settings for your system. On iSeries, QSQSRVR jobs process Java database access (JDBC). By default, five QSQSRVR jobs are initially active. When fewer than two QSQSRVR jobs are unused, two more jobs are created. An application that establishes a large number of database connections over a short period of time may be able to create connections more quickly if these values are increased. To increase the initial number of jobs, threshold, and additional number of jobs values, run this command on the CL command line:
CHGPJE SBSD(QSYS/QSYSWRK) PGM(QSYS/QSQSRVR)
Do not start more QSQSRVR jobs than your application requires, because there is some overhead associated with having QSQSRVR jobs active, even if they are not being used.
If you use a different database, refer to that product's documentation.