Phase 6: Database System Implementation and Tuning
After the logical and physical designs are completed, we can implement
the database system. This is typically the responsibility of the DBA and is
carried out in conjunction with the database designers. Language statements in
the DDL, including the SDL (storage definition language) of the selected DBMS,
are compiled and used to create the database schemas and (empty) database
files. The database can then be loaded (populated)
with the data. If data is to be converted from an earlier computerized system,
conversion routines may be needed to
reformat the data for loading into the new database.
Database programs are implemented by the application programmers, by
referring to the conceptual specifications of transactions, and then writing
and testing pro-gram code with embedded DML (data manipulation language) commands.
Once the transactions are ready and the data is loaded into the database, the
design and implementation phase is over and the operational phase of the
database system begins.
Most systems include a monitoring utility to collect performance
statistics, which are kept in the system catalog or data dictionary for later
analysis. These include statistics on the number of invocations of predefined
transactions or queries, input/output activity against files, counts of file
disk pages or index records, and frequency of index usage. As the database
system requirements change, it often becomes necessary to add or remove
existing tables and to reorganize some files by changing primary access methods
or by dropping old indexes and constructing new ones. Some queries or
transactions may be rewritten for better performance. Database tuning continues
as long as the database is in existence, as long as performance problems are
discovered, and while the requirements keep changing (see Chapter 20).