1. Database systems are partitioned into modules for di erent functions. Some functions (e.g. le systems) may be provided by the operating system.

2. Components include:

File manager manages allocation of disk space and data structures used to represent information on disk.

Figure 1.6: Database system structure.

Database manager: The interface between low-level data and application programs and queries.

Query processor translates statements in a query language into low-level instructions the database manager understands. (May also attempt to nd an equivalent but more ecient form.)

DML precompiler converts DML statements embedded in an application program to normal procedure calls in a host language. The precompiler interacts with the query processor.

DDL compiler converts DDL statements to a set of tables containing metadata stored in a data dictionary.

In addition, several data structures are required for physical system implementation:

• Data files: store the database itself.
• Data dictionary: stores information about the structure of the database. It is used heavily. Great emphasis should be placed on developing a good design and ecient implementation of the dictionary.
• Indices: provide fast access to data items holding particular values.

3. Figure 1.6 shows these components.