Chapter 5: Data Storage Structures

File Organization #

A database is mapped into a number of different files managed by the underlying operating system. These files are organized logically as a sequence of records and are mapped onto disk blocks.

Each file is also logically partitioned into fixed-length storage units called blocks, which are file system units for both storage allocation and data transfer.

Records are also stored as either fixed-length or variable-length in size in any given file depending on the database implementation.

In most cases records are stored in variable-length sizes. The ff. are two ways to structure variable-length records in a block:

Variable-Length Records #

The ff. are ways (out of many available) to structure variable-length records:

Fixed-Length Prefix, Variable-Length Suffix #

In this structure a block is divided into three sections: the beginning contains values for fixed-length data, the end contains values for variable-length data, and in-between is a section for the null bitmap which simply represents attributes that have null values in them.

Variable-length values are mapped from a record in the fixed-length section that contains information about its offset and length:

In this example the $ID$ attribute is represented in the fixed-length section as being at the offset position of $21$ and having a length of $5$.

Slotted-Page Structure #

The most common layout for variable-length records.

This structure contains three sections: the header section, the free section, and the record list section.

The block header commonly contains the ff. information:

• Total record entries
• The end of free space in the block
• An array containing the offset and length of each record