File Extensions

Each embedded database table in your database actually stores its information in several files that all share the same root name but have different extensions. Each extension and its meaning is explained below.

.DAT

This is the table. The table stores the physical data. A special portion of this file is reserved for storing information about the structure of the data and the indexes of the table, called the header.

If compression is used in the table, then the data is removed from the .DAT file and stored in a .VLD file (see below). Instead of data, the .DAT file now only contains pointers to the .VLD. For more information, see Compressing Data.

.DEF

This is the table-definition file. This file contains a printable copy of the structure of the table and its indexes. This can be used for documentation and can also be used for rebuilding the structure of a table.

.DEF files are not automatically created or updated when a table is changed. You must specifically output .DEF files using the appropriate Database Builder option for this. See "Output .DEF Files" for more information.

.FD

This is the column-definition file. This file contains symbolic replacements for all of the columns in the table. This file is compiled into your application with the DataFlex Open command.

.FLT

This is a column-template file. A column template is a record of the structure of a single table column. The templates store information about the column's name, type, length, and main index.

.HDR

This is the header-backup file. This file stores an up-to-date copy of the structure information that is stored in the .DAT file (see above). A .HDR file is only maintained for tables that use header-integrity checking.

.K

These are the index files. One .K file is created for each index. These are numbered according to the index number they represent, i.e., CUSTOMER.K2 stores Index 2 for the customer table.

.TAG

This is the column tags file. This file stores all the column names.

.VLD

This is the variable-length-table. This file stores the data when the table is employing data compression. See Compressing Data for more information.

Example

If the table name is CUSTOMER, then the following files are created to store all the information:

CUSTOMER.DAT
CUSTOMER.DEF
CUSTOMER.FD
CUSTOMER.HDR
CUSTOMER.K1
CUSTOMER.K2
CUSTOMER.K3
CUSTOMER.TAG
CUSTOMER.VLD

This example assumes that the customer table has 3 indexes, uses data compression, and file-header-integrity checking. Some of these files will only exist if certain settings are switched on (e.g., Compression and Header-Integrity checking). The .DEF file is only created if the user runs the Output .DEF/.FD option.