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each row store the information pertaining to each
defined column. Each column in a table has a unique
name, and is defined by field, count, and key type.
3.7.3
Indices. Index files are implemented for tables with
variable-length records, primitive tables, and feature tables in
tiled databases that require query response capabilities. In the
DCW, tables that have an associated index are all those with
variable-length records; area, line, point, and text feature
tables; and the face, edge, node and text primitive tables.
a. Variable-length field index. The index associated with
variable-length fields specifies the starting position
in bytes for variable-length text or coordinate records.
This index is a separate file containing a header and a
data array that specifies the byte offset from the
beginning of the file for each record. These indices
are implemented to permit direct data access, which
accelerates data retrieval.
b. Spatial index. The index associated with primitive
tables is a spatial index that enhances data access and
is based on the value of the coordinate column.
c. Thematic index. The index associated with tiled feature
tables is a thematic index that enhances data access and
is based on the value of the ID columns.
3.8 DCW record layout. The directories, tables, and indices
implemented for the DCW are presented in this section generally in
the order they are stored on the CD-ROM, beginning with the
database directory (see Appendix Section 50). A format and
content table illustration is used throughout this specification
to define the physical record structure required for each VPF
table in the DCW (FIGURE 2). The VPF field types and key types
implemented in the DCW are depicted in TABLE 2. The tables
implemented in the DCW are listed by directory level in TABLE 3.
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