SQL/MP Reference Manual

HP NonStop SQL/MP Reference Manual—523352-013
D-18
DCL (Data Control Language) Statements
DCL (Data Control Language) Statements
DCL (Data Control Language) is the set of SQL statements and directives that control
parallel processing, name resolution, and performance-related considerations such as
access paths, join methods, and locks and cursors. This table summarizes the DCL
statements and directives.
For information, see the specific statement or directive.
DCOMPRESS File Attribute
DCOMPRESS is a Guardian file attribute that controls key compression in data blocks.
DCOMPRESS applies only to key-sequenced tables and to indexes.
The table default is NO DCOMPRESS. The index default is the table value at index
creation. The default compression method is 1, the former compression method.
Considerations—DCOMPRESS

Use DCOMPRESS when you need to save disk space but do not require
maximum performance. Because a given amount of disk space holds more keys if
the keys are compressed, key compression reduces the use of disk space;
however, it also increases overhead for accessing records.

DCOMPRESS 2 extends the compression technique to cover non-key columns,
which is not possible with DCOMPRESS 1. This strategy results in significant disk
space saving at a relatively lower performance price.

For compression method 1, the former method, a file cannot use key compression
in data blocks unless its primary key meets these requirements:
DCL Statements and Directives
CONTROL EXECUTOR
Directive
Enables or disables parallel processing of queries
CONTROL QUERY
Directive
Specifies whether to resolve names at execution or SQL
startup, whether to consider use of hash joins, and whether to
optimize queries for few or many rows returned
CONTROL TABLE
Directive
Controls locks, opens, buffers, access paths, join methods, and
join sequences
FREE RESOURCES
Statement
Closes cursors and releases locks held by a process
LOCK TABLE Statement
Locks a table (or the underlying tables of a view) and
associated indexes
UNLOCK TABLE
Statement
Releases locks held on nonaudited tables or views
{ DCOMPRESS { 1 | 2 } | NO DCOMPRESS }