OpenEdge Data Management: DataServer for Microsoft SQL Server

LOOKAHEAD
NO–LOOKAHEAD
SEPARATE–CONNECTION
NO–SEPARATE–CONNECTION
Query tuning
Table 4–1: Query-tuning options (2 of 3)
Option Description
Specifies whether the DataServer uses lookahead or standard
cursors. Lookahead cursors fetch as many records as fit in the
allocated cache (see the CACHE–SIZE entry in this table). This
reduces the number of SQL statements and network messages
that are required, thereby improving performance.
Using lookahead cursors results in behavior that is different from
an OpenEdge database because changes made to the records in
the cache might not be immediately visible. Specify
NO–LOOKAHEAD for behavior that is consistent with OpenEdge.
Default: LOOKAHEAD, when statements use NO-LOCK or when
statements use SHARE-LOCK with TXN_ISOLATION level set to 1
(read uncommitted.)
Specifies whether each cursor should use a separate database
connection. Executing cursors in separate connections might
improve performance because the DataServer does not have to
restart the cursors and sort the results.
Do not specify SEPARATE–CONNECTION if you require behavior
that is consistent with OpenEdge.
Default: NO–SEPARATE–CONNECTION except in certain cases. For
details, see the “Managing connections to an MS SQL Server
database” section on page 6–29.
NO-QUERY-ORDER-ADDED Specifies that OpenEdge should not choose an index in the
absence of a USE-INDEX or BY clause in the query request.
OpenEdge may otherwise select an index if it is needed to provide
ABL language compatibility.
Note: If you elect to use this option to omit index selection on
the query, you may see better performance using the optimizer’s
sort selections. However, compatibility with OpenEdge
forward/backward scrolling and reposition capability may be
lost. Only use this option when compatibility is not required and
can be overlooked for the sake of better performance.
NO-QUERY-UNIQUE-ADDED Specifies that OpenEdge should omit the record identifier from
the end of the query’s generated ORDER BY clause when trying to
obtain record uniqueness from a selected non-unique index. A
sort order that is modified to derive uniqueness may produce a
query that can’t find a useful index to perform sorting thus
impacting query performance.
Note: If you elect to use this option, the query may find an index
match to provide better performance. However, turning off
uniqueness in a query where scrolling is required may result in
behavior that is incompatible with the OpenEdge ABL. Only use
this option when compatibility is not required and can be
overlooked for the sake of better performance.
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