OpenEdge Data Management: DataServer for Microsoft SQL Server

For example, the following statement returns the same results for an OpenEdge
database and a MS SQL Server data source:
DEFINE QUERY myquery FOR customer FIELDS (custnum name) SCROLLING.
OPEN QUERY myquery FOR EACH customer NO-LOCK WHERE custnum LT 6
BY customer.name.
ABL issues
Include the SCROLLING option to enable GET PREVIOUS. You must include the NO–LOCK
option when you open queries that are defined with field lists.
Similarly, you must include the NO–LOCK option in FOR EACH statements that include field
lists, as in the following example:
FOR EACH customer FIELDS (cust_num name) NO-LOCK:
Field lists are effective only when you also specify the NO–LOCK option. This option
ensures that the DataServer does not have to refetch rows, which can slow
performance. If a lock upgrade is required, the field list is ignored and all fields are
retrieved.
Use field lists to retrieve only those fields that your application requires. For
performance reasons, the DataServer retrieves the first index field even if you do not
include it in the field list. In cases where the DataServer can predict that a query will
require a refetch, it retrieves the entire record. The DataServer allocates memory
based on the maximum size defined for a field in a record. Omitting larger fields from a
query can enhance performance. In addition, combining lookahead cursors and field
lists greatly improves a query’s performance.
When you specify a field that has an extent, the query returns the entire array.
When the DataServer processes a query with a field list, it caches the fields that are
part of the field list and any other fields that the query specified, which you can then
access without making another call to the data source. For example, the DataServer
fetches the name and the postalcode field to process the following query:
FOR EACH customer FIELDS (name) WHERE customer.postalcode EQ 01730 NO-LOCK:
Note: Cached fields might have performance implications if you modify the record
later, as the DataServer must refetch the record to place a lock on it.
If you specify a field list in a join, you might have to adjust the cache size for lookahead
cursors, either with the CACHE–SIZE option in a QUERY–TUNING phrase or at the session
level with the -Dsrv qt_cache_size startup parameter.
Any performance gained through field lists is lost if you use nonlookahead cursors.
Lookahead and block cursors gain performance by prebinding the fields of your result
set. For maximum efficiency, any text or image fields should be explicitly excluded from
your field list if possible because MS SQL Server does not allow those fields to be
pre-bound.
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