OpenEdge Data Management: DataServer for Microsoft SQL Server

Query tuning
Table 24: Query-tuning options (4 of 4)
Option Description
SELECT-ON-JOIN When performing an outer join operation,
SELECT-ON-JOIN specifies that any search
condition separate from the join predicate be
bracketed after the join is performed. This option
eliminates all the records with NULL values
contained in the non-matching results, if the
columns with the NULL values appear in the join
search criteria.
JOIN-ON-SELECT When performing an outer join operation,
JOIN-ON-SELECT specifies that any search
condition separate from the join predicate be
bracketed before the join is performed. This
option includes all the records with NULL values
contained in the non-matching results and
causes records that join on NULL to be included
in the results of an OUTER-JOIN which is
consistent with OpenEdge join handling of the
unknown value.
Note: JOIN-ON-SELECT is the default behavior
for outer join operations.
All but two of the QUERY–TUNING options take effect at both compile time and run time.
The exceptions are JOIN–BY–SQLDB and NO–JOIN–BY–SQLDB, which apply only at
compile time. You can override query-tuning defaults (except JOIN–BY–SQLDB) at
run-time by specifying the appropriate startup parameters.
The following example shows how to use the QUERY–TUNING phrase to enhance
performance. It includes a join, JOIN-BY-SQLDB, that the DataServer instructs the MS
SQL Server data source to perform by default, as shown:
FOR EACH customer, EACH order OF customer WHERE order.ordernum GT 20
BY customer.custnum QUERY-TUNING (JOIN-BY-SQLDB)
The QUERY–TUNING options in this example specifies the following:
• Lookahead cursors are not used (the NO–LOOKAHEAD option)
• The DataServer writes an extended report on the SQL statements that it executes
(the DEBUG EXTENDED option)
When the DataServer constructs queries for a MS SQL Server data source, it uses the
QUERY–TUNING options that you specify as guidelines. This is because there might be
syntax considerations that prevent the DataServer from applying the QUERY–TUNING
options as specified. In such a case, the DataServer executes the query using the most
appropriate options.
Note: The DataServer does not issue errors or warnings if it does not apply the
QUERY–TUNING options that you specify.
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