OpenEdge Data Management: DataServer for Microsoft SQL Server

Additional Features to Enhance DataServer Performance
Join by SQLDB
4–14
For queries that include joins issued in FOR EACH and OPEN QUERY statements, the DataServer
evaluates the queries and in some cases instructs the MS SQL Server data source to perform the
joins. A join performed by the data source, called a join by SQLDB, can improve performance;
however, you receive the associated query results in an order consistent with the data source,
not with the OpenEdge database. To get results that are consistent with the OpenEdge database,
turn off JOIN–BY–SQLDB, either with the QUERY–TUNING phrase at the query level or with the
Server Join (-nojoinbysqldb) startup parameter when you compile. If the order of returned
records is important to your application, specify a sort order on the query.
For each join, the DataServer evaluates whether the MS SQL Server data source can perform it
and estimates whether doing so improves performance. To determine whether a join by SQLDB
is possible, the DataServer assesses whether the following criteria are true:
• All tables in the join are in the same logical OpenEdge database; that is, they are contained
in the same DataServer schema.
• Every table, except the innermost one, has a unique record identifier (ROWID or RECID
support).
• The query does not include a USING phrase for any of the inner tables. For example, a join
by SQLDB will not occur for this query:
FOR EACH customer, EACH order OF customer USING order.ordernum:
• The query does not include a BY phrase that contains expressions or array fields.
• The query does not include a request for an EXCLUSIVE-LOCK on any of the tables in the
join.
• The join does not exceed 10 levels.
To estimate whether performing a join by the data source might improve performance, the
DataServer assesses whether these additional criteria are true:
• The join uses an OF clause or a WHERE clause for each of the inner table loops. For example,
the following query requires a field-to-field correspondence between two tables:
FOR EACH customer, EACH order OF customer:
• The WHERE clause includes either an operator or the AND option. The following example
includes the equals (=) operator:
FOR EACH customer, EACH order
WHERE customer.custnum = order.custnum:
The DataServer also performs a join by SQLDB for the following query: