OpenEdge Data Management: DataServer for Microsoft SQL Server

Join by SQLDB
Join by SQLDB
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.
• There is no nested FOR block in 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:
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