Join by SQL DB

For queries that include joins issued in FOR EACH and OPEN QUERY statements, the DataServer evaluates the queries and in some cases instructs the ODBC data source to perform the joins. A join performed by a data source, called a join by SQL DB, can improve performance; however, you receive the associated query results in an order consistent with the data source, not with ABL. To get results that are consistent with ABL, 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 ODBC data source can perform it and estimates whether doing so improves performance. To determine whether a join by SQL DB 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 SQL DB 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 SQL DB for the following query:

FOR EACH customer, EACH order
WHERE customer.custnum = order.custnum AND customer.custnum > 100:

For the following query, however, the DataServer instructs the client to perform the join because of the OR option:

FOR EACH customer, EACH order
WHERE customer.custnum = order.custnum OR customer.custnum > 100:

By default, the DataServer instructs an ODBC data source to perform a join when possible and when desirable. However, you can control the default behavior by using either the QUERY–TUNING NO–JOIN–BY–SQLDB phrase or the Server Join (-nojoinbysqldb) startup parameter. The QUERY–TUNING phrase controls the behavior for a single query. The -nojoinbysqldb parameter controls it at the session level.

The following table describes how these controls interact and affect the behavior.

Table 12. Controlling join by SQLDB behavior
QUERY–TUNING	Startup parameter	Behavior
JOIN–BY–SQLDB	-nojoinbysqldb	The ODBC data source performs the join if possible
JOIN–BY–SQLDB	None	The ODBC data source performs the join if possible
NO–JOIN–BY–SQLDB	-nojoinbysqldb	The client performs the join
NO–JOIN–BY–SQLDB	None	The client performs the join
None	-nojoinbysqldb	The client performs the join
None	None	The ODBC data source performs the join if possible and the join contains the selection criteria

A join by SQL DB does not occur by default for the following query because the DataServer determines that it does not increase performance:

FOR EACH customer, EACH order:

You receive a warning if you specify JOIN–BY–SQLDB when the ODBC data source cannot perform the join and the DataServer performs the join instead. You receive a warning at compile time if you specify JOIN–BY–SQLDB when the data source can perform the join but it is not optimal for it to do so.