Cursors and connections

How to use database cursors across connections?

Several database connections can be opened simultaneously with the CONNECT TOCONNECT TO instruction. Once connected, you can DECLARE cursors or PREPARE statements to be used in parallel within different connection contexts. This section describes how to use SQL cursors and SQL statements in a multiple-connection program.

When you DECLARE a cursor or when you PREPARE a statement, you actually create an SQL statement handle; the runtime system allocates resources for that statement handle before sending the SQL text to the database server via the database driver.

The SQL statement handle is created in the context of the current connection, and must be used in that context, until it is freed or recreated with another DECLARE or PREPARE statement. Using an SQL statement handle in a different connection context than the one for which it was created will produce a runtime error.

The SET CONNECTION instruction changes the connection context. Connections are identified by a name. The AS clause of the CONNECT TO instruction allows you to specify a connection name. If the AS clause is omitted, the connection gets a default name based on the data source name.

This small program example illustrates the use of two cursors with two different connections:

MAIN
   CONNECT TO "db1" AS "s1"
   CONNECT TO "db2" AS "s2"
   SET CONNECTION "s1"
   DECLARE c1 CURSOR FOR SELECT tab1.* FROM tab1
   SET CONNECTION "s2"
   DECLARE c2 CURSOR FOR SELECT tab1.* FROM tab1
   SET CONNECTION "s1"
   OPEN c1
   SET CONNECTION "s2"
   OPEN c2
   ...
END MAIN

The DECLARE and PREPARE instructions are a type of creator instruction; if an SQL statement handle is recreated in a connection other than the original connection for which it was created, old resources are freed and new resources are allocated in the current connection context.

This allows you to re-execute the same cursor code in different connection contexts, as in this example:

MAIN
   CONNECT TO "db1" AS "s1"
   CONNECT TO "db2" AS "s2"
   SET CONNECTION "s1"
   IF checkForOrders() > 0 ...
   SET CONNECTION "s2"
   IF checkForOrders() > 0 ...
   ...
END MAIN

FUNCTION checkForOrders(d)
   DEFINE d DATE, i INTEGER
   DECLARE c1 CURSOR FOR SELECT COUNT(*) FROM orders WHERE ord_date = d
   OPEN c1
   FETCH c1 INTO i
   CLOSE c1
   FREE c1
   RETURN i
END FUNCTION

If the SQL statement handle was created in a different connection, the resources used in the old connection context are freed automatically, and new statement handle resources are allocated in the current connection context.