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ProDataSet Parameters : Passing a ProDataSet with BY-REFERENCE or BIND : Passing a ProDataSet parameter by reference : Importance of optimized code with BY-REFERENCE
Importance of optimized code with BY-REFERENCE
The information in this section applies to the use of BIND as well, which you will read about in the .
This diversion into a discussion of BY-REFERENCE might seem overly complex, but it has been introduced here for a reason. If you get into the habit of structuring your procedures and their ProDataSet parameters properly from the beginning, you will be well positioned to optimize most calls that are sometimes made locally and sometimes remotely by adding the BY-REFERENCE keyword to your calls without any undesirable consequences. It is always best to design your procedures so that they work properly when run locally—even when in a deployed application they may be distributed on different machines. This makes the initial development and testing of your application more straightforward, and supports the case (even if it is only for demo purposes) where everything is running in a single session. Since the BY-REFERENCE optimization is such a valuable one, it is worth making the effort to prepare for it right from the start, even if you first code and test your application without it and then add the keyword to your calls to improve performance.
You can pass a ProDataSet reference locally as a HANDLE PARAMETER as you can with other objects such as temp-tables. This gives the called procedure access to the ProDataSet instance defined in the caller, but has two essential limitations. First, the called procedure must be in the same OpenEdge session as the calling procedure. You cannot pass an object handle of any kind across an AppServer call and have it maintain its validity. Second, the called procedure can only reference the ProDataSet using dynamic attributes and methods if it receives it as a HANDLE. In other words, the called procedure cannot receive the HANDLE parameter into a static DATASET definition and reference it using static table and field names.
By contrast, when you pass or receive a ProDataSet as a DATASET-HANDLE, this simply means that the procedure using the DATASET-HANDLE parameter form sees the ProDataSet as a dynamic object. The procedure on the other side of the call can see it as a static object. The table below shows the different combinations. Normally, passing a ProDataSet as a DATASET-HANDLE causes its definition and data to be copied to the receiving procedure. Using the BY-REFERENCE option on the parameter makes passing a ProDataSet as a DATASET-HANDLE cost no more within a single session than passing a reference to it as a HANDLE. For this reason, we recommend that you normally use the DATASET-HANDLE form. In this way, if the call is ever moved to a remote procedure, it will still work properly, whereas the HANDLE parameter will fail on a remote call.
Passing a ProDataSet BY-REFERENCE is particularly valuable when the called procedure uses the DATASET parameter form to receive the ProDataSet into a static definition. This will be the norm in most server-side procedures, and especially in most event handling procedures. The business logic in these procedures is much simpler to write if it uses static ABL statements to refer to and manipulate the records in the ProDataSet's temp-tables. All the elements of the ProDataSet definition, along with all the data in its temp-tables, are available to the called procedure. The default buffers that are part of the ProDataSet, which have the same names as their temp-tables, are in fact shared between caller and callee. Therefore, if the called procedure changes the record position in any of these buffers, this will be visible to the caller after the procedure returns. The called procedure can define buffers of its own to avoid this. Any changes to the ProDataSet data made by the called procedure are visible to the caller, as if the object were SHARED.
There are several restrictions in ProDataSet usage that are at least partially related to this parameter support:
*You cannot define a ProDataSet inside an internal procedure or trigger. This is the same restriction that applies to temp-tables.
*The static parameter form (that is, DEFINE INPUT/OUTPUT PARAMETER DATASET FORdataset-name) cannot be used in the main block of a procedure that is run PERSISTENT.
As a final note, it is possible to specify a ProDataSet as a parameter for a dynamic call object, which lets you set up the entire definition of a RUN statement dynamically. You can also specify the BY-REFERENCE qualifier in the arguments to the dynamic CALL. For more information, see the reference documentation or online help for the SET-PARAMETER( ) method on the dynamic call object.