Update Procedures (Triggers)

Views are abstractions above physical sources. They typically union or join information from multiple tables, often from multiple data sources or other views. Teiid can perform update operations against views. Update commands - INSERT, UPDATE, or DELETE - against a view require logic to define how the tables and views integrated by the view are affected by each type of command. This transformation logic, also referred to as a trigger, is invoked when an update command is issued against a view. Update procedures define the logic for how a user’s update command against a view should be decomposed into the individual commands to be executed against the underlying physical sources. Similar to Virtual Procedures, update procedures have the ability to execute queries or other commands, define temporary tables, add data to temporary tables, walk through result sets, use loops, and use conditional logic.

Table of Contents

Update Procedure Processing
For Each Row
Definition
Special Variables
Examples

Update Procedure Processing

The user application submits the SQL command through one of SOAP, JDBC, or ODBC.
The view this SQL command is executed against is detected.
The correct procedure is chosen depending upon whether the command is an INSERT, UPDATE, or DELETE.
The procedure is executed. The procedure itself can contain SQL commands of its own which can be of different types than the command submitted by the user application that invoked the procedure.
Commands, as described in the procedure, are issued to the individual physical data sources or other views.
A value representing the number of rows changed is returned to the calling application.

For Each Row

A FOR EACH ROW procedure will evaluate its block for each row of the view affected by the update statement. For UPDATE and DELETE statements this will be every row that passes the WHERE condition. For INSERT statements there will be 1 new row for each set of values from the VALUES or query expression. The rows updated is reported as this number regardless of the affect of the underlying procedure logic.

Teiid FOR EACH ROW update procedures function like INSTEAD OF triggers in traditional databases. There may only be 1 FOR EACH ROW procedure for each INSERT, UPDATE, or DELETE operation against a view. FOR EACH ROW update procedures can also be used to emulate BEFORE/AFTER each row triggers while still retaining the ability to perform an inherent update. This BEFORE/AFTER trigger behavior with an inherent update can be achieved by creating an additional updatable view over the target view with update procedures of the form:

FOR EACH ROW
    BEGIN ATOMIC
    --before row logic

    --default insert/update/delete against the target view
    INSERT INTO VW (c1, c2, c3) VALUES (NEW.c1, NEW.c2, NEW.c3);

    --after row logic
    END

Definition

Usage:

FOR EACH ROW
   BEGIN ATOMIC
      ...
   END

The BEGIN and END keywords are used to denote block boundaries. Within the body of the procedure, any valid statement may be used.

Tip	The use of the atomic keyword is currently optional for backward compatibility, but unlike a normal block, the default for instead of triggers is atomic.

Special Variables

You can use a number of special variables when defining your update procedure.

NEW Variables

Every attribute in the view whose UPDATE and INSERT transformations you are defining has an equivalent variable named NEW.<column_name>

When an INSERT or an UPDATE command is executed against the view, these variables are initialized to the values in the INSERT VALUES clause or the UPDATE SET clause respectively.

In an UPDATE procedure, the default value of these variables, if they are not set by the command, is the old value. In an INSERT procedure, the default value of these variables is the default value of the virtual table attributes. See CHANGING Variables for distinguishing defaults from passed values.

OLD Variables

Every attribute in the view whose UPDATE and DELETE transformations you are defining has an equivalent variable named OLD.<column_name>

When a DELETE or UPDATE command is executed against the view, these variables are initialized to the current values of the row being deleted or updated respectively.

CHANGING Variables

Every attribute in the view whose UPDATE and INSERT transformations you are defining has an equivalent variable named CHANGING.<column_name>

When an INSERT or an UPDATE command is executed against the view, these variables are initialized to true or false depending on whether the INPUT variable was set by the command. A CHANGING variable is commonly used to differentiate between a default insert value and one specified in the user query.

For example, for a view with columns A, B, C:

If User Executes… Then…

If User Executes…	Then…
`INSERT INTO VT (A, B) VALUES (0, 1)`	CHANGING.A = true, CHANGING.B = true, CHANGING.C = false
`UPDATE VT SET C = 2`	CHANGING.A = false, CHANGING.B = false, CHANGING.C = true

INSERT INTO VT (A, B) VALUES (0, 1)

CHANGING.A = true, CHANGING.B = true, CHANGING.C = false

UPDATE VT SET C = 2

CHANGING.A = false, CHANGING.B = false, CHANGING.C = true

Examples

For example, for a view with columns A, B, C:

Sample DELETE Procedure

FOR EACH ROW
BEGIN
    DELETE FROM X WHERE Y = OLD.A;
    DELETE FROM Z WHERE Y = OLD.A; // cascade the delete
END

Sample UPDATE Procedure

FOR EACH ROW
BEGIN
    IF (CHANGING.B)
    BEGIN
        UPDATE Z SET Y = NEW.B WHERE Y = OLD.B;
    END
END

Update Procedures