Triggers

View 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 that you run against a view (INSERT, UPDATE, or DELETE) 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 the update command that you run against a view is 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. For more inmformation about virtual procedures, see Virtual procedures.

You can use INSTEAD OF triggers on views in a way that is similar to the way that you might use them with traditional databases. You can have only one FOR EACH ROW procedure for each INSERT, UPDATE, or DELETE operation against a view.

Usage
CREATE TRIGGER ON view_name INSTEAD OF INSERT|UPDATE|DELETE AS
FOR EACH ROW
...
Update procedure processing

  1. The user application submits the SQL command.

  2. The command detects the view that it is executed against.

  3. The correct procedure is chosen depending upon the command type (INSERT, UPDATE, or DELETE).

  4. The procedure is executed. The procedure might contain SQL commands of its own. Commands in the procedure can be different in type from the command that is received from the calling application.

  5. Commands, as described in the procedure, are issued to the individual physical data sources or other views.

  6. A value representing the number of rows changed is returned to the calling application.

Source triggers

Teiid can use AFTER triggers on source tables. AFTER triggers are called by events from a change data capture (CDC) system.

Usage:
CREATE TRIGGER ON source_table AFTER INSERT|UPDATE|DELETE AS
FOR EACH ROW
...
FOR EACH ROW triggers

Only the FOR EACH ROW construct serves as a trigger handler. A FOR EACH ROW trigger procedure will evaluate its block for each row of the view/source 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 one new row for each set of values from the VALUES or query expression. For a view, the rows updated is reported as this number, regardless of the affect of the underlying procedure logic.

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.

Note
 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 for update procedures

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

NEW variables

Every attribute in the view/table 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, or the event is received, 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, later in this list for distinguishing defaults from passed values.

OLD variables

Every attribute on the view/table 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, or the event is received, these variables are initialized to the current values of the row being deleted or updated respectively.

CHANGING variables

Every attribute on the view/table 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, or an the event is received, 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 that is specified in the user query.

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

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
Key variables

To return generated keys from an INSERT trigger, a KEY group is made available that can be assigned the value to be returned. Typically this requires using the generated_key system function. However, not all inserts provide generated keys, because not all sources return generated keys.

create view v1 (i integer, k integer not null auto_increment primary key)
OPTIONS (UPDATABLE true) as
   select x, y from tbl;
create trigger on v1 instead of insert as
   for each row begin atomic
      -- ... some logic
      insert into tbl (x) values (new.i);
      key.k = cast(generated_key('y') as integer);
   end;
Example update procedures

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
Other usages

FOR EACH ROW update procedures in a view 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:

CREATE TRIGGER ON outerVW INSTEAD OF INSERT AS
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

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