CREATE TRIGGER

CREATE TRIGGER

Defines a new trigger. User-defined triggers are not supported in Greenplum Database.

Synopsis

CREATE TRIGGER name {BEFORE | AFTER} {event [OR ...]}
       ON table [ FOR [EACH] {ROW | STATEMENT} ]
       EXECUTE PROCEDURE funcname ( arguments )

Description

CREATE TRIGGER creates a new trigger. The trigger will be associated with the specified table and will execute the specified function when certain events occur.

Due to the distributed nature of a Greenplum Database system, the use of triggers is very limited in Greenplum Database. The function used in the trigger must be IMMUTABLE, meaning it cannot use information not directly present in its argument list. The function specified in the trigger also cannot execute any SQL or modify distributed database objects in any way. Given that triggers are most often used to alter tables (for example, update these other rows when this row is updated), these limitations offer very little practical use of triggers in Greenplum Database. For that reason, Greenplum does not support the use of user-defined triggers in Greenplum Database. Triggers cannot be used on append-optimized tables.

If multiple triggers of the same kind are defined for the same event, they will be fired in alphabetical order by name.

SELECT does not modify any rows so you can not create SELECT triggers. Rules and views are more appropriate in such cases.

Parameters

name
The name to give the new trigger. This must be distinct from the name of any other trigger for the same table.
BEFORE AFTER
Determines whether the function is called before or after the event. If the trigger fires before the event, the trigger may skip the operation for the current row, or change the row being inserted (for INSERT and UPDATE operations only). If the trigger fires after the event, all changes, including the last insertion, update, or deletion, are visible to the trigger.
event
Specifies the event that will fire the trigger (INSERT, UPDATE, or DELETE). Multiple events can be specified using OR.
table
The name (optionally schema-qualified) of the table the trigger is for.
FOR EACH ROW
FOR EACH STATEMENT
This specifies whether the trigger procedure should be fired once for every row affected by the trigger event, or just once per SQL statement. If neither is specified, FOR EACH STATEMENT is the default. A trigger that is marked FOR EACH ROW is called once for every row that the operation modifies. In contrast, a trigger that is marked FOR EACH STATEMENT only executes once for any given operation, regardless of how many rows it modifies.
funcname
A user-supplied function that is declared as IMMUTABLE, taking no arguments, and returning type trigger, which is executed when the trigger fires. This function must not execute SQL or modify the database in any way.
arguments
An optional comma-separated list of arguments to be provided to the function when the trigger is executed. The arguments are literal string constants. Simple names and numeric constants may be written here, too, but they will all be converted to strings. Please check the description of the implementation language of the trigger function about how the trigger arguments are accessible within the function; it may be different from normal function arguments.

Notes

To create a trigger on a table, the user must have the TRIGGER privilege on the table.

Examples

Declare the trigger function and then a trigger:

CREATE FUNCTION sendmail() RETURNS trigger AS 
'$GPHOME/lib/emailtrig.so' LANGUAGE C IMMUTABLE;

CREATE TRIGGER t_sendmail AFTER INSERT OR UPDATE OR DELETE 
ON mytable FOR EACH STATEMENT EXECUTE PROCEDURE sendmail();

Compatibility

The CREATE TRIGGER statement in Greenplum Database implements a subset of the SQL standard. The following functionality is currently missing:

  • Greenplum Database has strict limitations on the function that is called by a trigger, which makes the use of triggers very limited in Greenplum Database. For this reason, triggers are not officially supported in Greenplum Database.
  • SQL allows triggers to fire on updates to specific columns (e.g., AFTER UPDATE OF col1, col2).
  • SQL allows you to define aliases for the 'old' and 'new' rows or tables for use in the definition of the triggered action (e.g., CREATE TRIGGER ... ON tablename REFERENCING OLD ROW AS somename NEW ROW AS othername ...). Since Greenplum Database allows trigger procedures to be written in any number of user-defined languages, access to the data is handled in a language-specific way.
  • Greenplum Database only allows the execution of a user-defined function for the triggered action. The standard allows the execution of a number of other SQL commands, such as CREATE TABLE as the triggered action. This limitation is not hard to work around by creating a user-defined function that executes the desired commands.
  • SQL specifies that multiple triggers should be fired in time-of-creation order. Greenplum Database uses name order, which was judged to be more convenient.
  • SQL specifies that BEFORE DELETE triggers on cascaded deletes fire after the cascaded DELETE completes. The Greenplum Database behavior is for BEFORE DELETE to always fire before the delete action, even a cascading one. This is considered more consistent.
  • The ability to specify multiple actions for a single trigger using OR is a Greenplum Database extension of the SQL standard.