43.6. Trigger Functions in PL/Tcl #
Trigger functions can be written in PL/Tcl. Postgres Pro requires that a function that is to be called as a trigger must be declared as a function with no arguments and a return type of trigger
.
The information from the trigger manager is passed to the function body in the following variables:
$TG_name
The name of the trigger from the
CREATE TRIGGER
statement.$TG_relid
The object ID of the table that caused the trigger function to be invoked.
$TG_table_name
The name of the table that caused the trigger function to be invoked.
$TG_table_schema
The schema of the table that caused the trigger function to be invoked.
$TG_relatts
A Tcl list of the table column names, prefixed with an empty list element. So looking up a column name in the list with Tcl's
lsearch
command returns the element's number starting with 1 for the first column, the same way the columns are customarily numbered in Postgres Pro. (Empty list elements also appear in the positions of columns that have been dropped, so that the attribute numbering is correct for columns to their right.)$TG_when
The string
BEFORE
,AFTER
, orINSTEAD OF
, depending on the type of trigger event.$TG_level
The string
ROW
orSTATEMENT
depending on the type of trigger event.$TG_op
The string
INSERT
,UPDATE
,DELETE
, orTRUNCATE
depending on the type of trigger event.$NEW
An associative array containing the values of the new table row for
INSERT
orUPDATE
actions, or empty forDELETE
. The array is indexed by column name. Columns that are null will not appear in the array. This is not set for statement-level triggers.$OLD
An associative array containing the values of the old table row for
UPDATE
orDELETE
actions, or empty forINSERT
. The array is indexed by column name. Columns that are null will not appear in the array. This is not set for statement-level triggers.$args
A Tcl list of the arguments to the function as given in the
CREATE TRIGGER
statement. These arguments are also accessible as$1
...$
in the function body.n
The return value from a trigger function can be one of the strings OK
or SKIP
, or a list of column name/value pairs. If the return value is OK
, the operation (INSERT
/UPDATE
/DELETE
) that fired the trigger will proceed normally. SKIP
tells the trigger manager to silently suppress the operation for this row. If a list is returned, it tells PL/Tcl to return a modified row to the trigger manager; the contents of the modified row are specified by the column names and values in the list. Any columns not mentioned in the list are set to null. Returning a modified row is only meaningful for row-level BEFORE
INSERT
or UPDATE
triggers, for which the modified row will be inserted instead of the one given in $NEW
; or for row-level INSTEAD OF
INSERT
or UPDATE
triggers where the returned row is used as the source data for INSERT RETURNING
or UPDATE RETURNING
clauses. In row-level BEFORE
DELETE
or INSTEAD OF
DELETE
triggers, returning a modified row has the same effect as returning OK
, that is the operation proceeds. The trigger return value is ignored for all other types of triggers.
Tip
The result list can be made from an array representation of the modified tuple with the array get
Tcl command.
Here's a little example trigger function that forces an integer value in a table to keep track of the number of updates that are performed on the row. For new rows inserted, the value is initialized to 0 and then incremented on every update operation.
CREATE FUNCTION trigfunc_modcount() RETURNS trigger AS $$ switch $TG_op { INSERT { set NEW($1) 0 } UPDATE { set NEW($1) $OLD($1) incr NEW($1) } default { return OK } } return [array get NEW] $$ LANGUAGE pltcl; CREATE TABLE mytab (num integer, description text, modcnt integer); CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab FOR EACH ROW EXECUTE FUNCTION trigfunc_modcount('modcnt');
Notice that the trigger function itself does not know the column name; that's supplied from the trigger arguments. This lets the trigger function be reused with different tables.