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>Trigger Procedures in PL/Tcl</TITLE
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><H1
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><A
NAME="PLTCL-TRIGGER"
>40.6. Trigger Procedures in PL/Tcl</A
></H1
><P
> Trigger procedures can be written in PL/Tcl.
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> requires that a procedure that is to be called
as a trigger must be declared as a function with no arguments
and a return type of <TT
CLASS="LITERAL"
>trigger</TT
>.
</P
><P
> The information from the trigger manager is passed to the procedure body
in the following variables:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="VARNAME"
>$TG_name</TT
></DT
><DD
><P
> The name of the trigger from the <TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
> statement.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_relid</TT
></DT
><DD
><P
> The object ID of the table that caused the trigger procedure
to be invoked.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_table_name</TT
></DT
><DD
><P
> The name of the table that caused the trigger procedure
to be invoked.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_table_schema</TT
></DT
><DD
><P
> The schema of the table that caused the trigger procedure
to be invoked.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_relatts</TT
></DT
><DD
><P
> A Tcl list of the table column names, prefixed with an empty list
element. So looking up a column name in the list with <SPAN
CLASS="APPLICATION"
>Tcl</SPAN
>'s
<CODE
CLASS="FUNCTION"
>lsearch</CODE
> command returns the element's number starting
with 1 for the first column, the same way the columns are customarily
numbered in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. (Empty list
elements also appear in the positions of columns that have been
dropped, so that the attribute numbering is correct for columns
to their right.)
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_when</TT
></DT
><DD
><P
> The string <TT
CLASS="LITERAL"
>BEFORE</TT
>, <TT
CLASS="LITERAL"
>AFTER</TT
>, or
<TT
CLASS="LITERAL"
>INSTEAD OF</TT
>, depending on the type of trigger event.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_level</TT
></DT
><DD
><P
> The string <TT
CLASS="LITERAL"
>ROW</TT
> or <TT
CLASS="LITERAL"
>STATEMENT</TT
> depending on the
type of trigger event.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$TG_op</TT
></DT
><DD
><P
> The string <TT
CLASS="LITERAL"
>INSERT</TT
>, <TT
CLASS="LITERAL"
>UPDATE</TT
>,
<TT
CLASS="LITERAL"
>DELETE</TT
>, or <TT
CLASS="LITERAL"
>TRUNCATE</TT
> depending on the type of
trigger event.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$NEW</TT
></DT
><DD
><P
> An associative array containing the values of the new table
row for <TT
CLASS="COMMAND"
>INSERT</TT
> or <TT
CLASS="COMMAND"
>UPDATE</TT
> actions, or
empty for <TT
CLASS="COMMAND"
>DELETE</TT
>. The array is indexed by column
name. Columns that are null will not appear in the array.
This is not set for statement-level triggers.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$OLD</TT
></DT
><DD
><P
> An associative array containing the values of the old table
row for <TT
CLASS="COMMAND"
>UPDATE</TT
> or <TT
CLASS="COMMAND"
>DELETE</TT
> actions, or
empty for <TT
CLASS="COMMAND"
>INSERT</TT
>. 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.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>$args</TT
></DT
><DD
><P
> A Tcl list of the arguments to the procedure as given in the
<TT
CLASS="COMMAND"
>CREATE TRIGGER</TT
> statement. These arguments are also accessible as
<TT
CLASS="LITERAL"
>$1</TT
> ... <TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
> in the procedure body.
</P
></DD
></DL
></DIV
><P>
</P
><P
> The return value from a trigger procedure can be one of the strings
<TT
CLASS="LITERAL"
>OK</TT
> or <TT
CLASS="LITERAL"
>SKIP</TT
>, or a list of column name/value pairs.
If the return value is <TT
CLASS="LITERAL"
>OK</TT
>,
the operation (<TT
CLASS="COMMAND"
>INSERT</TT
>/<TT
CLASS="COMMAND"
>UPDATE</TT
>/<TT
CLASS="COMMAND"
>DELETE</TT
>)
that fired the trigger will proceed
normally. <TT
CLASS="LITERAL"
>SKIP</TT
> 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 <TT
CLASS="LITERAL"
>BEFORE</TT
> <TT
CLASS="COMMAND"
>INSERT</TT
> or <TT
CLASS="COMMAND"
>UPDATE</TT
>
triggers, for which the modified row will be inserted instead of the one
given in <TT
CLASS="VARNAME"
>$NEW</TT
>; or for row-level <TT
CLASS="LITERAL"
>INSTEAD OF</TT
>
<TT
CLASS="COMMAND"
>INSERT</TT
> or <TT
CLASS="COMMAND"
>UPDATE</TT
> triggers where the returned row
is used as the source data for <TT
CLASS="COMMAND"
>INSERT RETURNING</TT
> or
<TT
CLASS="COMMAND"
>UPDATE RETURNING</TT
> clauses.
In row-level <TT
CLASS="LITERAL"
>BEFORE</TT
> <TT
CLASS="COMMAND"
>DELETE</TT
> or <TT
CLASS="LITERAL"
>INSTEAD
OF</TT
> <TT
CLASS="COMMAND"
>DELETE</TT
> triggers, returning a modified row has the same
effect as returning <TT
CLASS="LITERAL"
>OK</TT
>, that is the operation proceeds.
The trigger return value is ignored for all other types of triggers.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> The result list can be made from an array representation of the
modified tuple with the <TT
CLASS="LITERAL"
>array get</TT
> Tcl command.
</P
></BLOCKQUOTE
></DIV
><P
> Here's a little example trigger procedure 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.
</P><PRE
CLASS="PROGRAMLISTING"
>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 PROCEDURE trigfunc_modcount('modcnt');</PRE
><P>
Notice that the trigger procedure itself does not know the column
name; that's supplied from the trigger arguments. This lets the
trigger procedure be reused with different tables.
</P
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