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>Chapter 35. Triggers</TD
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><A
NAME="TRIGGER-INTERFACE"
>35.3. Writing Trigger Functions in C</A
></H1
><A
NAME="AEN31070"
></A
><P
> This section describes the low-level details of the interface to a
trigger function. This information is only needed when writing a
trigger function in C. If you are using a higher-level
language then these details are handled for you. The documentation
of each procedural language explains how to write a trigger in that
language.
</P
><P
> Trigger functions must use the <SPAN
CLASS="QUOTE"
>"version 1"</SPAN
> function manager
interface.
</P
><P
> When a function is called by the trigger manager, it is not passed
any normal arguments, but it is passed a <SPAN
CLASS="QUOTE"
>"context"</SPAN
>
pointer pointing to a <TT
CLASS="STRUCTNAME"
>TriggerData</TT
> structure. C
functions can check whether they were called from the trigger
manager or not by executing the macro
</P><PRE
CLASS="PROGRAMLISTING"
>CALLED_AS_TRIGGER(fcinfo)</PRE
><P>
which expands to
</P><PRE
CLASS="PROGRAMLISTING"
>((fcinfo)->context != NULL && IsA((fcinfo)->context, TriggerData))</PRE
><P>
If this returns true, then it is safe to cast
<TT
CLASS="LITERAL"
>fcinfo->context</TT
> to type <TT
CLASS="LITERAL"
>TriggerData
*</TT
> and make use of the pointed-to
<TT
CLASS="STRUCTNAME"
>TriggerData</TT
> structure. The function must
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> alter the <TT
CLASS="STRUCTNAME"
>TriggerData</TT
>
structure or any of the data it points to.
</P
><P
> <TT
CLASS="STRUCTNAME"
>struct TriggerData</TT
> is defined in
<TT
CLASS="FILENAME"
>commands/trigger.h</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct TriggerData
{
NodeTag type;
TriggerEvent tg_event;
Relation tg_relation;
HeapTuple tg_trigtuple;
HeapTuple tg_newtuple;
Trigger *tg_trigger;
} TriggerData;</PRE
><P>
where the members are defined as follows:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="STRUCTFIELD"
>type</TT
></DT
><DD
><P
> Always <TT
CLASS="LITERAL"
>T_TriggerData</TT
>.
</P
></DD
><DT
><TT
CLASS="STRUCTFIELD"
>tg_event</TT
></DT
><DD
><P
> Describes the event for which the function is called. You may use the
following macros to examine <TT
CLASS="LITERAL"
>tg_event</TT
>:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_BEFORE(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger fired before the operation.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_AFTER(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger fired after the operation.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_FOR_ROW(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger fired for a row-level event.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_FOR_STATEMENT(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger fired for a statement-level event.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_BY_INSERT(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger was fired by an <TT
CLASS="COMMAND"
>INSERT</TT
> command.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_BY_UPDATE(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger was fired by an <TT
CLASS="COMMAND"
>UPDATE</TT
> command.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TRIGGER_FIRED_BY_DELETE(tg_event)</TT
></DT
><DD
><P
> Returns true if the trigger was fired by a <TT
CLASS="COMMAND"
>DELETE</TT
> command.
</P
></DD
></DL
></DIV
><P>
</P
></DD
><DT
><TT
CLASS="STRUCTFIELD"
>tg_relation</TT
></DT
><DD
><P
> A pointer to a structure describing the relation that the trigger fired for.
Look at <TT
CLASS="FILENAME"
>utils/rel.h</TT
> for details about
this structure. The most interesting things are
<TT
CLASS="LITERAL"
>tg_relation->rd_att</TT
> (descriptor of the relation
tuples) and <TT
CLASS="LITERAL"
>tg_relation->rd_rel->relname</TT
>
(relation name; the type is not <TT
CLASS="TYPE"
>char*</TT
> but
<TT
CLASS="TYPE"
>NameData</TT
>; use
<TT
CLASS="LITERAL"
>SPI_getrelname(tg_relation)</TT
> to get a <TT
CLASS="TYPE"
>char*</TT
> if you
need a copy of the name).
</P
></DD
><DT
><TT
CLASS="STRUCTFIELD"
>tg_trigtuple</TT
></DT
><DD
><P
> A pointer to the row for which the trigger was fired. This is
the row being inserted, updated, or deleted. If this trigger
was fired for an <TT
CLASS="COMMAND"
>INSERT</TT
> or
<TT
CLASS="COMMAND"
>DELETE</TT
> then this is what you should return
to from the function if you don't want to replace the row with
a different one (in the case of <TT
CLASS="COMMAND"
>INSERT</TT
>) or
skip the operation.
</P
></DD
><DT
><TT
CLASS="STRUCTFIELD"
>tg_newtuple</TT
></DT
><DD
><P
> A pointer to the new version of the row, if the trigger was
fired for an <TT
CLASS="COMMAND"
>UPDATE</TT
>, and <TT
CLASS="SYMBOL"
>NULL</TT
> if
it is for an <TT
CLASS="COMMAND"
>INSERT</TT
> or a
<TT
CLASS="COMMAND"
>DELETE</TT
>. This is what you have to return
from the function if the event is an <TT
CLASS="COMMAND"
>UPDATE</TT
>
and you don't want to replace this row by a different one or
skip the operation.
</P
></DD
><DT
><TT
CLASS="STRUCTFIELD"
>tg_trigger</TT
></DT
><DD
><P
> A pointer to a structure of type <TT
CLASS="STRUCTNAME"
>Trigger</TT
>,
defined in <TT
CLASS="FILENAME"
>utils/rel.h</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct Trigger
{
Oid tgoid;
char *tgname;
Oid tgfoid;
int16 tgtype;
bool tgenabled;
bool tgisconstraint;
Oid tgconstrrelid;
bool tgdeferrable;
bool tginitdeferred;
int16 tgnargs;
int16 tgattr[FUNC_MAX_ARGS];
char **tgargs;
} Trigger;</PRE
><P>
where <TT
CLASS="STRUCTFIELD"
>tgname</TT
> is the trigger's name,
<TT
CLASS="STRUCTFIELD"
>tgnargs</TT
> is number of arguments in
<TT
CLASS="STRUCTFIELD"
>tgargs</TT
>, and <TT
CLASS="STRUCTFIELD"
>tgargs</TT
> is an array of
pointers to the arguments specified in the <TT
CLASS="COMMAND"
>CREATE
TRIGGER</TT
> statement. The other members are for internal use
only.
</P
></DD
></DL
></DIV
><P>
</P
><P
> A trigger function must return either <TT
CLASS="SYMBOL"
>NULL</TT
> or a
<TT
CLASS="STRUCTNAME"
>HeapTuple</TT
> pointer. Be careful to return either
<TT
CLASS="STRUCTFIELD"
>tg_trigtuple</TT
> or <TT
CLASS="STRUCTFIELD"
>tg_newtuple</TT
>,
as appropriate, if you don't want to modify the row being operated on.
</P
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