Execute dynamic prepared statement inside a trigger - postgresql

Question

I want to use my trigger for logging purposes - every time, when a user inserts something or makes updates, there should appear a new row in my history table. I already have a trigger, which works good. Now it runs every time, when I do inserts. This is how it looks like:
CREATE OR REPLACE FUNCTION public.trigger_history_insert()
RETURNS trigger AS
$BODY$
DECLARE
...
BEGIN
... it does a lot of things here and works absolutely correctly
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION public.trigger_history_insert()
OWNER TO postgres;
Now I want to add logging into this trigger. I tried to follow these two official tutorials ([1], [2]), but ended in failure. This was my first attempt:
DECLARE
...
BEGIN
... everything remains unchanged except this part
EXEC SQL BEGIN DECLARE SECTION;
const char *stmt = 'INSERT INTO history (field1) VALUES (?)';
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE mystmt FROM :stmt;
EXEC SQL EXECUTE mystmt USING new.field1;
END;
In this case I get syntax error pointing to this line of code:
EXEC SQL BEGIN DECLARE SECTION;
^
This was my second attempt:
BEGIN
... everything remains unchanged except this part
BEGIN DECLARE SECTION;
const char *stmt = 'INSERT INTO history (field1) VALUES (?)';
END DECLARE SECTION;
PREPARE mystmt FROM :stmt;
EXECUTE mystmt USING new.field1;
END;
Now, I get an error message pointing to this line:
BEGIN DECLARE SECTION;
^
This was my third attempt:
$BODY$
DECLARE
...
const char *stmt = 'INSERT INTO history (field1) VALUES (?)'
BEGIN
...
PREPARE mystmt FROM :stmt;
EXECUTE mystmt USING new.field1;
END
Now I get an error message pointing to this line:
const char *stmt = 'INSERT INTO history ...
^
This was my last attempt:
$BODY$
DECLARE
...
stmt text := 'INSERT INTO history (field1) VALUES (?)'
BEGIN
...
PREPARE mystmt FROM :stmt;
EXECUTE mystmt USING new.field1;
END
And in this final case, the error message points to this line:
PREPARE mystmt FROM :stmt;
^
So, what am I doing wrong and how can I fix it?

Answer 1

You are mixing up PL/pgSQL, a language for writing functions in the database, and ecpg, which is used for embedding database access in C client code.
The correct solution would look like this:
INSERT INTO history (field1) VALUES (NEW.field1);