Check if trigger exists - postgresql

Question

I have the following query to triggers on all tables in schema public:
SELECT 'CREATE TRIGGER ' || tab_name|| '_if_modified_trg INSERT OR UPDATE OR DELETE ON ' || tab_name|| ' FOR EACH ROW EXECUTE PROCEDURE audit.if_modified_func(); ' AS trigger_creation_query
FROM (
SELECT quote_ident(table_schema) || '.' || quote_ident(table_name) as tab_name
FROM information_schema.tables
WHERE table_schema='public'
) AS foo;
And I know how to check if a trigger exists:
SELECT tgname
from pg_trigger
where not tgisinternal AND tgname='randomname'
But how can I check in the first query whether a trigger with the same name already exists - and skip creating it and just continue? Here is my solution but it doesn't work:
SELECT 'CREATE TRIGGER ' || tab_name|| '_if_modified_trg INSERT OR UPDATE OR DELETE ON ' || tab_name|| ' FOR EACH ROW EXECUTE PROCEDURE audit.if_modified_func(); ' AS trigger_creation_query
FROM (
SELECT quote_ident(table_schema) || '.' || quote_ident(table_name) as tab_name
FROM information_schema.tables
WHERE table_schema='public'
) AS foo
WHERE tab_name||'if_modified_trg' NOT IN (
SELECT tgname
from pg_trigger
where not tgisinternal );

Answer 1

Using this you can check if the trigger exists and create it if not. Don't forget the last ";".
DO $$
BEGIN
IF NOT EXISTS (SELECT 1 FROM pg_trigger WHERE tgname = 'randomname') THEN
CREATE TRIGGER randomname
AFTER INSERT OR UPDATE OR DELETE ON randomtable
FOR EACH ROW EXECUTE PROCEDURE randomfunction();
END IF;
END
$$;
I hope this can help you.

Answer 2

You can use a DO statement or a plpgsql function to execute the trigger creation conditionally:
DO
$do$
BEGIN
IF EXISTS (
SELECT 1
FROM pg_trigger
WHERE NOT tgisinternal AND tgname = 'randomname'
) THEN
-- do nothing
ELSE
-- create trigger
END IF;
END
$do$
On closer inspection, the rest of your code also has various problems.
Seems like you are trying to do this:
DO
$do$
DECLARE
_tbl regclass;
_trg text;
BEGIN
FOR _tbl, _trg IN
SELECT c.oid::regclass, relname || '_if_modified_trg'
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE n.nspname = 'public'
AND c.relkind = 'r' -- only regular tables
LOOP
IF EXISTS (
SELECT
FROM pg_trigger
WHERE tgname = _trg
AND tgrelid = _tbl -- check only for respective table
) THEN
-- do nothing
ELSE
-- create trigger
EXECUTE format(
'CREATE TRIGGER %I
BEFORE INSERT OR UPDATE OR DELETE ON %s
FOR EACH ROW EXECUTE PROCEDURE audit.if_modified_func()'
, _trg, _tbl::text
);
END IF;
END LOOP;
END
$do$;
I use the system catalog pg_class instead of information_schema.tables for multiple reasons. Most importantly, it includes the oid of the table, which makes the check on pg_trigger simpler and less error-prone.
We can actually simplify further and check for existence of the trigger in the same query. Substantially faster, yet:
DO
$do$
DECLARE
_tbl text;
_trg text;
BEGIN
FOR _tbl, _trg IN
SELECT c.oid::regclass::text, relname || '_if_modified_trg'
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
LEFT JOIN pg_trigger t ON t.tgname = c.relname || '_if_modified_trg'
AND t.tgrelid = c.oid -- check only respective table
WHERE n.nspname = 'public'
AND c.relkind = 'r' -- only regular tables
AND t.tgrelid IS NULL -- trigger does not exist yet
LOOP
EXECUTE format(
'CREATE TRIGGER %I
BEFORE INSERT OR UPDATE OR DELETE ON %s
FOR EACH ROW EXECUTE PROCEDURE audit.if_modified_func()'
, _trg_name, _tbl_oid::text
);
END LOOP;
END
$do$;
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