I'm trying to create a Trigger/Function in Postgres that will check, upon an insert to a table, whether or not there is already another post by a different member with the same content. If there is a post, this function will not insert the new one and leave the table unchanged. Otherwise, it will be added.
So far, the trigger and function look like:
Trigger:
CREATE TRIGGER isPostUnique
AFTER INSERT ON posts
FOR EACH ROW
EXECUTE PROCEDURE deletePost();
Function:
CREATE FUNCTION deletePost() RETURNS TRIGGER AS $isPostUnique$
BEGIN
IF (EXISTS (SELECT * FROM posts p1, posts p2
WHERE (p1.userID <> p2.userID)
AND (p1.content LIKE p2.content)))
THEN
DELETE FROM NEW WHERE (posts.postID = NEW.postID);
RETURN NEW;
END IF;
END;
$isPostUnique$ LANGUAGE plpgsql;
Adding the function and trigger works without any errors, but when I try to run the following query to test it: INSERT INTO posts VALUES (7, 3, 'test redundant post', 10, 1); I get this error
ERROR: relation "new" does not exist
LINE 1: DELETE FROM NEW WHERE (posts.postID = NEW.postID)
^
QUERY: DELETE FROM NEW WHERE (posts.postID = NEW.postID)
CONTEXT: PL/pgSQL function dp() line 7 at SQL statement
I am aware that you can't use 'NEW' in FOR EACH ROW inserts, but I have no other idea of how to accomplish this.
Updated answer for updated question
Of course you can use NEW in FOR EACH ROW trigger function. You just can't direct a DELETE statement at it. It's a row type (data type HeapTuple to be precise), not a table.
To abort the INSERT silently (no exception raised) if the same content is already there ...
CREATE FUNCTION deletePost()
RETURNS TRIGGER AS
$func$
BEGIN
IF EXISTS (
SELECT 1
FROM posts p
WHERE p.content = NEW.content
-- AND p.userID <> NEW.userID -- I doubt you need this, too?
) THEN
RETURN NULL; -- cancel INSERT
ELSE
RETURN NEW; -- go ahead
END IF;
END
$func$ LANGUAGE plpgsql;
Of course this only works for a trigger ...
...
BEFORE INSERT ON posts
...
Unique index
A UNIQUE constraint or a unique index (almost the same effect) might be a superior solution:
CREATE UNIQUE INDEX posts_content_uni_idx (content);
Would raise an exception at the attempt to insert a duplicate value. No trigger necessary.
It also provides the very well needed index to speed up things.
Related
Is there any variant to create a trigger before insert and if the value is a value that exists already in table, just update it. I know about 'ON DUBLICATE KEY' or a 'ON CONFLICT' in PostgreSQL but I need a trigger just because it's a task in my university.
I tried to create it, but I get just an error about duplicate keys.
CREATE or replace FUNCTION trigger_function()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS $$
BEGIN
IF new.name in (select name from "Test")
then
update "Test" set intt = new.intt where name = new.name;
end if ;
return new;
END;
$$
CREATE TRIGGER trigger_name
BEFORE insert
ON "Test"
for each row
EXECUTE PROCEDURE trigger_function();
Is it possible to create such trigger?
Read the docs plpgsql trigger function:
Row-level triggers fired BEFORE can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the INSERT/UPDATE/DELETE does not occur for this row).
So RETURN NULL after the UPDATE statement inside the if andRETURN NEW when the if is false. Then the INSERT will happen if the name is unique.
I have a very simple trigger, that updates a column in table_a whenever table_b is updated:
create or replace function trigger_update_status()
returns trigger as
$body$
begin
-- UPDATE TABLE A
update table_a
set a_status = new.b_status,
date_updated = now()
from table_b
where table_a.serial_number = table_b.serial_number;
return new;
end;
$body$
language plpgsql;
create trigger "currentStatus" after update or insert on table_b
for each row
execute procedure trigger_update_status();
However, I am getting the error that there is no RETURN value:
ERROR: control reached end of trigger procedure without RETURN
I'm confused on whether or not NEW is appropriate here as I've been reading conflicting information.
On the one hand, the answer here (Postgres trigger after insert accessing NEW) makes it clear that: "The return value of a row-level trigger fired AFTER or a statement-level trigger fired BEFORE or AFTER is always ignored; it might as well be null. However, any of these types of triggers might still abort the entire operation by raising an error."
On the other hand, my trigger here essentially matches the one here (https://dba.stackexchange.com/questions/182678/postgresql-trigger-to-update-a-column-in-a-table-when-another-table-gets-inserte), which calls NEW & AFTER together. So I am not sure why mine is not working. Any help is very greatly appreciated!
Future answer for anybody with same issue: it was a second (similarly named, which is why I didn't catch it until much later) trigger that was called AFTER this one that was causing the issue. The second trigger could not RETURN NEW because in this case, there was no new value. I fixed this by adding an IF/ELSE statement to my second trigger:
IF NEW.current_location <> OLD.current_location
THEN
INSERT INTO table_x(serial_number, foo, bar)
VALUES(OLD.serial_number, OLD.foo, old.bar);
return new;
-- ADDED THIS LINE:
else return null;
END IF;
Lesson learned thanks to #sticky bit - if the trigger works in an isolated dbfiddle, it's something else causing the issue.
I want to check if the id I want to insert into tableA exists in tableB into an if statement
Can I do something like this
if new.id exists (select id from tableB where stat = '0' ) then
some code here
end if;
When I try this I get an error message, any thoughts?
Why not do it like this? I'm not very knowledgeable about PostgreSQL but this would work in T-SQL.
INSERT INTO TargetTable(ID)
SELECT ID
FROM TableB
WHERE ID NOT IN (SELECT DISTINCT ID FROM TargetTable)
This is usually done with a trigger. A trigger function does the trick:
CREATE FUNCTION "trf_insert_tableA"() RETURNS trigger AS $$
BEGIN
PERFORM * FROM "tableB" WHERE id = NEW.id AND stat = '0';
IF FOUND THEN
-- Any additional code to go here, optional
RETURN NEW;
ELSE
RETURN NULL;
END IF;
END; $$ LANGUAGE plpgsql;
CREATE TRIGGER "tr_insert_tableA"
BEFORE INSERT ON "tableA"
FOR EACH ROW EXECUTE PROCEDURE "trf_insert_tableA"();
A few notes:
Identifiers in PostgreSQL are case-insensitive. PostgreSQL by default makes them lower-case. To maintain the case, use double-quotes. To make your life easy, use lower-case only.
A trigger needs a trigger function, this is always a two-step affair.
In an INSERT trigger, you can use the NEW implicit parameter to access the column values that are attempted to be inserted. In the trigger function you can modify these values and those values are then inserted. This only works in a BEFORE INSERT trigger, obviously; AFTER INSERT triggers are used for side effects such as logging, auditing or cascading inserts to other tables.
The PERFORM statement is a special form of a SELECT statement to test for the presence of data; it does not return any data, but it does set the FOUND implicit parameter that you can use in a conditional statement.
Depending on your logic, you may want the insert to succeed or to fail. RETURN NEW to make the insert succeed, RETURN NULL to make it fail.
After you defined the trigger, you can simply issue an INSERT statement: the trigger function is invoked automatically.
Presumably, you want something like this:
if exists (select 1 from tableB b where stat = '0' and b.id = new.id) then
some code here
end if;
I want to write a Postgres SQL trigger that will basically find if a number appears in a column 5 or more times. If it appears a 5th time, I want to throw an exception. Here is how the table looks:
create table tab(
first integer not null constraint pk_part_id primary key,
second integer constraint fk_super_part_id references bom,
price integer);
insert into tab values(1,NULL,100), (2,1,50), (3,1,30), (4,2,20), (5,2,10), (6,3,20);
Above are the original inserts into the table. My trigger will occur upon inserting more values into the table.
Basically if a number appears in the 'second' column more than 4 times after inserting into the table, I want to raise an exception. Here is my attempt at writing the trigger:
create function check() return trigger as '
begin
if(select first, second, price
from tab
where second in (
select second from tab
group by second
having count(second) > 4)
) then
raise exception ''Error, there are more than 5 parts.'';
end if;
return null;
end
'language plpgsql;
create trigger check
after insert or update on tab
for each row execute procedure check();
Could anyone help me out? If so that would be great! Thanks!
CREATE FUNCTION trg_upbef()
RETURN trigger as
$func$
BEGIN
IF (SELECT count(*)
FROM tab
WHERE second = NEW.second ) > 3 THEN
RAISE EXCEPTION 'Error: there are more than 5 parts.';
END IF;
RETURN NEW; -- must be NEW for BEFORE trigger
END
$func$ LANGUAGE plpgsql;
CREATE TRIGGER upbef
BEFORE INSERT OR UPDATE ON tab
FOR EACH ROW EXECUTE procedure trg_upbef();
Major points
Keyword is RETURNS, not RETURN.
Use the special variable NEW to refer to the newly inserted / updated row.
Use a BEFORE trigger. Better skip early in case of an exception.
Don't count everything for your test, just what you need. Much faster.
Use dollar-quoting. Makes your live easier.
Concurrency:
If you want to be absolutely sure, you'll have to take an exclusive lock on the table before counting. Else, concurrent inserts / updates might outfox each other under heavy concurrent load. While this is rather unlikely, it's possible.
I am trying to create a trigger, so that when ever I add a new record it adds another record in the same table. The session field will only take values between 1 and 4. So when I add a 1 in session I want it to add another record but with session 3 blocked. But the problem is that it leads to cascading triggers and it inserts itself again and again because the trigger is triggered when inserted.
I have for example a simple table:
CREATE TABLE example
(
id SERIAL PRIMARY KEY
,name VARCHAR(100) NOT NULL
,session INTEGER
,status VARCHAR(100)
);
My trigger function is:
CREATE OR REPLACE FUNCTION add_block() RETURNS TRIGGER AS $$
BEGIN
INSERT INTO example VALUES (NEW.id + 1, NEW.name, NEW.session+2, 'blocked');
RETURN NULL;
END;
$$ LANGUAGE 'plpgsql';
Trigger is:
CREATE TRIGGER add_block
AFTER INSERT OR UPDATE
ON example
FOR EACH ROW
EXECUTE PROCEDURE add_block();
I get error:
SQL statement "INSERT INTO example VALUES ( $1 +1, $2 , $3 + 2, $4)"
PL/pgSQL function "add_block" line 37 at SQL statement
This error repeats itself so many times that I can't see the top.
How would I solve this?
EDIT:
CREATE TABLE block_rules
(
id SERIAL PRIMARY KEY
,session INTEGER
,block_session INTEGER
);
This table holds the block rules. So if a new record is inserted into the EXAMPLE table with session 1 then it blocks session 3 accordingly by inserting a new record with blocked status in the same (EXAMPLE) table above (not block_rules). Same for session 2 but it blocks session 4.
The block_rules table holds the rules (or pattern) to block a session by. It holds
id | session | block_session
------------------------------
1 | 1 | 3
2 | 2 | 4
3 | 3 | 2
How would I put that in the WHEN statement of the trigger going with Erwin Branstetter's answer below?
Thanks
New answer to edited question
This trigger function adds blocked sessions according to the information in table block_rules.
I assume that the tables are linked by id - information is missing in the question.
I now assume that the block rules are general rules for all sessions alike and link by session. The trigger is only called for non-blocked sessions and inserts a matching blocked session.
Trigger function:
CREATE OR REPLACE FUNCTION add_block()
RETURNS TRIGGER AS
$BODY$
BEGIN
INSERT INTO example (name, session, status)
VALUES (NEW.name
,(SELECT block_session
FROM block_rules
WHERE session = NEW.session)
,'blocked');
RETURN NULL;
END;
$BODY$ LANGUAGE plpgsql;
Trigger:
CREATE TRIGGER add_block
AFTER INSERT -- OR UPDATE
ON example
FOR EACH ROW
WHEN (NEW.status IS DISTINCT FROM 'blocked')
EXECUTE PROCEDURE add_block();
Answer to original question
There is still room for improvement. Consider this setup:
CREATE OR REPLACE FUNCTION add_block()
RETURNS TRIGGER AS
$BODY$
BEGIN
INSERT INTO example (name, session, status)
VALUES (NEW.name, NEW.session + 2, 'blocked');
RETURN NULL;
END;
$BODY$ LANGUAGE plpgsql;
CREATE TRIGGER add_block
AFTER INSERT -- OR UPDATE
ON example
FOR EACH ROW
WHEN (NEW.session < 3)
-- WHEN (status IS DISTINCT FROM 'blocked') -- alternative guess at filter
EXECUTE PROCEDURE add_block();
Major points:
For PostgreSQL 9.0 or later you can use a WHEN condition in the trigger definition. This would be most efficient. For older versions you use the same condition inside the trigger function.
There is no need to add a column, if you can define criteria to discern auto-inserted rows. You did not tell, so I assume that only auto-inserted rows have session > 2 in my example. I added an alternative WHEN condition for status = 'blocked' as comment.
You should always provide a column list for INSERTs. If you don't, later changes to the table may have unexpected side effects!
Do not insert NEW.id + 1 in the trigger manually. This won't increment the sequence and the next INSERT will fail with a duplicate key violation.
id is a serial column, so don't do anything. The default nextval() from the sequence is inserted automatically.
Your description only mentions INSERT, yet you have a trigger AFTER INSERT OR UPDATE. I cut out the UPDATE part.
The keyword plpgsql doesn't have to be quoted.
OK so can't you just add another column, something like this:
ALTER TABLE example ADD COLUMN trig INTEGER DEFAULT 0;
CREATE OR REPLACE FUNCTION add_block() RETURNS TRIGGER AS $$
BEGIN
IF NEW.trig = 0 THEN
INSERT INTO example VALUES (NEXTVAL('example_id_seq'::regclass), NEW.name, NEW.session+2, 'blocked', 1);
END IF;
RETURN NULL;
END;
$$ LANGUAGE 'plpgsql';
it's not great, but it works :-)
CREATE OR REPLACE FUNCTION add_block() RETURNS TRIGGER AS $$
BEGIN
SET SESSION session_replication_role = replica;
INSERT INTO example VALUES (NEXTVAL('example_id_seq'::regclass), NEW.name, NEW.session+2, 'blocked');
SET SESSION session_replication_role = origin;
RETURN NULL;
END;
$$ LANGUAGE 'plpgsql';