All we need is an easy explanation of the problem, so here it is.
In the trigger body, how can I assign a value to
NEW by it’s field name?
That’s what I want to do:
some_key = "some_column"; NEW[some_key] = 5;
How to solve :
I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.
First of all, there is no "trigger body" (unlike Oracle). In Postgres you have a trigger function (also, misleadingly, called "procedure") with a function body and 0-n triggers (without body) calling this function.
The special variable
NEW in plpgsql trigger functions is neither a map nor an array; it’s a record holding the new row:
RECORD; variable holding the new database row for
UPDATEoperations in row-level triggers. This variable is
unassigned in statement-level triggers and for
NEW.some_key := 5;
What you seem to be looking for is to parameterize the column name, which isn’t quite as simple.
The additional module
hstore provides the
#= operator. (It’s included in pretty much all standard distributions.) Install the module once per database with:
CREATE EXTENSION hstore;
Then you can:
NEW := NEW #= '"some_key"=>"5"'::hstore;
Sets the column
some_key to ‘5’ – if the column exists.
- An explicit cast to
hstoreis optional. The operator
#=coerces a string literal to the right data type automatically.
hstoreonly stores text strings, so a given literal for the value may have to be cast twice – a very minor drawback compared to alternative solutions.
- The given string literal has to fit the data type of the column, or an exception is raised.
- If no column with the given name exists, nothing is changed, no exception raised.
Related answer with details and an alternative solution:
CREATE OR REPLACE FUNCTION trg_tbl_ins_bef() RETURNS trigger AS $func$ BEGIN NEW := NEW #= '"some_key"=>"5"'; RETURN NEW; END $func$ LANGUAGE plpgsql; CREATE TRIGGER ins_bef BEFORE INSERT ON tbl FOR EACH ROW EXECUTE PROCEDURE trg_tbl_ins_bef();
Postgres 11 or later allows to replace the misleding term
PROCEDURE (the old syntax will keep working for the foreseeable future):
... FOR EACH ROW EXECUTE FUNCTION trg_tbl_ins_bef();
I must admit that this is no easy way of solving it, but at least it’s a way. I created the below example as a standalone one, to avoid all clutter with trigger creation and such. If you use it in a trigger, you can remove the declaration and initialization of
p and replace the remaining use with
DO $$ DECLARE p members_test; BEGIN p := (1,2,3); CREATE TEMP TABLE t ON COMMIT DROP AS SELECT p.*; -- a one row table holding -- the values of the variable EXECUTE format($e$UPDATE t SET %s = 43$e$, 'b'); -- this way you can access -- the columns dynamically SELECT * INTO p FROM t; -- assign the new values back to the variable RAISE INFO 'p: %', p; END; $$; INFO: p: (1,43,3)
Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂