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;
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1I know that, thats why I raised this question. There is a hacky way to access variable from NEW dynamically like this EXECUTE 'SELECT ($1).'||column_name||' ;' INTO some_var USING NEW; but I cant assign to NEW like that. @dezsoBen– Ben2014-11-06 09:19:09 +00:00Commented Nov 6, 2014 at 9:19
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And @dezso , whenever someone calls map an array - my heart bleedsBen– Ben2014-11-06 09:21:24 +00:00Commented Nov 6, 2014 at 9:21
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Yes, but can you access in postgres array by key? Im just saying that some scripts don't show difference between array and map, and I know people that programm such scripts and don't know the difference between those two.Ben– Ben2014-11-06 09:27:38 +00:00Commented Nov 6, 2014 at 9:27
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Thanks for your comments @dezso, I guess you just can't achieve this in postgres.Ben– Ben2014-11-06 09:37:08 +00:00Commented Nov 6, 2014 at 9:37
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2 Answers
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:
NEWData type
RECORD; variable holding the new database row forINSERT/UPDATEoperations in row-level triggers. This variable is unassigned in statement-level triggers and forDELETEoperations.
Assigning to a field (or column) of NEW is simple. The documented assignment operator is :=. (Since Postgres 9.4 also =.)
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:
Code example
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();
answered Nov 6, 2014 at 10:12
Erwin Brandstetter
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I haven't thought about
hstore, it's much simpler than my approach.András Váczi– András Váczi2014-11-06 10:15:10 +00:00Commented Nov 6, 2014 at 10:15 -
2Check out the linked answer. Pavel and I worked hard on a solution before the
#=operator was introduced - and found one, too.Erwin Brandstetter– Erwin Brandstetter2014-11-06 10:17:21 +00:00Commented Nov 6, 2014 at 10:17 -
I didn't said NEW is a map or array, writing this NEW[some_key] = 5; I was just giving a hint what I want to achieve. The thing with hstore is genius, would never think of it by myself.Ben– Ben2014-11-06 10:47:16 +00:00Commented Nov 6, 2014 at 10:47
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 NEW.
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)
