PostgreSQL plpgsql - variable column names - postgresql

Question

I am creating a trigger, which uses dynamic names for columns
NEW.name:=2222; -- works fine !
but
dynamic_column:='name';
EXECUTE '$1.'||dynamic_column||':=2222 ' USING NEW; -- raises error
gives an error:
ERROR: syntax error at or near "$1" LINE 1: $1.name:=2222

Answer 1

I found info here: Assign to NEW by key in a Postgres trigger
If we enable the module hstore by:
CREATE EXTENSION hstore;
We can do this:
dynamic_column:='name';
temp_sql_string:='"'||dynamic_column||'"=>"2222"';
NEW := NEW #= temp_sql_string::hstore;
And the RECORD NEW.name now is set to the value 2222.
Thank you tough for making an effort to find a solution #Laurenz Albe

Answer 2

The problem is that this is not a valid SQL statement.
You can access the columns in new with dynamic SQL like this:
EXECUTE 'SELECT $1.id' INTO v_id USING NEW;
There is no comfortable way like that for changing individual columns in NEW.
You could use TG_RELID to get the OID of the table, query pg_attribute for the columns, compose a row literal string composed of the values in NEW and your new value, cast this to the table type and assign the result to NEW. Quite cumbersome.
Here is sample code that does that (I tested it, but there may be bugs left):
CREATE OR REPLACE FUNCTION dyntrig() RETURNS trigger
LANGUAGE plpgsql AS
$$DECLARE
colname text;
colval text;
newrow text := '';
fieldsep text := 'ROW(';
BEGIN
/* loop through the columns of the table */
FOR colname IN
SELECT attname
FROM pg_catalog.pg_attribute
WHERE attrelid = TG_RELID
AND attnum > 0
AND NOT attisdropped
ORDER BY attnum
LOOP
IF colname = 'name' THEN
colval = '2222';
ELSE
/* all other columns than 'name' retain their value */
EXECUTE 'SELECT CAST($1.' || quote_ident(colname) || ' AS text)'
INTO colval USING NEW;
END IF;
/* compose a string that represents the new table row */
IF colval IS NULL THEN
newrow := newrow || fieldsep || 'NULL';
ELSE
newrow := newrow || fieldsep || '''' || colval || '''';
END IF;
fieldsep := ',';
END LOOP;
newrow := newrow || ')';
/* assign the new table row to NEW */
EXECUTE 'SELECT (CAST(' || newrow || ' AS '
|| quote_ident(TG_TABLE_SCHEMA) || '.' || quote_ident(TG_TABLE_NAME)
|| ')).*'
INTO NEW;
RETURN NEW;
END;$$;

Answer 3

You already found my answer recommending the hstore operator #= on dba.SE. You may also be interested in the corresponding reference answer here on SO:
How to set value of composite variable field using dynamic SQL
Since you construct the auxiliary hstore value from variables I suggest the simple function hstore():
CREATE OR REPLACE FUNCTION dyn_trigger_func()
RETURNS TRIGGER AS
$func$
DECLARE
dyn_col_name text := 'name';
dyn_col_val text := '2222';
BEGIN
NEW := NEW #= hstore(dyn_col_name, dyn_col_val);
RETURN NEW;
END
$func$ LANGUAGE plpgsql;
Faster / simpler / clearer / more secure this way.
Or, since it's obviously a trigger function, you may want to pass column name and value in CREATE TRIGGER statements:
CREATE OR REPLACE FUNCTION dyn_trigger_func()
RETURNS TRIGGER AS
$func$
BEGIN
NEW := NEW #= hstore(TG_ARGV[0], TG_ARGV[1]);
RETURN NEW;
END
$func$ LANGUAGE plpgsql;
And:
CREATE TRIGGER ins_bef
BEFORE INSERT ON tbl
FOR EACH ROW EXECUTE PROCEDURE dyn_trigger_func('name', '2222');
Provide column name unquoted and case-sensitive.
Related:
Get values from varying columns in a generic trigger
Trigger with dynamic field name