Dynamic SELECT in trigger function gives syntax error - postgresql

Question

Just need help as to debug some syntax errors in the below code.The code is as below and have only couple of syntax errors near keywords like Insert, Select, etc
CREATE OR REPLACE FUNCTION audit_temp() RETURNS TRIGGER LANGUAGE plpgsql AS $BODY$
DECLARE
ri RECORD;
oldValue TEXT;
newValue TEXT;
isColumnSignificant BOOLEAN;
isValueModified BOOLEAN;
BEGIN
IF TG_OP = 'INSERT' OR TG_OP = 'UPDATE' THEN
NEW.record_modified_ = clock_timestamp();
FOR ri IN
-- Fetch a ResultSet listing columns defined for this trigger's table.
SELECT ordinal_position, column_name, data_type
FROM information_schema.columns
WHERE table_schema = quote_ident(TG_TABLE_SCHEMA)
AND table_name = quote_ident(TG_TABLE_NAME)
ORDER BY ordinal_position
LOOP
-- For each column in this trigger's table, copy the OLD & NEW values into respective variables.
-- NEW value
EXECUTE 'SELECT ($1).' || ri.column_name || '::text' INTO newValue USING NEW;
-- OLD value
IF TG_OP = 'INSERT' THEN -- If operation is an INSERT, we have no OLD value, so use an empty string.
oldValue := ''::varchar;
ELSE -- Else operation is an UPDATE, so capture the OLD value.
EXECUTE 'SELECT ($1).' || ri.column_name || '::text' INTO oldValue USING OLD;
END IF;
isColumnSignificant := (position( '_x_' in ri.column_name ) < 1) AND
(ri.column_name <> 'pkey_') AND
(ri.column_name <> 'record_modified_');
IF isColumnSignificant THEN
isValueModified := oldValue <> newValue; -- If this nthField in the table was modified, make history.
IF isValueModified THEN
/*RAISE NOTICE E'Inserting history_ row for INSERT or UPDATE.\n';*/
INSERT INTO audit_temp( operation_, table_oid_, table_name_, uuid_, column_name_, ordinal_position_of_column_, old_value_, new_value_ )
VALUES ( TG_OP, TG_RELID, TG_TABLE_NAME, NEW.pkey_, ri.column_name::VARCHAR, ri.ordinal_position, oldValue::VARCHAR, newValue::VARCHAR);
END IF;
END IF;
END LOOP;
RETURN NEW;
ELSIF TG_OP = 'DELETE' THEN
/*RAISE NOTICE E'Inserting history_ row for DELETE.\n';*/
-- Similar to INSERT above, but refers to OLD instead of NEW, and passes empty values for last 4 fields.
INSERT INTO audit_temp ( operation_, table_oid_, table_name_, uuid_, column_name_, ordinal_position_of_column_, old_value_, new_value_ )
VALUES ( TG_OP, TG_RELID, TG_TABLE_NAME, OLD.pkey_, ''::VARCHAR, 0, ''::VARCHAR, ''::VARCHAR );
RETURN OLD;
END IF;
/* Should never reach this point. Branching in code above should always reach a call to RETURN. */
RAISE EXCEPTION 'Unexpectedly reached the bottom of this function without calling RETURN.';
END; $BODY$;
The error is as follows & mostly around Select Insert keywords only:
>[Error] Script lines: 1-42 -------------------------
ERROR: syntax error at or near "SELECT"
Any suggestions?????

Answer 1

Syntax error
The offending statement is this (and the others like it):
EXECUTE 'SELECT ($1).' || ri.column_name || '::text' INTO newValue USING NEW;
According to the documentation parameter symbols can only be used for data values — if you want to use dynamically determined table or column names, you must insert them into the command string textually. So the solution would be:
EXECUTE 'SELECT (' || NEW || ').' || ri.column_name || '::text' INTO newValue;
Improvements
You can make a few improvements to your trigger function to make it faster and more efficient:
You should check isColumnSignficant before you populate oldValue and newValue.
When updating, you do not have to record the OLD values: they are already in the audit table when the data was first inserted or later updated.
When deleting, don't store empty strings and 0, just leave the columns NULL.