PostgreSQL log trigger optimalization - postgresql

Question

I spent a lot of time trying to optimize our pgsql log trigger which started to be a problem. I did huge progress (from 18min to 2.5min by inserting 3M rows) but I would like to know if some pgSql masters will be able to do it even better.
CREATE OR REPLACE FUNCTION table_log_trig()
RETURNS trigger AS
$BODY$
DECLARE
col TEXT; -- Single column name to save
newVal TEXT; -- New value for column
oldVal TEXT; -- Old value for column
colLimit TEXT[]; -- Columns that should be logged
BEGIN
IF TG_ARGV[0] IS NOT NULL THEN
-- Trigger specifies columns to log
SELECT array_agg(unnest)
FROM unnest(string_to_array(TG_ARGV[0], ','))
INTO colLimit;
ELSE
-- Trigger with no params. Log all columns
SELECT array_agg(json_object_keys)
FROM json_object_keys(row_to_json(NEW))
WHERE json_object_keys NOT IN ('id', 'created_at', 'updated_at') -- Exceptions
INTO colLimit;
END IF;
-- Loop over columns that should be saved in log
FOREACH col IN ARRAY colLimit
LOOP
-- INSERT & UPDATE
EXECUTE 'SELECT ($1).' || col || '::text' INTO newVal USING NEW;
-- UPDATE
IF TG_OP = 'UPDATE' THEN
EXECUTE 'SELECT ($1).' || col || '::text' INTO oldVal USING OLD;
END iF;
-- Add only new or changed data
IF
newVal != oldVal OR
(oldVal IS NULL AND newVal IS NOT NULL) OR
(oldVal IS NOT NULL AND newVal IS NULL)
THEN
INSERT INTO tab_logs (record_id, field_name, old_value, new_value, created_at, created_by, action)
VALUES (NEW.id, col, oldVal, newVal, NOW(), 999, 'O');
END IF;
END LOOP;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

Answer 1

row_to_json() returns both column names and values; you may as well make use of these values, rather than extracting them later via dynamic SQL.
I haven't thoroughly tested this, let alone benchmarked it, but here's the gist of it:
CREATE OR REPLACE FUNCTION table_log_trig() RETURNS trigger AS
$$
DECLARE
OldJson JSONB = NULL;
BEGIN
IF TG_OP <> 'INSERT' THEN
OldJson := to_jsonb(old);
END IF;
INSERT INTO tab_logs (record_id, field_name, old_value, new_value, created_at, created_by, action)
SELECT new.id, key, OldValues.value, NewValues.value, now(), 999, 'O'
FROM jsonb_each(to_jsonb(new)) NewValues
LEFT JOIN jsonb_each(OldJson) OldValues USING (key)
WHERE
(
(TG_ARGV[0] IS NULL AND key NOT IN ('id', 'created_at', 'updated_at')) OR
(TG_ARGV[0] IS NOT NULL AND key = ANY(string_to_array(TG_ARGV[0], ',')))
) AND
OldValues.value::text IS DISTINCT FROM NewValues.value::text;
RETURN NULL;
END
$$
LANGUAGE plpgsql VOLATILE;