PostgreSQL : RULES OR TRIGGERS TO LOG BULK UPDATES?

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About rules

While triggers are well known to most application developers and database administrators, rules are less well known. The full name “query rewrite rule” explains what they are doing: Before the query is optimized, a rule can either replace the query with a different one or add additional queries. These are then planned and executed instead of or together with the original query.

While rules are very powerful, they are also tricky to get right, particularly when data modification is involved. It is usually recommended to use the more modern triggers, and there have been attempts to deprecate them. But since rules are used to implement views, they are probable here to stay.

Moreover, they are said to be faster than triggers when many rows are affected. That’s what I tried to measure.

The test case

We have a simple table:

CREATE UNLOGGED TABLE upd (
   id  integer NOT NULL,
   val text    NOT NULL
) WITH (autovacuum_enabled = off);

Since we are only interested in the speed of processing, I decided to use an unlogged table to bypass disk I/O as much as possible. Autovacuum is disabled to avoid inferences with it.
Every update to this table should be logged to a second table:

CREATE UNLOGGED TABLE log (
   id       integer     NOT NULL,
   log_time timestamptz NOT NULL
            DEFAULT current_timestamp,
   new_val  text        NOT NULL
) WITH (autovacuum_enabled = off);

The test will repeatedly perform the following in a loop:

TRUNCATE upd;
TRUNCATE log;
 
INSERT INTO upd (id, val)
   SELECT i, 'text number ' || i
   FROM generate_series(1, 100000) i;
 
VACUUM (ANALYZE) upd;
 
UPDATE upd SET val = val;

baseline

I’ll perform two baseline measurements: one without the UPDATE (called “zero”), and one with just the UPDATE, but no logging (called “base”). This will enable me to measure the net time spent in the UPDATE by subtraction so I can calculate how much performance is slowed down by the different logging techniques.

logging with a rule

For this measurement, which I call “rule”, I’ll use the following rule to log updates:

CREATE RULE upd_rule AS ON UPDATE TO upd
   DO ALSO INSERT INTO log (id, new_val)
           VALUES (NEW.id, NEW.val);

logging with a row level trigger

For this measurement, which I call “trig_row”, I’ll use the following trigger to log updates:

CREATE FUNCTION upd_row_trig() RETURNS trigger
   LANGUAGE plpgsql AS
$$BEGIN
   INSERT INTO log (id, new_val)
   VALUES (NEW.id, NEW.val);
 
   RETURN NEW;
END;$$;
 
CREATE TRIGGER upd_row_trig
   AFTER UPDATE ON upd FOR EACH ROW
   EXECUTE PROCEDURE upd_row_trig();

logging with a statement level trigger

For this measurement, which I call “trig_stmt”, I’ll use the following trigger to log updates:

CREATE FUNCTION upd_stmt_trig() RETURNS trigger
   LANGUAGE plpgsql AS
$$BEGIN
   INSERT INTO log (id, new_val)
   SELECT id, val FROM newrows;
 
   RETURN NEW;
END;$$;
 
CREATE TRIGGER upd_row_trig AFTER UPDATE ON upd
   REFERENCING NEW TABLE AS newrows FOR EACH STATEMENT
   EXECUTE PROCEDURE upd_stmt_trig();

This uses transition tables which are new in PostgreSQL v10. The REFERENCING clause will make all rows modified by the statement available in a “temporary” table newrows.

Test environment

All commands are run from a shell script on my Linux laptop using psql -c "...". The script loops through the statements twenty times, and the elapsed time is measured with time.
Each test is executed three times to ensure that the execution time does not vary, then the middle value is used.

Test results

The zero measurements have been subtracted and the result divided by twenty to obtain the following numbers:

Performance of rule and triggers for logging bulk updates
	baseline	rule	row level trigger	statement level trigger
execution time	295 ms	454 ms	3816 ms	642 ms
overhead percentage	0 %	54 %	1193 %	117 %

Discussion

The test confirms that rules perform better than anything else when it comes to logging bulk updates. However, while logging with row level triggers caused a slowdown by a forbidding factor of 13, statement level triggers with transition tables only doubled the execution time.
So with PostgreSQL v10 we have a good alternative that performs half as well as rules, and there is no need to use rules unless you need cutting edge performance.

Thursday, September 17, 2020