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Inspired by my co-worker's recent blog post, I decided to revisit the old question of rules vs. triggers and run a little benchmark to see which one does better.
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.
We have a simple table:
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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:
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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:
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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; |
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.
For this measurement, which I call “rule”, I'll use the following rule to log updates:
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CREATE RULE upd_rule AS ON UPDATE TO upd DO ALSO INSERT INTO log (id, new_val) VALUES (NEW.id, NEW.val); |
For this measurement, which I call “trig_row”, I'll use the following trigger to log updates:
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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(); |
For this measurement, which I call “trig_stmt”, I'll use the following trigger to log updates:
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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.
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.
The zero measurements have been subtracted and the result divided by twenty to obtain the following numbers:
| baseline | rule | row level trigger | statement level trigger | |
|---|---|---|---|---|
| execution time | 295 ms | 454 ms | 3816 ms | 642 ms |
| overhead percentage | 0 % | 54 % | 1193 % | 117 % |
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.
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Very useful and informative article, Thank you.
Note that this kind of usage of rules may result in unexpected behavior. For instance a simple query like
update upd set val = val || random();will result in different values actually written tovaland logged intologtable.You are absolutely right.
Rules are not easy to use correctly, particularly with data modifications. That is why it is recommended to use triggers.
I was only out to compare performance, not usability.
What about using logical decoding to retrieve logs?
https://www.postgresql.org/docs/9.5/logicaldecoding.html
Good read -- thanks much!
Thank you for posting this Benchmark.
for each rowhas one saving grace:when. For example,for each row when (new.val = 'bla')seems to outperform rules.