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Rules or triggers to log bulk updates?

07.2018 / Category: / Tags: | |
Triggers with transition tables rule!
 © Laurenz Albe 2018

 

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.

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:

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:

The test will repeatedly perform the following in a loop:

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:

logging with a row level trigger

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

logging with a statement level trigger

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

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.

 


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Wellspring
Wellspring
4 years ago

Good read -- thanks much!

Chris Savvopoulos
Chris Savvopoulos
1 year ago

Thank you for posting this Benchmark. for each row has one saving grace: when. For example, for each row when (new.val = 'bla') seems to outperform rules.

Shivam Rathore
Shivam Rathore
5 years ago

What about using logical decoding to retrieve logs?
https://www.postgresql.org/docs/9.5/logicaldecoding.html

Nick
Nick
6 years ago

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 to val and logged into log table.

laurenz
laurenz
5 years ago
Reply to  Nick

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.

Wutikrai Pornchai
Wutikrai Pornchai
6 years ago

Very useful and informative article, Thank you.

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