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This month's #PGSQLPhriday is hosted by Lætita Avrot, and she asks about triggers.

History time. Shortly after I started using PostgreSQL, I had a need to track changes in tables. Back then - this was the early version 7.x days - there was no such option available. I set out to write a tool for it. The logical choice to do that was to pick triggers to implement this. Today the world is different, PostgreSQL gained replication, and along with this, one can hook tools into the replication and stream all the changes. Back then there was no replication.

The second thing I discovered was that pl/pgSQL can't do the job. That was a rather big disappointment. My idea was to have one function which can be used with a trigger, the function figures out the columns and writes the changes into a separate table. However one can't access arbitrary column names in NEW and OLD in pl/pgSQL trigger functions. Something like this doesn't work:

columnname1 := "created_at";
columnname2 := "changed_at";

NEW.$columnname1 := OLD.$columnname1;
NEW.$columnname2 := OLD.$columnname2;

In pl/pgSQL, you have to "know" the column names in advance. Depesz recently posted some workarounds for this, but these options also have not been available back then.

Which made me write the tool in C. This at least allowed me to access the NEW and OLD values, and recoed changes. The tool is called "table_log", I also presented it at the first PGDay in Prato, Italy in 2007, and originally it was hosted on pgfoundry. This site is also long gone, I later copied the code to GitHub. But I also know that PostgreSQL 9.x had some internal changes which renders the tool non-working. However because the entire ecosystem had improved, and other tools are available, I did not update the old code anymore.

My conclusion: Triggers were one of the first "advanced" features I used in PostgreSQL, and I like them very much. They allowed me to implement an audit feature I need.

I just answered a question in the usenet about triggers in Mysql and discovered something very scary:

You can have only one trigger action time and event at a time for a table. So for example you can only have one "BEFORE INSERT" trigger.