On Tue, Apr 10, 2012 at 5:10 PM, Jeff Davis <pgsql@j-davis.com> wrote:
On Tue, 2012-04-10 at 16:15 -0400, Andy Chambers wrote:
> Does anyone know the time complexity of the algorithm used to handle > triggers with a when clause?
It's done with a linear scan of all triggers, testing the WHEN clause for each.
> To make this a little more concrete, what is likely to perform better > > > a) A single trigger with "n" if/else clauses > b) A set of "n" triggers each using a different when clause.
Both are essentially linear.
If you want to scale to a large number of conditions, I would recommend using one trigger in a fast procedural language, and searching for the matching conditions using something better than a linear search.
To beat a linear search, you need something resembling an index, which is dependent on the types of conditions. For instance, if your conditions are:
00 <= x < 10 10 <= x < 20 20 <= x < 30 ...
you can use a tree structure. But, obviously, postgres won't know enough about the conditions to know that a tree structure is appropriate from a given sequence of WHEN clauses. So, you should use one trigger and code the condition matching yourself.
