Monday, November 30 2009
PostgreSQL, Extensions, prefix, 9.1

prefix 1.1.0

So I had two bug reports about prefix in less than a week. It means several things, one of them is that my code is getting used in the wild, which is nice. The other side of the coin is that people do find bugs in there. This one is about the behavior of the btree opclass of the type prefix range. We cheat a lot there by simply having written one, because a range does not have a strict ordering: is [1-3] before of after [2-4]? But when you know you have no overlapping intervals in your prefix_range column, being able to have it part of a primary key is damn useful.

Note: in 8.5 we should have a way to express contraint exclusion and have PostgreSQL forbids overlapping entries for us. Not being there yet, you could write a constraint trigger and use the GiST index to have nice speed there, which is exactly what this constraint exclusion support is about.

It turns out the code change required is pretty simple:

-    return (a->first == b->first) ? (a->last - b->last) : (a->first - b->first);
+    /*
+     * we are comparing e.g. '1' and '12' (the shorter contains the
+     * smaller), so let's pretend '12' < '1' as it contains less elements.
+     */
+    return (alen == mlen) ? 1 : -1;

This happens in the compare support function (see Interfacing Extensions To Indexes) so that means you now have to rebuild your prefix_range btree indexes, hence the version number bump.