Just this week I had my own go at plpgsql again a few years after my last attempts. I have come away with the same conclusions I did last time "oh wow, this is really cool and fast and great; oh bummer, this is taking way too many of my own cycles to get something working".

The pg docs unfortunately weren't much beyond selecting some data. Trying to write any logic is something that left me struggling (in my case, I needed a hash table that could store a key value pair of numbers).

From my own experience a few years ago, however, go look at the stored procedures in PostGIS. There are many, many of them methodically written and you could probably learn most of what there is to know from detailed inspection.