Plurrrr

Error messages are part of our daily lives online. Every time a server is down or we don’t have internet, or we forget to add some info in a form, we get an error message. “Something went wrong” is the classic. But what went wrong? What happened? And, most importantly, how can I fix it?

With PostgreSQL 15 comes a new SQL Command called MERGE. MERGE has been in the SQL standard for quite a while, but it just made its way into the PostgreSQL codebase. Prior to MERGE, if you wanted to refresh a target table from a source table, prior to Postgres 15, you could use the "upsert" method with the ON CONFLICT clause.

Now, MERGE can be used instead! Some situations where MERGE makes a lot of sense are:

• data loading from external sources, thru foreign data wrappers
• staged and batched process jobs

The interesting part of this program is the breadth-first tree traversal, and the tree traversal part now has only two arguments instead of three; the filter operation afterwards is trivial. Tree search in Haskell is mostly tree, and hardly any search!