How to UPSERT (MERGE, INSERT … ON DUPLICATE UPDATE) in PostgreSQL?

Question

A very frequently asked question here is how to do an upsert, which is what MySQL calls INSERT ... ON DUPLICATE UPDATE and the standard supports as part of the

Answer 1

I am trying to contribute with another solution for the single insertion problem with the pre-9.5 versions of PostgreSQL. The idea is simply to try to perform first the insertion, and in case the record is already present, to update it:

do $$
begin 
  insert into testtable(id, somedata) values(2,'Joe');
exception when unique_violation then
  update testtable set somedata = 'Joe' where id = 2;
end $$;

Note that this solution can be applied only if there are no deletions of rows of the table.

I do not know about the efficiency of this solution, but it seems to me reasonable enough.