You’re getting the 55P03 because you are requesting it - the NOWAIT tells Postgres to return an error if it can’t immediately acquire the lock.

This is not going to handle a raised exception; catch with a single value on the left of the -> will only capture values emitted with throw. You’d need the two value shape instead.

BUT what you likely want instead is rescue, with a specific exception:

try do
  # ... some SQL bits ...
rescue
  Postgrex.Error ->
    # error handling
end

Other general notes:

• children_count |> Kernel.+(1) pipes are not magic, this is just children_count + 1 with extra steps
• the code around convert_ecto_datetime_to_epoc looks like it could be shortened to DateTime.add(snapshot.started_at, snapshot.interval, :second)

This is an error message being returned from Postgrex, after it was returned by the database. This error can happen when you do a SELECT ... FOR UPDATE or similar that interacts with the DB’s locking systems.

You’ll need to provide more information about what your application is doing when this error occurs (and/or source code) to get specific help.