Tom has pointed me at this proposal, since on another thread I asked for something very similar. (No need to reprise that discussion, but I wanted prepared queries to be able to do SET work_mem = X; SELECT). This idea looks a good way forward to me.
Since we're all in roughly the same place, I'd like to propose that we proceed with the following syntax... whether or not this precisely solves OP's issue on this thread.
1. Allow SET to set multiple parameters... SET guc1 = x, guc2 = y This looks fairly straightforward
2. Allow a SET to apply only for a single statement SET guc1 = x, guc2 = y FOR stmt e.g. SET max_parallel_workers = 4 FOR SELECT count(*) FROM bigtable Internally a GUC setting already exists for a single use, via GUC_ACTION_SAVE, so we just need to invoke it.
Quick prototype seems like it will deliver quite quickly. I couldn't see a reason to use "LET" rather than just "SET" which would be the POLA choice.
