Re: Using threads in FDW for read-ahead

Noah Watkins  writes:
> I have created a FDW for a storage backend and it is working well, and now
> I would like to overlap processing with I/O by performing read-ahead. I
> started by using a thread to do background I/O and this worked, but
> problems started to arise when I tried to do predicate filtering in the
> thread.

> In particular, it seems as though `check_stack_depth` is built to assume a
> single threaded environment (`stack_base_ptr` is global).

That is not even the tip of the iceberg of what will break if you try
to use multiple threads in a Postgres backend.  It's not supported.
You might possibly manage to not break things if you keep the extra
threads sufficiently narrowly scoped --- which for starters would include
no use of palloc nor elog.  Executing query predicates is right out.
		regards, tom lane