Re: COPY FROM WHEN condition

Right now we have 2 syntaxes for filtering rows in queries, both of
which use WHERE immediately before the condition:

1). SELECT ... FROM ... WHERE condition

2). SELECT agg_fn FILTER (WHERE condition) FROM ...

I'm not a huge fan of (2), but that's the SQL standard, so we're stuck
with it. There's a certain consistency in it's use of WHERE to
introduce the condition, and preceding that with FILTER helps to
distinguish it from any later WHERE clause. But what you'd be adding
here would be a 3rd syntax

3). COPY ... FROM ... FILTER condition

which IMO will just lead to confusion.