Is there any way to embed a flag in a select that indicates that it is the first or the last row of a result set? I'm thinking something to the effect of:
> SELECT is_first_row() AS f, is_last_row() AS l FROM blah;
f | l
-----------
t | f
f | f
f | f
f | f
f | t
The answer might be in window functions but I've only just learned about them, and I question their efficiency.
SELECT first_value(unique_column) OVER () = unique_column, last_value(unique_column) OVER () = unique_column, * FROM blah;
seems to do what I want. Unfortunately, I don't even fully understand that syntax, but since unique_column is unique and NOT NULL it should deliver unambiguous results. But if it does sorting, then the cure might be worse than the disease. (Actually, in my tests, unique_column is not sorted, so that's something.)
EXPLAIN ANALYZE doesn't indicate there's an efficiency problem, but when has it ever told me what I needed to know?
And I might need to use this in an aggregate function, but I've just been told window functions aren't allowed there.
