"John D. Burger" <john@mitre.org> writes:
> It seemed reasonable to me that a select on the first element of an
> array column could use an index on the column, but, as seen in this
> example, I can't get it to do so:
Nope. The operators that go along with a btree index are equality,
less than, etc on the whole indexed column. btree knows nothing
about extracting array elements.
You might find that contrib/intarray would help, though the operators it
can index are not exactly "array[x] = y".
regards, tom lane