Custom collations, collation modifiers (eg case-insensitive)

One of the big appeals of the new COLLATE feature was, to me, the
possibility that we'd be able to support custom collations including
case-insensitive collations in future.

It's something I'd like to tackle one day, and in the mean time want to
pop on the TODO so it's not lost and forgotten. Everyone OK with that?

[TODO] User-defined collations or collation modifiers/filters

[TODO] Provide a built-in case-insensitive collation modifier, i.e.
COLLATE ... CASE INSENSITIVE, or current-collation case insensitive as
COLLATE CASE INSENSITIVE.

Case insensitive collation could be implemented as a collation filter,
where both inputs are transformed using the supplied function before
being passed to the system collation support. The most obvious being
"lower() both inputs".

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services