Collation-aware comparisons in GIN opclasses

Hackers,

some GIN opclasses uses collation-aware comparisons while they don't need
to do especially collation-aware comparison. Examples are text[] and hstore
opclasses. Depending on collation this may make them a much slower.

See example.

# show lc_collate ;
lc_collate
─────────────
ru_RU.UTF-8
(1 row)

# create table test as (select array_agg(i::text) from
generate_series(1,1000000) i group by (i-1)/10);
SELECT 100000

# create index test_idx on test using gin(array_agg);
CREATE INDEX
Time: *26930,423 ms*

# create index test_idx2 on test using gin(array_agg collate "C");
CREATE INDEX
Time: *5143,682 ms*

Index creation with collation "ru_RU.UTF-8" is 5 times slower while
collation has absolutely no effect on index functionality.

However, we can just replace comparison function for those opclasses
because it would break binary compatibility for pg_upgrade. I see following
solution:

1. Rename such opclasses and make them not default.
2. Create new default opclasses with bitwise comparison functions.
3. Write recommendation to re-create indexes with default opclasses into
documentation.

------
With best regards,
Alexander Korotkov.