Please see:
> # create table t (a text collate "cs_CZ");
> CREATE TABLE
>
> # insert into t values ('c'), ('ch');
> INSERT 0 2
>
> # select * from t where a < 'd';
> a
> ---
> c
> (1 row)
>
> xx2_game=# create index on t using spgist (a);
> CREATE INDEX
>
> xx2_game=# set enable_seqscan = 0;
> SET
>
> xx2_game=# select * from t where a < 'd';
> a
> ----
> c
> ch
> (2 rows)
I assume we don't have any option than removing support for comparison
operators with collations just like btree.
The bug may apply to the other access methods.