When NULLS FIRST is the default order, the index should be used for sorting when NULLS FIRST is specified, as it does not change anything. But currently the index is ignored.

Example:

CREATE TABLE my_table(K1 INT);
CREATE INDEX my_index ON my_table(K1);
EXPLAIN PLAN FOR SELECT * FROM my_table ORDER BY K1 ASC NULLS FIRST;

This should lead to an explanation containing

/* index sorted */

See also

• this H2 Google Group post
• Issue An available index is never used for ordering, when the requested order is DESC #701 (index not used for ordering when order direction is DESC)