Item_func_coalesce::fix_length_and_dec() calls
Item_func::count_string_result_length()) which called agg_arg_charsets()
with wrong flags, so the collation derivation of the COALESCE result was
not properly set to DERIVATION_COERCIBLE. It erroneously stayed
DERIVATION_NUMERIC. So GREATEST() misinterpreted the argument as
a number rather that a string and did not calculate its own length properly.