B ORA-20000: Oracle Text error: DRG-10599: column is not indexed

SELECT :search_id, 100, :item_type, c.search_college, c.search_colname, TRUNC(dbms_random.value(900000000000, 999999999999)) rand, COUNT(1), c.college_rating, min(DECODE(c.search_postcode, NULL, 99999, NULL)) search_distance FROM w_search_text c , hot_mail.m_postcode_prefix_tmp p WHERE c.search_course_type is not null and search_region = postcode_prefix and (contains(c.search_text,:search_text,2) > 0) AND c.search_item_type = :item_bind GROUP BY c.search_college, c.search_colname, c.college_rating ORDER BY c.search_college courses: evening classes part time courses: undergraduate postgraduate courses

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