Você pode tentar um índice parcial:
CREATE INDEX idx_partial ON othertable (m_id)
WHERE (col1 is not null and col2 is not null and col3 is not null);
Dos documentos:http://www.postgresql.org/docs /current/interactive/indexes-partial.html
CREATE INDEX idx_partial ON othertable (m_id)
WHERE (col1 is not null and col2 is not null and col3 is not null);
