text: CREATE TABLE memos (
    id integer,
    content text
  );
  
  INSERT INTO memos VALUES (1, '');
  INSERT INTO memos VALUES (2, 'a');
  INSERT INTO memos VALUES (3, 'ab');
  
  CREATE INDEX grnindex ON memos
    USING pgroonga (content pgroonga.text_full_text_search_ops);
  
  SET enable_seqscan = off;
  SET enable_indexscan = on;
  SET enable_bitmapscan = off;
  
  SELECT id, content
    FROM memos
   WHERE content ILIKE '_';
  
  DROP TABLE memos;
  