unknown ca5383cfe5 fix for one of the bugs spotted by #1274 ...

sql/sql_select.cc:
  back to the bug #1274:
  the following query
  EXPLAIN SELECT SQL_CALC_FOUND_ROWS race_name 
  FROM races WHERE race_name LIKE '%Madison%' ORDER BY race_date DESC LIMIT 0,100
  
  +-------+------+---------------+------+---------+------+--------+-----------------------------+
  | table | type | possible_keys | key  | key_len | ref  | rows   | Extra                       |
  +-------+------+---------------+------+---------+------+--------+-----------------------------+
  | races | ALL  | NULL          | NULL |    NULL | NULL | 505821 | Using where; Using filesort |
  +-------+------+---------------+------+---------+------+--------+-----------------------------+
  
  The query returns no rows.
  
  There are two problems with it: 
  - wrong access plan is chosed  (sequential index scan in reverse order,
  which is VERY SLOW in case of MyISAM table + packed keys)
  It's wrong, because it doesn't take into account that SQL_CALC_FOUND_ROWS 
  is present, in other words, is based on assumtion that LIMIT
  clause decrease number of rows to access significantly, which is
  not true as all rows are accessed.
  - the access plan is not shown in the EXPLAIN (bug #1560). I'm not
  fixing it here

2003-10-16 17:00:09 +04:00

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