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fix for one of the bugs spotted by #1274

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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
This commit is contained in:
unknown
2003-10-16 17:00:09 +04:00
parent 8ddb4b7c55
commit ca5383cfe5
1 changed files with 11 additions and 1 deletions
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12
sql/sql_select.cc
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@ -968,10 +968,20 @@ mysql_select(THD *thd,TABLE_LIST *tables,List<Item> &fields,COND *conds,
} }
} }
} }
/*
Here we sort rows for ORDER BY/GROUP BY clause, if the optimiser
chose FILESORT to be faster than INDEX SCAN or there is no
suitable index present.
Note, that create_sort_index calls test_if_skip_sort_order and may
finally replace sorting with index scan if there is a LIMIT clause in
the query. XXX: it's never shown in EXPLAIN!
OPTION_FOUND_ROWS supersedes LIMIT and is taken into account.
*/
if (create_sort_index(&join.join_tab[join.const_tables], if (create_sort_index(&join.join_tab[join.const_tables],
group ? group : order, group ? group : order,
select_limit, select_limit,
thd->select_limit)) join.select_options & OPTION_FOUND_ROWS ?
HA_POS_ERROR : thd->select_limit))
goto err; goto err;
} }
join.having=having; // Actually a parameter join.having=having; // Actually a parameter