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Bug #38745: MySQL 5.1 optimizer uses filesort for ORDER BY

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when it should use index

Sometimes the LEFT/RIGHT JOIN with an empty table caused an
unnecessary filesort.

Sample query, where t1.i1 is indexed and t3 is empty:

  SELECT t1.*, t2.* FROM t1 JOIN t2 ON t1.i1 = t2.i2
                       LEFT JOIN t3 ON t2.i2 = t3.i3
    ORDER BY t1.i1 LIMIT 5;

The server erroneously used an item of empty outer-joined
table as a common constant of a Item_equal (multi-equivalence
expression).
By the fix for the bug 16590 the constant status of such
an item has been propagated to st_table::const_key_parts
map bits related to other Item_equal argument-related
key parts (those are obviously not constant in our case).
As far as test_if_skip_sort_order function skips constant
prefixes of testing keys, this caused an ignorance of
available indices, since some prefixes were marked as
constant by mistake.


mysql-test/r/order_by.result:
  Test case for bug #38745.
mysql-test/t/order_by.test:
  Test case for bug #38745.
sql/item.h:
  Bug #38745: MySQL 5.1 optimizer uses filesort for ORDER BY
              when it should use index
  
  Item::is_outer_field() has been added and overloaded for
  Item_field and Item_ref classes.
sql/item_cmpfunc.cc:
  Bug #38745: MySQL 5.1 optimizer uses filesort for ORDER BY
              when it should use index
  
  Item_equal::update_const() and Item_equal::update_used_tables()
  have been updated to not take into account the constantness
  of outer-joined table items.
This commit is contained in:
Gleb Shchepa
2010-05-31 16:52:19 +04:00
parent 77e744f844
commit fefbd756db
4 changed files with 85 additions and 2 deletions
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24
mysql-test/t/order_by.test
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@ -1468,3 +1468,27 @@ SELECT * FROM t1 FORCE INDEX FOR JOIN (a), t2 WHERE t1.a < 2 ORDER BY t1.a;
DROP TABLE t1, t2;
--echo End of 5.1 tests
--echo #
--echo # Bug #38745: MySQL 5.1 optimizer uses filesort for ORDER BY
--echo # when it should use index
--echo #
CREATE TABLE t1 (i1 integer NOT NULL PRIMARY KEY);
CREATE TABLE t2 (i2 integer NOT NULL PRIMARY KEY);
CREATE TABLE t3 (i3 integer);
INSERT INTO t1 VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12);
INSERT INTO t2 SELECT * FROM t1;
EXPLAIN EXTENDED
SELECT t1.*, t2.* FROM t1 JOIN t2 ON t1.i1 = t2.i2
LEFT JOIN t3 ON t2.i2 = t3.i3
ORDER BY t1.i1 LIMIT 5;
SELECT t1.*, t2.* FROM t1 JOIN t2 ON t1.i1 = t2.i2
LEFT JOIN t3 ON t2.i2 = t3.i3
ORDER BY t1.i1 LIMIT 5;
DROP TABLE t1, t2, t3;