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BUG#18068: SELECT DISTINCT (with duplicates and covering index)

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When converting DISTINCT to GROUP BY where the columns are from the covering
index and they are quoted twice in the SELECT list the optimizer is creating
improper processing sequence. This is because of the fact that the columns
of the covering index are not recognized as such and treated as non-index
columns.

Generally speaking duplicate columns can safely be removed from the GROUP
BY/DISTINCT list because this will not add or remove new rows in the
resulting set. Duplicates can be removed even if they are not consecutive
(as is the case for ORDER BY, where the duplicate columns can be removed
only if they are consecutive).

So we can safely transform "SELECT DISTINCT a,a FROM ... ORDER BY a" to
"SELECT a,a FROM ... GROUP BY a ORDER BY a" instead of 
"SELECT a,a FROM .. GROUP BY a,a ORDER BY a". We can even transform 
"SELECT DISTINCT a,b,a FROM ... ORDER BY a,b" to
"SELECT a,b,a FROM ... GROUP BY a,b ORDER BY a,b".

The fix to this bug consists of checking for duplicate columns in the SELECT
list when constructing the GROUP BY list in transforming DISTINCT to GROUP
BY and skipping the ones that are already in.


mysql-test/r/distinct.result:
  test case for the bug without loose index scan
mysql-test/r/group_min_max.result:
  test case for the bug
mysql-test/t/distinct.test:
  test case for the bug without loose index scan
mysql-test/t/group_min_max.test:
  test case for the bug
sql/sql_select.cc:
  duplicates check and removal
This commit is contained in:
unknown
2006-05-09 18:13:01 +03:00
parent 33417297c5
commit baae7a97d9
5 changed files with 77 additions and 0 deletions
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22
mysql-test/r/group_min_max.result
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@@ -2116,3 +2116,25 @@ COUNT(DISTINCT a)
1
DROP TABLE t1;
DROP PROCEDURE a;
CREATE TABLE t1 (a varchar(64) NOT NULL default '', PRIMARY KEY(a));
INSERT INTO t1 (a) VALUES
(''), ('CENTRAL'), ('EASTERN'), ('GREATER LONDON'),
('NORTH CENTRAL'), ('NORTH EAST'), ('NORTH WEST'), ('SCOTLAND'),
('SOUTH EAST'), ('SOUTH WEST'), ('WESTERN');
EXPLAIN SELECT DISTINCT a,a FROM t1 ORDER BY a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range NULL PRIMARY 66 NULL 12 Using index for group-by
SELECT DISTINCT a,a FROM t1 ORDER BY a;
a a
CENTRAL CENTRAL
EASTERN EASTERN
GREATER LONDON GREATER LONDON
NORTH CENTRAL NORTH CENTRAL
NORTH EAST NORTH EAST
NORTH WEST NORTH WEST
SCOTLAND SCOTLAND
SOUTH EAST SOUTH EAST
SOUTH WEST SOUTH WEST
WESTERN WESTERN
DROP TABLE t1;