[MDEV-15433] Optimizer does not use group by optimization with distinct - Jira

Details

Type: Bug
Status: Closed (View Workflow)
Priority: Critical
Resolution: Fixed
Affects Version/s: 10.1.31
Fix Version/s: 10.1.35
Component/s: Optimizer, Storage Engine - InnoDB
Labels:
- optimizer

Description

This is similar to the following upstream bug that was fixed in MySQL 5.6, but this problem seems to effect all InnoDB tables in MariaDB 10.1, not just partitioned ones:

https://bugs.mysql.com/bug.php?id=60023

For example, run the following test:

CREATE TABLE tbl1 (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, a INT NOT NULL, KEY(a)) ENGINE=InnoDB;

INSERT INTO tbl1(a) VALUES (1), (2), (3), (4);

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

OPTIMIZE TABLE tbl1;

EXPLAIN SELECT DISTINCT a FROM tbl1;

SELECT DISTINCT a FROM tbl1;

EXPLAIN SELECT a FROM tbl1 GROUP BY a;

SELECT a FROM tbl1 GROUP BY a;

Here's the output seen on MariaDB 10.1.31:

MariaDB [db1]> EXPLAIN SELECT DISTINCT a FROM tbl1;

+------+-------------+-------+-------+---------------+------+---------+------+-------+-------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows  | Extra       |

+------+-------------+-------+-------+---------------+------+---------+------+-------+-------------+

|    1 | SIMPLE      | tbl1  | index | NULL          | a    | 4       | NULL | 65895 | Using index |

+------+-------------+-------+-------+---------------+------+---------+------+-------+-------------+

1 row in set (0.00 sec)

MariaDB [db1]> SELECT DISTINCT a FROM tbl1;

+---+

| a |

+---+

| 1 |

| 2 |

| 3 |

| 4 |

+---+

4 rows in set (0.02 sec)

MariaDB [db1]> EXPLAIN SELECT a FROM tbl1 GROUP BY a;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

1 row in set (0.00 sec)

MariaDB [db1]> SELECT a FROM tbl1 GROUP BY a;

+---+

| a |

+---+

| 1 |

| 2 |

| 3 |

| 4 |

+---+

4 rows in set (0.00 sec)

Compare this to the output seen on 5.6.38:

mysql> EXPLAIN SELECT DISTINCT a FROM tbl1;

+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|  1 | SIMPLE      | tbl1  | range | a             | a    | 4       | NULL |    7 | Using index for group-by |

+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

1 row in set (0.00 sec)

mysql> SELECT DISTINCT a FROM tbl1;

+---+

| a |

+---+

| 1 |

| 2 |

| 3 |

| 4 |

+---+

4 rows in set (0.00 sec)

mysql> EXPLAIN SELECT a FROM tbl1 GROUP BY a;

+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|  1 | SIMPLE      | tbl1  | range | a             | a    | 4       | NULL |    7 | Using index for group-by |

+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

1 row in set (0.00 sec)

mysql> SELECT a FROM tbl1 GROUP BY a;

+---+

| a |

+---+

| 1 |

| 2 |

| 3 |

| 4 |

+---+

4 rows in set (0.00 sec)

Attachments

Issue Links

relates to

MDEV-10880 Assertions `keypart_map' or `prebuilt->search_tuple->n_fields > 0' fail on DISTINCT and GROUP BY constant

Closed

links to

MySQL Bug #60023 - No Loose Index Scan for GROUP BY / DISTINCT on InnoDB partitioned table

Activity

Ascending order - Click to sort in descending order

Richard Stracke added a comment - 2018-03-02 15:15

On MariaDB 10.2 it works with

SET SESSION use_stat_tables='NEVER';

On 10.1. a workarround is to combine group by and distinct.

EXPLAIN SELECT DISTINCT a FROM tbl1 GROUP BY a;

Richard Stracke added a comment - 2018-03-02 15:15 On MariaDB 10.2 it works with SET SESSION use_stat_tables= 'NEVER' ; On 10.1. a workarround is to combine group by and distinct. EXPLAIN SELECT DISTINCT a FROM tbl1 GROUP BY a;

Alice Sherepa added a comment - 2018-03-06 12:14

Not reproducible on MariaDB 5.5, 10.2, 10.3 - Using index for group-by in all cases;
on 10.1 - reproducible with Innodb and MyIsam, with 10.0, 10.1, with Innodb, if query "optimize table" was not executed, then-"using index" -for both queries;
with MyISAM/10.1

ANALYZE SELECT DISTINCT a FROM tbl1

+------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows  | r_rows   | filtered | r_filtered | Extra       |

+------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+

|    1 | SIMPLE      | tbl1  | index | NULL          | a    | 4       | NULL | 65536 | 65536.00 |   100.00 |     100.00 | Using index |

+------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+

1 row in set (0.06 sec)

ANALYZE SELECT a FROM tbl1 GROUP BY a

+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | r_rows | filtered | r_filtered | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    5 |   4.00 |   100.00 |     100.00 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+

1 row in set (0.00 sec)

Innodb/10.1

--source include/have_innodb.inc

CREATE TABLE tbl1 (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, a INT NOT NULL, KEY(a)) ENGINE=Innodb;

INSERT INTO tbl1(a) VALUES (1), (2), (3), (4);

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

INSERT INTO tbl1(a) SELECT a FROM tbl1;

OPTIMIZE TABLE tbl1;

ANALYZE SELECT DISTINCT a FROM tbl1;

ANALYZE SELECT a FROM tbl1 GROUP BY a;

DROP TABLE tbl1;

Sporadic, not using group-by optimization
1)

ANALYZE SELECT DISTINCT a FROM tbl1

+------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows  | r_rows   | filtered | r_filtered | Extra       |

+------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+

|    1 | SIMPLE      | tbl1  | index | NULL          | a    | 4       | NULL | 64244 | 65536.00 |   100.00 |     100.00 | Using index |

+------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+

1 row in set (0.18 sec)

ANALYZE SELECT a FROM tbl1 GROUP BY a

+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | r_rows | filtered | r_filtered | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 |   4.00 |   100.00 |     100.00 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+

1 row in set (0.00 sec)

or sometimes like this:

 ANALYZE SELECT DISTINCT a FROM tbl1;

 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra

-1	SIMPLE	tbl1	index	NULL	a	4	NULL	66012	65536.00	100.00	100.00	Using index

+1	SIMPLE	tbl1	index	NULL	a	4	NULL	63323	65536.00	100.00	100.00	Using index

 ANALYZE SELECT a FROM tbl1 GROUP BY a;

 id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra

-1	SIMPLE	tbl1	range	NULL	a	4	NULL	3	4.00	100.00	100.00	Using index for group-by

+1	SIMPLE	tbl1	range	NULL	a	4	NULL	15	4.00	100.00	100.00	Using index for group-by

Alice Sherepa added a comment - 2018-03-06 12:14 Not reproducible on MariaDB 5.5, 10.2, 10.3 - Using index for group-by in all cases; on 10.1 - reproducible with Innodb and MyIsam, with 10.0, 10.1, with Innodb, if query "optimize table" was not executed, then-"using index" -for both queries; with MyISAM/10.1 ANALYZE SELECT DISTINCT a FROM tbl1 +------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra | +------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+ | 1 | SIMPLE | tbl1 | index | NULL | a | 4 | NULL | 65536 | 65536.00 | 100.00 | 100.00 | Using index | +------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+ 1 row in set (0.06 sec) ANALYZE SELECT a FROM tbl1 GROUP BY a +------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 5 | 4.00 | 100.00 | 100.00 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+ 1 row in set (0.00 sec) Innodb/10.1 --source include/have_innodb.inc CREATE TABLE tbl1 (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY , a INT NOT NULL , KEY (a)) ENGINE=Innodb; INSERT INTO tbl1(a) VALUES (1), (2), (3), (4); INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; INSERT INTO tbl1(a) SELECT a FROM tbl1; OPTIMIZE TABLE tbl1; ANALYZE SELECT DISTINCT a FROM tbl1; ANALYZE SELECT a FROM tbl1 GROUP BY a; DROP TABLE tbl1; Sporadic, not using group-by optimization 1) ANALYZE SELECT DISTINCT a FROM tbl1 +------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra | +------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+ | 1 | SIMPLE | tbl1 | index | NULL | a | 4 | NULL | 64244 | 65536.00 | 100.00 | 100.00 | Using index | +------+-------------+-------+-------+---------------+------+---------+------+-------+----------+----------+------------+-------------+ 1 row in set (0.18 sec) ANALYZE SELECT a FROM tbl1 GROUP BY a +------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 7 | 4.00 | 100.00 | 100.00 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+--------------------------+ 1 row in set (0.00 sec) or sometimes like this: ANALYZE SELECT DISTINCT a FROM tbl1; id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra -1 SIMPLE tbl1 index NULL a 4 NULL 66012 65536.00 100.00 100.00 Using index +1 SIMPLE tbl1 index NULL a 4 NULL 63323 65536.00 100.00 100.00 Using index ANALYZE SELECT a FROM tbl1 GROUP BY a; id select_type table type possible_keys key key_len ref rows r_rows filtered r_filtered Extra -1 SIMPLE tbl1 range NULL a 4 NULL 3 4.00 100.00 100.00 Using index for group-by +1 SIMPLE tbl1 range NULL a 4 NULL 15 4.00 100.00 100.00 Using index for group-by

Varun Gupta (Inactive) added a comment - 2018-08-02 11:55 - edited

Patch
http://lists.askmonty.org/pipermail/commits/2018-August/012754.html

this patch reverts the commit for ~~MDEV-10880~~

commit b76b69cd5fe634d8ddb9406aa2c82ef2a375b4d8

Author: Oleksandr Byelkin <sanja@mariadb.com>

Date:   Tue Jun 20 14:55:30 2017 +0200

    MDEV-10880: Assertions `keypart_map' or `prebuilt->search_tuple->n_fields > 0' fail on DISTINCT and GROUP BY constant

    add_group_and_distinct_keys() should take into account JOIN::simple_group.

Varun Gupta (Inactive) added a comment - 2018-08-02 11:55 - edited Patch http://lists.askmonty.org/pipermail/commits/2018-August/012754.html this patch reverts the commit for MDEV-10880 commit b76b69cd5fe634d8ddb9406aa2c82ef2a375b4d8 Author: Oleksandr Byelkin <sanja@mariadb.com> Date: Tue Jun 20 14:55:30 2017 +0200 MDEV-10880: Assertions `keypart_map' or `prebuilt->search_tuple->n_fields > 0' fail on DISTINCT and GROUP BY constant add_group_and_distinct_keys() should take into account JOIN::simple_group.

Varun Gupta (Inactive) added a comment - 2018-08-03 07:01

After discussion with igor we have concluded that the fix for ~~MDEV-10880~~ is not correct, so we will have to revert the fix.

Varun Gupta (Inactive) added a comment - 2018-08-03 07:01 After discussion with igor we have concluded that the fix for MDEV-10880 is not correct, so we will have to revert the fix.

Varun Gupta (Inactive) added a comment - 2018-08-03 13:49 - edited

When do we use GROUP BY optimization

1) MIN , MAX with GROUP BY does GROUP BY optimization

MariaDB [test]> explain select max(b) , min(b) from tbl1 group by a;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

2) DISINCT without GROUP BY does GROUP BY optimization

MariaDB [test]> explain select distinct a from tbl1;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

3) With GROUP BY we do GROUP BY optimization

MariaDB [test]> explain select b+4 from tbl1 group by a;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

4) With Distinct and GROUP by , we can do GROUP BY optimization only if the rules are followed by the GROUP BY clause

MariaDB [test]> explain select distinct a from tbl1 group by a;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

5) AGG_FUNC(DISTINCT column_list) does GROUP BY optimization.

MariaDB [test]> explain select count(distinct a) from tbl1;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 4       | NULL |    7 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

Varun Gupta (Inactive) added a comment - 2018-08-03 13:49 - edited When do we use GROUP BY optimization 1) MIN , MAX with GROUP BY does GROUP BY optimization MariaDB [test]> explain select max(b) , min(b) from tbl1 group by a; +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 7 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ 2) DISINCT without GROUP BY does GROUP BY optimization MariaDB [test]> explain select distinct a from tbl1; +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 7 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ 3) With GROUP BY we do GROUP BY optimization MariaDB [test]> explain select b+4 from tbl1 group by a; +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 7 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ 4) With Distinct and GROUP by , we can do GROUP BY optimization only if the rules are followed by the GROUP BY clause MariaDB [test]> explain select distinct a from tbl1 group by a; +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 7 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ 5) AGG_FUNC(DISTINCT column_list) does GROUP BY optimization. MariaDB [test]> explain select count(distinct a) from tbl1; +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl1 | range | NULL | a | 4 | NULL | 7 | Using index for group-by | +------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

Varun Gupta (Inactive) added a comment - 2018-08-03 13:56

In ~~MDEV-10880~~ the query is

SELECT DISTINCT a FROM tbl1 GROUP BY 'foo';

MariaDB [test]> explain SELECT DISTINCT a FROM tbl1 GROUP BY 'foo';

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | tbl1  | range | NULL          | a    | 0       | NULL |    2 | Using index for group-by |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

The key_len= 0 which tells us that the length of the key on the GROUP BY column is 0. For such a case I think we should add a case of not allowing the GROUP BY optimization.

This case comes under the category (4) as mentioned above.

Varun Gupta (Inactive) added a comment - 2018-08-03 13:57 - edited

the code in get_best_group_min_max

/*

   Check (GA2) if this is a DISTINCT query.

   If GA2, then Store a new ORDER object in group_fields_array at the

   position of the key part of item_field->field. Thus we get the ORDER

   objects for each field ordered as the corresponding key parts.

   Later group_fields_array of ORDER objects is used to convert the query

   to a GROUP query.

*/

   if ((!join->group && join->select_distinct)

So here we check if we have GROUP BY or not and if we don't have GROUP BY and distinct then we convert DISTINCT to GROUP BY.

So for our case we don't enter the IF condition here and so we don't have any groups over which we will create the group by prefix and that is why we end up with key_len=0. So we should not allow such cases.

Varun Gupta (Inactive) added a comment - 2018-08-03 13:57 - edited the code in get_best_group_min_max /* Check (GA2) if this is a DISTINCT query. If GA2, then Store a new ORDER object in group_fields_array at the position of the key part of item_field->field. Thus we get the ORDER objects for each field ordered as the corresponding key parts. Later group_fields_array of ORDER objects is used to convert the query to a GROUP query. */ if ((!join->group && join->select_distinct) So here we check if we have GROUP BY or not and if we don't have GROUP BY and distinct then we convert DISTINCT to GROUP BY. So for our case we don't enter the IF condition here and so we don't have any groups over which we will create the group by prefix and that is why we end up with key_len=0. So we should not allow such cases.

Varun Gupta (Inactive) added a comment - 2018-08-04 00:31

Second patch
http://lists.askmonty.org/pipermail/commits/2018-August/012769.html

Varun Gupta (Inactive) added a comment - 2018-08-04 00:31 Second patch http://lists.askmonty.org/pipermail/commits/2018-August/012769.html

People

Assignee:: Varun Gupta (Inactive)

Reporter:: Geoff Montee (Inactive)

Votes:: 3 Vote for this issue

Watchers:: 10 Start watching this issue

Dates

Created:: 2018-02-27 22:16

Updated:: 2024-07-08 00:22

Resolved:: 2018-08-05 10:34

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