Description:
create table ten(a int);
insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
create table twenty (a int, filler char(200), key(a));
insert into twenty select A.a + B.a* 10, 'AAAAAAAAAAAAAAAAAAAA' from ten A, ten B where B.a in (0,1);
MySQL [test]> explain select A.a from ten A, twenty B where A.a+B.a> 0;
+----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------------------------------------------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 10 | NULL |
| 1 | SIMPLE | B | index | NULL | a | 5 | NULL | 20 | Using where; Using index; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------------------------------------------------+
2 rows in set (0.00 sec)
MySQL [test]> explain select distinct A.a from ten A, twenty B where A.a+B.a> 0;
+----+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 10 | Using temporary |
| 1 | SIMPLE | B | index | NULL | a | 5 | NULL | 20 | Using where; Using index; Distinct; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------------+
Look at the second query. It has "Distinct". I think this wrong. "Distinct"
doesn't work with "Using join buffer".
To make sure we are on the same page: "Distinct" in table B means that as soon
as we've got a match for current record in table A, we don't have to look for
any other matches. The query's SELECT list is "SELECT DISTINCT A.a", there is
no point to look for additional A.row-B.row pairs with the same A.row.
How to repeat:
You can see Distinct in action: put a breakpoint in end_write and end_send.
Then:
set optimizer_switch='block_nested_loop=off';
# Put a breakpoint in end_send
select A.a from ten A, twenty B where A.a+B.a> 0;
# observe that breakpoint was hit 200 times.
# Run EXPLAIN to be sure about what's going on:
MySQL [test]> explain select A.a from ten A, twenty B where A.a+B.a> 0;
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 10 | NULL |
| 1 | SIMPLE | B | index | NULL | a | 5 | NULL | 20 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
Then,
set optimizer_switch='block_nested_loop=off';
# Put a breakpoint in end_write
select distinct A.a from ten A, twenty B where A.a+B.a> 0;
# Observe that breakpoint was hit 11 times.
# This is how Distinct optimization works.
# (just in case, let's check EXPLAIN: )
MySQL [test]> explain select distinct A.a from ten A, twenty B where A.a+B.a> 0;
+----+-------------+-------+-------+---------------+------+---------+------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+------------------------------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 10 | Using temporary |
| 1 | SIMPLE | B | index | NULL | a | 5 | NULL | 20 | Using where; Using index; Distinct |
+----+-------------+-------+-------+---------------+------+---------+------+------+------------------------------------+
Ok, now with "Using join buffer":
set optimizer_switch='block_nested_loop=on';
# Put a breakpoint in end_send
select A.a from ten A, twenty B where A.a+B.a> 0;
# observe that breakpoint was hit 200 times.
# Put a breakpoint in end_write
select distinct A.a from ten A, twenty B where A.a+B.a> 0;
# Observe that breakpoint was hit 200 times, again.
When one takes time to think about, it becomes obvious that "Distinct"
optimization cannot be used with Join buffering. And it is not used. But EXPLAIN shows it's used.
