慢SQL日志里看到一个三张表的关联查询，如下：
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SELECT
COUNT
(1)
FROM
refund_order_item i, artisan a,
user
u
WHERE
u.userid = i.user_id
AND
a.artisan_id = i.artisan_id;
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测试查询时间：
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mysql>
SELECT
COUNT
(1)
->
FROM
refund_order_item i, artisan a,
user
u
->
WHERE
u.userid = i.user_id
->
AND
a.artisan_id = i.artisan_id;
+
----------+
|
COUNT
(1) |
+
----------+
| 260605 |
+
----------+
1 row
in
set
(2.30 sec)
|
查看执行计划：
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mysql> explain
SELECT
COUNT
(1)
->
FROM
refund_order_item i, artisan a,
user
u
->
WHERE
u.userid = i.user_id
->
AND
a.artisan_id = i.artisan_id;
+
----+-------------+-------+------------+--------+----------------------------+---------+---------+------------------+--------+----------+-------------+
| id | select_type |
table
| partitions | type | possible_keys |
key
| key_len | ref |
rows
| filtered | Extra |
+
----+-------------+-------+------------+--------+----------------------------+---------+---------+------------------+--------+----------+-------------+
| 1 | SIMPLE | i |
NULL
|
ALL
| idx_user_id,idx_artisan_id |
NULL
|
NULL
|
NULL
| 255599 | 100.00 |
NULL
|
| 1 | SIMPLE | a |
NULL
| eq_ref |
PRIMARY
|
PRIMARY
| 122 | hlj.i.artisan_id | 1 | 100.00 | Using
index
|
| 1 | SIMPLE | u |
NULL
| eq_ref | userid | userid | 122 | hlj.i.user_id | 1 | 100.00 | Using
index
|
+
----+-------------+-------+------------+--------+----------------------------+---------+---------+------------------+--------+----------+-------------+
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可以看到refund_order_item表没有走索引。
创建联合索引：
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ALTER
TABLE
refund_order_item
ADD
INDEX
idx_aid_uid (artisan_id, user_id);
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查看执行计划：
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explain
SELECT
COUNT
(1)
FROM
refund_order_item i, artisan a,
user
u
WHERE
u.userid = i.user_id
AND
a.artisan_id = i.artisan_id;
+
----+-------------+-------+------------+--------+----------------------------------------+-------------+---------+------------------+--------+----------+-------------+
| id | select_type |
table
| partitions | type | possible_keys |
key
| key_len | ref |
rows
| filtered | Extra |
+
----+-------------+-------+------------+--------+----------------------------------------+-------------+---------+------------------+--------+----------+-------------+
| 1 | SIMPLE | i |
NULL
|
index
| idx_user_id,idx_artisan_id,idx_aid_uid | idx_aid_uid | 244 |
NULL
| 255599 | 100.00 | Using
index
|
| 1 | SIMPLE | a |
NULL
| eq_ref |
PRIMARY
|
PRIMARY
| 122 | hlj.i.artisan_id | 1 | 100.00 | Using
index
|
| 1 | SIMPLE | u |
NULL
| eq_ref | userid | userid | 122 | hlj.i.user_id | 1 | 100.00 | Using
index
|
+
----+-------------+-------+------------+--------+----------------------------------------+-------------+---------+------------------+--------+----------+-------------+
|
可以看到执行计划已经走索引。
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测试查询时间：
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mysql>
SELECT
COUNT
(1)
->
FROM
refund_order_item i, artisan a,
user
u
->
WHERE
u.userid = i.user_id
->
AND
a.artisan_id = i.artisan_id;
+
----------+
|
COUNT
(1) |
+
----------+
| 260605 |
+
----------+
1 row
in
set
(1.15 sec)
|