導致MySQL做全表掃描的幾種情況
這兩天看到瞭兩種可能會導致全表掃描的sql，這裡給大傢看一下，希望可以避免踩坑：
情況1：
強制類型轉換的情況下，不會使用索引，會走全表掃描。
舉例如下：
首先我們創建一個表
CREATE TABLE `test` ( `id` int(11) NOT NULL AUTO_INCREMENT, `age` int(11) DEFAULT NULL, `score` varchar(20) NOT NULL DEFAULT '', PRIMARY KEY (`id`), KEY `idx_score` (`score`) ) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8
我們可以看到，這個表有三個字段，其中兩個int類型，一個varchar類型。varchar類型的字段score是一個索引，而id是主鍵。
然後我們給這個表裡面插入一些數據，插入數據之後的表如下：
mysql：yeyztest 21:43:12>>select * from test; +----+------+-------+ | id | age | score | +----+------+-------+ | 1 | 1 | 5 | | 2 | 2 | 10 | | 5 | 5 | 25 | | 8 | 8 | 40 | | 9 | 2 | 45 | | 10 | 5 | 50 | | 11 | 8 | 55 | +----+------+-------+ 7 rows in set (0.00 sec)
這個時候，我們使用explain語句來查看兩條sql的執行情況，分別是：
explain select * from test where score ='10'; explain select * from test where score =10;
結果如下：
mysql:yeyztest 21:42:29>>explain select * from test where score ='10'; +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+ | 1 | SIMPLE | test | NULL | ref | idx_score | idx_score | 62 | const | 1 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql:yeyztest 21:43:06>>explain select * from test where score =10; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | test | NULL | ALL | idx_score | NULL | NULL | NULL | 7 | 14.29 | Using where | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 3 warnings (0.00 sec)
可以看到，如果我們使用的是varchar類型的值，那麼結果中掃描的行數rows就是1，而當我們使用的是整數值10的時候，掃描行數變為瞭7，證明，如果出現瞭強制類型轉換，則會導致索引失效。
情況2：
反向查詢不能使用索引，會導致全表掃描。
創建一個表test1，它的主鍵是score，然後插入6條數據：
CREATE TABLE `test1` ( `score` varchar(20) not null default '' , PRIMARY KEY (`score`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 mysql:yeyztest 22:09:37>>select * from test1; +-------+ | score | +-------+ | 111 | | 222 | | 333 | | 444 | | 555 | | 666 | +-------+ 6 rows in set (0.00 sec)
當我們使用反向查找的時候，不會使用到索引，來看下面兩條sql：
explain select * from test1 where score='111'; explain select * from test1 where score!='111';
mysql:yeyztest 22:13:01>>explain select * from test1 where score='111'; +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | test1 | NULL | const | PRIMARY | PRIMARY | 62 | const | 1 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql:yeyztest 22:13:08>>explain select * from test1 where score!='111'; +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | 1 | SIMPLE | test1 | NULL | index | PRIMARY | PRIMARY | 62 | NULL | 6 | 100.00 | Using where; Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ 1 row in set, 1 warning (0.00 sec)
可以看到，使用！=作為條件的時候，掃描的行數是表的總記錄行數。因此如果想要使用索引，我們就不能使用反向匹配規則。
情況3：
某些or值條件可能導致全表掃描。
首先我們創建一個表，並插入幾條數據：
CREATE TABLE `test4` ( `id` int(11) DEFAULT NULL, `name` varchar(20) DEFAULT NULL, KEY `idx_id` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 1 row in set (0.00 sec) [email protected]:yeyztest 22:23:44>>select * from test4; +------+------+ | id | name | +------+------+ | 1 | aaa | | 2 | bbb | | 3 | ccc | | 4 | yeyz | | NULL | yeyz | +------+------+ 5 rows in set (0.00 sec)
其中表test4包含兩個字段，id字段是一個索引，而name字段是varchar類型，我們來看下面三個語句的掃描行數：
explain select * from test4 where id=1; explain select * from test4 where id is null; explain select * from test4 where id=1 or id is null;
mysql:yeyztest 22:24:12>>explain select * from test4 where id is null; +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | test4 | NULL | ref | idx_id | idx_id | 5 | const | 1 | 100.00 | Using index condition | +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) mysql:yeyztest 22:24:17>>explain select * from test4 where id=1; +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+ | 1 | SIMPLE | test4 | NULL | ref | idx_id | idx_id | 5 | const | 1 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) mysql:yeyztest 22:24:28>>explain select * from test4 where id=1 or id is null; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | test4 | NULL | ALL | idx_id | NULL | NULL | NULL | 5 | 40.00 | Using where | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
可以看到單獨使用id=1和id is null，都隻會掃描一行記錄，而使用or將二者連接起來就會導致掃描全表而不使用索引。
簡單總結一下：
1.強制類型轉換的情況下，不會使用索引，會走全表掃描
2.反向查詢不能使用索引，會導致全表掃描。
3.某些or值條件可能導致全表掃描。
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