MySQL从8.0.13版本开始支持一种新的range scan方式，称为Loose Skip Scan。该特性由Facebook贡献。我们知道在之前的版本中，如果要使用到索引进行扫描，条件必须满足索引前缀列，比如索引idx(col1,col2), 如果where条件只包含col2的话，是无法有效的使用idx的, 它需要扫描索引上所有的行，然后再根据col2上的条件过滤。
新的优化可以避免全量索引扫描，而是根据每个col1上的值+col2上的条件，启动多次range scan。每次range scan根据构建的key值直接在索引上定位，直接忽略了那些不满足条件的记录。
示例
下例是从官方文档上摘取的例子:
root@test 11:03:28>CREATE TABLE t1 (f1 INT NOT NULL, f2 INT NOT NULL, PRIMARY KEY(f1, f2));
Query OK, 0 rows affected (0.00 sec)
root@test 11:03:29>INSERT INTO t1 VALUES
-> (1,1), (1,2), (1,3), (1,4), (1,5),
-> (2,1), (2,2), (2,3), (2,4), (2,5);
Query OK, 10 rows affected (0.00 sec)
Records: 10 Duplicates: 0 Warnings: 0
root@test 11:03:29>INSERT INTO t1 SELECT f1, f2 + 5 FROM t1;
Query OK, 10 rows affected (0.00 sec)
Records: 10 Duplicates: 0 Warnings: 0
root@test 11:03:29>INSERT INTO t1 SELECT f1, f2 + 10 FROM t1;
Query OK, 20 rows affected (0.00 sec)
Records: 20 Duplicates: 0 Warnings: 0
root@test 11:03:29>INSERT INTO t1 SELECT f1, f2 + 20 FROM t1;
Query OK, 40 rows affected (0.00 sec)
Records: 40 Duplicates: 0 Warnings: 0
root@test 11:03:29>INSERT INTO t1 SELECT f1, f2 + 40 FROM t1;
Query OK, 80 rows affected (0.00 sec)
Records: 80 Duplicates: 0 Warnings: 0
root@test 11:03:29>ANALYZE TABLE t1;
+---------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------+---------+----------+----------+
| test.t1 | analyze | status | OK |
+---------+---------+----------+----------+
1 row in set (0.00 sec)
root@test 11:03:29>EXPLAIN SELECT f1, f2 FROM t1 WHERE f2 > 40;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------+
| 1 | SIMPLE | t1 | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 53 | 100.00 | Using where; Using index for skip scan |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------+
1 row in set, 1 warning (0.00 sec)
也可以从optimizer trace里看到如何选择的skip scan:
"skip_scan_range": {
"potential_skip_scan_indexes": [
{
"index": "PRIMARY",
"tree_travel_cost": 0.4,
"num_groups": 3,
"rows": 53,
"cost": 10.625
}
]
},
"best_skip_scan_summary": {
"type": "skip_scan",
"index": "PRIMARY",
"key_parts_used_for_access": [
"f1",
"f2"
],
"range": [
"40 < f2"
],
"chosen": true
},
我们从innodb的角度来看看这个SQL是如何执行的，我们知道每个index scan都会走到ha_innobase::index_read来构建search tuple，上述查询的执行步骤：
- 第一次从Index left side开始scan
- 第二次使用key(1,40) 扫描index，直到第一个range结束
- 使用key(1), find_flag =HA_READ_AFTER_KEY, 找到下一个Key值2
- 使用key(2,40)，扫描Index， 直到range结束
- 使用Key(2)，去找大于2的key值，上例中没有，因此结束扫描
笔者在代码注入了日志，打印search_tuple(dtuple_print())
STEP 1: no search_tuple
STEP 2:
DATA TUPLE: 2 fields;
0: len 4; hex 80000001; asc ;;
1: len 4; hex 80000028; asc (;;
STEP 3:
DATA TUPLE: 1 fields;
0: len 4; hex 80000001; asc ;;
STEP 4:
DATA TUPLE: 2 fields;
0: len 4; hex 80000002; asc ;;
1: len 4; hex 80000028; asc (;;
STEP 5:
DATA TUPLE: 1 fields;
0: len 4; hex 80000002; asc ;;
从上述描述可以看到使用skip-scan的方式避免了全索引扫描，从而提升了性能，尤其是在索引前缀列区分度比较低的时候
条件
skip scan可以通过Hint或者optimizer_switch来控制(skip_scan)，默认是打开的。根据worklog的描述，对于如下query:
SELECT A_1,...,A_k, B_1,...,B_m, C
FROM T
WHERE
EQ(A_1,...,A_k)
AND RNG(C);
需要满足如下条件才能使用skip scan:
A) Table T has at least one compound index I of the form:
I = <A_1,...,A_k, B_1,..., B_m, C ,[D_1,...,D_n]>
Key parts A and D may be empty, but B and C must be non-empty.
B) Only one table referenced.
C) Cannot have group by/select distinct
D) Query must reference fields in the index only.
E) The predicates on A_1...A_k must be equality predicates and they need
to be constants. This includes the 'IN' operator.
F) The query must be a conjunctive query.
In other words, it is a AND of ORs:
(COND1(kp1) OR COND2(kp1)) AND (COND1(kp2) OR ...) AND ...
G) There must be a range condition on C.
H) Conditions on D columns are allowed. Conditions on D must be in
conjunction with range condition on C.
ref: get_best_skip_scan()
当skip scan拥有更低的cost时，会被选择，计算cost的函数是cost_skip_scan()，由于索引统计信息中已经基于不同的前缀列值估算了distinct value的个数(rec_per_key), 可以基于此去预估可能需要读的行数。 更具体的可以参考wl#11322中的描述，笔者对此不甚了解，故不做笔墨
ref: cost_skip_scan()
参考
官方文档：Skip Scan Range Access Method
WL#11322: SUPPORT LOOSE INDEX RANGE SCANS FOR LOW CARDINALITY
Bug#88103
相关代码
本文作者：zhaiwx_yinfeng
本文为云栖社区原创内容，未经允许不得转载。