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Index condition pushdown(ICP)是直到mysql5.6才引入的特性，主要是为了减少通过二级索引查找主键索引的次数。目前ICP相关的文章也比较多，本文主要从源码角度介绍ICP的实现。讨论之前，我们先再温习下。
以下图片来自mariadb
- 引入ICP之前
- 引入ICP之后
再来看个例子
1. CREATE TABLE `t1` (
2. `a` int(11) DEFAULT NULL,
3. `b` char(8) DEFAULT NULL,
4. `c` int(11) DEFAULT '0',
5. `pk` int(11) NOT NULL AUTO_INCREMENT,
6. PRIMARY KEY (`pk`),
7. KEY `idx1` (`a`,`b`)
8. ) ENGINE=ROCKSDB;
9. INSERT INTO t1 (a,b) VALUES (1,'a'),(2,'b'),(3,'c');
10. INSERT INTO t1 (a,b) VALUES (4,'a'),(4,'b'),(4,'c'),(4,'d'),(4,'e'),(4,'f');
12. set optimizer_switch='index_condition_pushdown=off';
14. ## 关闭ICP(Using where)
15. explain select * from t1 where a=4 and b!='e';
16. +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
17. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
18. +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
19. | 1 | SIMPLE | t1 | range | idx1 | idx1 | 14 | NULL | 2 | Using where |
20. +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
22. ## 关闭ICP走cover index(Using where; Using index)
23. explain select a,b from t1 where a=4 and b!='e';
24. +----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
25. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
26. +----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
27. | 1 | SIMPLE | t1 | ref | idx1 | idx1 | 5 | const | 4 | Using where; Using index |
28. +----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
30. set optimizer_switch='index_condition_pushdown=on';
32. ## 开启ICP(Using index conditione)
33. explain select * from t1 where a=4 and b!='e';
34. +----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------+
35. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
36. +----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------+
37. | 1 | SIMPLE | t1 | range | idx1 | idx1 | 14 | NULL | 2 | Using index condition |
38. +----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------+
40. ## 开启ICP仍然是cover index(Using where; Using index)
41. explain select a,b from t1 where a=4 and b!='e';
42. +----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
43. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
44. +----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
45. | 1 | SIMPLE | t1 | ref | idx1 | idx1 | 5 | const | 4 | Using where; Using index |
46. +----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+
这里总结下ICP的条件
- 适用于以下类型，range, ref, eq_ref, and ref_or_null 的二级索引
- 不能是覆盖索引(cover index)
server层主要负责判断是否符合ICP的条件，符合ICP则把需要的condition push到engine层。 engine层通过二级索引查找数据时，用server层push的condition再做一次判断，如果符合条件才会去查找主索引。
目前mysql支持ICP的引擎有MyISAM和InnoDB,MyRocks引入rocksdb后，也支持了ICP。 server层实现是一样的，engine层我们主要介绍innodb和rocksdb的实现。
server层
关键代码片段如下
1. make_join_readinfo()
3. switch (tab->type) {
4. case JT_EQ_REF:
5. case JT_REF_OR_NULL:
6. case JT_REF:
7. if (tab->select)
8. tab->select->set_quick(NULL);
9. delete tab->quick;
10. tab->quick=0;
11. /* fall through */
12. case JT_SYSTEM:
13. case JT_CONST:
14. /* Only happens with outer joins */
15. if (setup_join_buffering(tab, join, options, no_jbuf_after,
16. &icp_other_tables_ok))
17. DBUG_RETURN(true);
18. if (tab->use_join_cache != JOIN_CACHE::ALG_NONE)
19. tab[-1].next_select= sub_select_op;
21. if (table->covering_keys.is_set(tab->ref.key) &&
22. !table->no_keyread)
23. table->set_keyread(TRUE);
24. else
25. push_index_cond(tab, tab->ref.key, icp_other_tables_ok,
26. &trace_refine_table);
27. break;
从代码中看出只有符合的类型range, ref, eq_ref, and ref_or_null 二级索引才可能会push_index_cond。
而这里通过covering_keys来判断并排除使用了cover index的情况。covering_keys是一个bitmap，保存了所有可能用到的覆盖索引。在解析查询列以及条件列时会设置covering_keys，详细可以参考setup_fields，setup_wild，setup_conds。
engine层
innodb
innodb在扫描二级索引时会根据是否有push condition来检查记录是否符合条件(row_search_idx_cond_check) 逻辑如下：
1. row_search_for_mysql()
2. ......
3. if (prebuilt->idx_cond)
4. {
5. row_search_idx_cond_check //检查condition
6. row_sel_get_clust_rec_for_mysql //检查通过了才会去取主索引数据
7. }
8. ....
典型的堆栈如下
1. handler::compare_key_icp
2. innobase_index_cond
3. row_search_idx_cond_check
4. row_search_for_mysql
5. ha_innobase::index_read
6. ha_innobase::index_first
7. ha_innobase::rnd_next
8. handler::ha_rnd_next
9. rr_sequential
10. join_init_read_record
11. sub_select
12. do_select
rocksdb
rocksdb在扫描二级索引时也会根据是否有push condition来检查记录是否符合条件
逻辑如下
1. read_row_from_secondary_key()
2. {
3. find_icp_matching_index_rec//push了condition才会检查condition
4. get_row_by_rowid//检查通过了才会去取主索引数据
5. }
典型的堆栈如下
1. handler::compare_key_icp
2. myrocks::ha_rocksdb::check_index_cond
3. myrocks::ha_rocksdb::find_icp_matching_index_rec
4. myrocks::ha_rocksdb::read_row_from_secondary_key
5. myrocks::ha_rocksdb::index_read_map_impl
6. myrocks::ha_rocksdb::read_range_first
7. handler::multi_range_read_next
other
ICP对cover index作出了严格的限制，而实际上应该可以放开此限制，这样可以减少enging层传第给server层的数据量，至少可以减少server层的内存使用。欢迎指正！
