###一、索引数据下探
http://blog.itpub.net/7728585/viewspace-2660796/
handler::multi_range_read_info_const
- 等值条件，根据参数 eq_range_index_dive_limit 来判断是否进行下探 0 始终 1 始终不 >1 判断 or 的个数
- 范围 始终下探
疑问：下探的采样范围和原理
```
下探栈
(gdb) bt
#0 handler::multi_range_read_info_const (this=0x7fff0576eb00, keyno=2, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges_arg=0, bufsz=0x7fffe8d3dd00, flags=0x7fffe8d3dd04, cost=0x7fffe8d3e1e0)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:6651
#1 0x0000000000ec4364 in DsMrr_impl::dsmrr_info_const (this=0x7fff0576ef70, keyno=2, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges=0, bufsz=0x7fffe8d3e2e0, flags=0x7fffe8d3e2e4,
cost=0x7fffe8d3e1e0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:7391
#2 0x000000000198cbc1 in ha_innobase::multi_range_read_info_const (this=0x7fff0576eb00, keyno=2, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges=0, bufsz=0x7fffe8d3e2e0, flags=0x7fffe8d3e2e4,
cost=0x7fffe8d3e1e0) at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/handler/ha_innodb.cc:23185
#3 0x0000000001733252 in check_quick_select (param=0x7fffe8d3e550, idx=0, index_only=false, tree=0x7fff040c0ed0, update_tbl_stats=true, mrr_flags=0x7fffe8d3e2e4, bufsize=0x7fffe8d3e2e0, cost=0x7fffe8d3e1e0)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/opt_range.cc:10099
#4 0x000000000172a110 in get_key_scans_params (param=0x7fffe8d3e550, tree=0x7fff040c0e08, index_read_must_be_used=false, update_tbl_stats=true, cost_est=0x7fffe8d3e430)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/opt_range.cc:5854
#5 0x0000000001723c21 in test_quick_select (thd=0x7fff04000bf0, keys_to_use=..., prev_tables=0, limit=18446744073709551615, force_quick_range=false, interesting_order=st_order::ORDER_NOT_RELEVANT,
tab=0x7fff057734a8, cond=0x7fff04007538, needed_reg=0x7fff057734e8, quick=0x7fffe8d40a38, ignore_table_scan=false) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/opt_range.cc:3108
#6 0x00000000014b2aa9 in get_quick_record_count (thd=0x7fff04000bf0, tab=0x7fff057734a8, limit=18446744073709551615) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:6013
#7 0x00000000014b2172 in JOIN::estimate_rowcount (this=0x7fff057730d0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:5760
#8 0x00000000014b05eb in JOIN::make_join_plan (this=0x7fff057730d0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:5117
#9 0x00000000014a4d06 in JOIN::optimize (this=0x7fff057730d0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:394
#10 0x000000000151f92b in st_select_lex::optimize (this=0x7fff04005d50, thd=0x7fff04000bf0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_select.cc:1018
#11 0x000000000151e053 in handle_query (thd=0x7fff04000bf0, lex=0x7fff040032a0, result=0x7fff04007780, added_options=0, removed_options=0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_select.cc:172
#12 0x00000000014d1d93 in execute_sqlcom_select (thd=0x7fff04000bf0, all_tables=0x7fff04006e58) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:5475
#13 0x00000000014cb119 in mysql_execute_command (thd=0x7fff04000bf0, first_level=true) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:3016
#14 0x00000000014d2e1b in mysql_parse (thd=0x7fff04000bf0, parser_state=0x7fffe8d424a0, update_userstat=false) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:5927
#15 0x00000000014c7a55 in dispatch_command (thd=0x7fff04000bf0, com_data=0x7fffe8d42c90, command=COM_QUERY) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:1539
#16 0x00000000014c688a in do_command (thd=0x7fff04000bf0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:1060
#17 0x00000000015fab28 in handle_connection (arg=0x3443230) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/conn_handler/connection_handler_per_thread.cc:325
#18 0x00000000018cad34 in pfs_spawn_thread (arg=0x3dd00c0) at /cdh/mysqldebug/percona-server-5.7.29-32/storage/perfschema/pfs.cc:2198
#19 0x00007ffff7bc6e65 in start_thread () from /lib64/libpthread.so.0
#20 0x00007ffff5fa088d in clone () from /lib64/libc.so.6
#0 btr_cur_search_to_nth_level (index=0x7fff0494e320, level=0, tuple=0x7fff04a619b0, mode=PAGE_CUR_GE, latch_mode=1025, cursor=0x7fffe8d39310, has_search_latch=0,
file=0x2311530 "/cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/btr/btr0cur.cc", line=5913, mtr=0x7fffe8d393b0) at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/btr/btr0cur.cc:798
#1 0x0000000001c047e9 in btr_estimate_n_rows_in_range_low (index=0x7fff0494e320, tuple1=0x7fff04a619b0, mode1=PAGE_CUR_GE, tuple2=0x7fff04a61a40, mode2=PAGE_CUR_G, nth_attempt=1)
at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/btr/btr0cur.cc:5913
#2 0x0000000001c05239 in btr_estimate_n_rows_in_range (index=0x7fff0494e320, tuple1=0x7fff04a619b0, mode1=PAGE_CUR_GE, tuple2=0x7fff04a61a40, mode2=PAGE_CUR_G)
at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/btr/btr0cur.cc:6248
#3 0x0000000001981cd7 in ha_innobase::records_in_range (this=0x7fff04954b00, keynr=1, min_key=0x7fffe8d3dc00, max_key=0x7fffe8d3dc20)
at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/handler/ha_innodb.cc:15147
#4 0x0000000000ec2adb in handler::multi_range_read_info_const (this=0x7fff04954b00, keyno=1, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges_arg=0, bufsz=0x7fffe8d3dd00, flags=0x7fffe8d3dd04,
cost=0x7fffe8d3e1e0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:6716
#5 0x0000000000ec4364 in DsMrr_impl::dsmrr_info_const (this=0x7fff04954f70, keyno=1, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges=0, bufsz=0x7fffe8d3e2e0, flags=0x7fffe8d3e2e4,
cost=0x7fffe8d3e1e0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:7391
#6 0x000000000198cbc1 in ha_innobase::multi_range_read_info_const (this=0x7fff04954b00, keyno=1, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges=0, bufsz=0x7fffe8d3e2e0, flags=0x7fffe8d3e2e4,
cost=0x7fffe8d3e1e0) at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/handler/ha_innodb.cc:23185
#7 0x0000000001733252 in check_quick_select (param=0x7fffe8d3e550, idx=0, index_only=false, tree=0x7fff04a26af0, update_tbl_stats=true, mrr_flags=0x7fffe8d3e2e4, bufsize=0x7fffe8d3e2e0, cost=0x7fffe8d3e1e0)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/opt_range.cc:10099
```
###二、唯一索引的特别执行计划
root@localhost:test:06:04:57>select *from t_un;
+----+------+---------+
| id | id2 | name |
+----+------+---------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | gaopeng |
+----+------+---------+
5 rows in set (2.74 sec)
但是实际都是做的唯一索引，不会导致全表扫描。
测试：
id2是唯一索引
root@localhost:test:06:04:57>select *from t_un;
+----+------+---------+
| id | id2 | name |
+----+------+---------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | gaopeng |
+----+------+---------+
5 rows in set (2.74 sec)
- 唯一索引没有适合的值
root@localhost:test:05:56:54>desc select *from t_un where id2=10 ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
1 row in set, 1 warning (1.75 sec)
- 唯一索引有适合的值，但是where条件过滤掉了
root@localhost:test:05:57:03>desc select *from t_un where id2=1 and name='test' ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+
1 row in set, 1 warning (2.18 sec)
ERROR:
No query specified
我看了交互信息，只看到一条数据，所以这种情况实际上也是用的唯一索引没有问题。
实际访问数据栈
```
#0 row_search_mvcc (buf=0x7fff0576e210 "\376\001", mode=PAGE_CUR_GE, prebuilt=0x7fff0414dc80, match_mode=1, direction=0) at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/row/row0sel.cc:4755
#1 0x0000000001978a27 in ha_innobase::index_read (this=0x7fff0576eb00, buf=0x7fff0576e210 "\376\001", key_ptr=0x7fff057746e0 "", key_len=5, find_flag=HA_READ_KEY_EXACT)
at /cdh/mysqldebug/percona-server-5.7.29-32/storage/innobase/handler/ha_innodb.cc:9970
#2 0x0000000000ec9c08 in handler::index_read_map (this=0x7fff0576eb00, buf=0x7fff0576e210 "\376\001", key=0x7fff057746e0 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.h:2990
#3 0x0000000000ec576f in handler::index_read_idx_map (this=0x7fff0576eb00, buf=0x7fff0576e210 "\376\001", index=1, key=0x7fff057746e0 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:8051
#4 0x0000000000ebb3b2 in handler::ha_index_read_idx_map (this=0x7fff0576eb00, buf=0x7fff0576e210 "\376\001", index=1, key=0x7fff057746e0 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT)
at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:3336
#5 0x00000000014862a9 in read_const (table=0x7fff0546eb00, ref=0x7fff05773b50) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_executor.cc:2020
#6 0x0000000001485d8c in join_read_const_table (tab=0x7fff05773a80, pos=0x7fff05773c18) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_executor.cc:1905
#7 0x00000000014b1aeb in JOIN::extract_func_dependent_tables (this=0x7fff05773498) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:5645
#8 0x00000000014b058d in JOIN::make_join_plan (this=0x7fff05773498) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:5109
#9 0x00000000014a4d06 in JOIN::optimize (this=0x7fff05773498) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_optimizer.cc:394
#10 0x000000000151f92b in st_select_lex::optimize (this=0x7fff04005d50, thd=0x7fff04000bf0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_select.cc:1018
#11 0x000000000151e053 in handle_query (thd=0x7fff04000bf0, lex=0x7fff040032a0, result=0x7fff04007ba8, added_options=0, removed_options=0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_select.cc:172
#12 0x00000000014d1e8d in execute_sqlcom_select (thd=0x7fff04000bf0, all_tables=0x7fff04006fd0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:5490
#13 0x00000000014cb119 in mysql_execute_command (thd=0x7fff04000bf0, first_level=true) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:3016
#14 0x00000000014d2e1b in mysql_parse (thd=0x7fff04000bf0, parser_state=0x7fffe8d424a0, update_userstat=false) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:5927
#15 0x00000000014c7a55 in dispatch_command (thd=0x7fff04000bf0, com_data=0x7fffe8d42c90, command=COM_QUERY) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:1539
#16 0x00000000014c688a in do_command (thd=0x7fff04000bf0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/sql_parse.cc:1060
#17 0x00000000015fab28 in handle_connection (arg=0x3443230) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/conn_handler/connection_handler_per_thread.cc:325
#18 0x00000000018cad34 in pfs_spawn_thread (arg=0x3dd00c0) at /cdh/mysqldebug/percona-server-5.7.29-32/storage/perfschema/pfs.cc:2198
#19 0x00007ffff7bc6e65 in start_thread () from /lib64/libpthread.so.0
#20 0x00007ffff5fa088d in clone () from /lib64/libc.so.6
(gdb) c
Continuing.
Breakpoint 7, handler::multi_range_read_info_const (this=0x7fff0576eb00, keyno=2, seq=0x7fffe8d3ddb0, seq_init_param=0x7fffe8d3ddf0, n_ranges_arg=0, bufsz=0x7fffe8d3dd00, flags=0x7fffe8d3dd04,
cost=0x7fffe8d3e1e0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/handler.cc:6651
6651 ha_rows rows, total_rows= 0;
```
深入理解MySQL主从原理：https://www.jianshu.com/nb/43148932
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