MySQL:2020 端午节随笔（索引下探和唯一索引特殊执行计划）

###一、索引数据下探

http://blog.itpub.net/7728585/viewspace-2660796/

/*

      Get the number of rows in the range. This is done by calling

      records_in_range() unless:

        1) The range is an equality range and the index is unique.

           There cannot be more than one matching row, so 1 is

           assumed. Note that it is possible that the correct number

           is actually 0, so the row estimate may be too high in this

           case. Also note: ranges of the form "x IS NULL" may have more

           than 1 mathing row so records_in_range() is called for these.

        2) a) The range is an equality range but the index is either 

              not unique or all of the keyparts are not used. 

           b) The user has requested that index statistics should be used

              for equality ranges to avoid the incurred overhead of 

              index dives in records_in_range().

           c) Index statistics is available.

           Ranges of the form "x IS NULL" will not use index statistics 

           because the number of rows with this value are likely to be 

           very different than the values in the index statistics.

    */

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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