歡迎關注我的《深入理解MySQL主從原理 32講 》，如下：

![image.png](https://upload-images.jianshu.io/upload_images/7398834-0ffa3bdc078cddf4.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
如果圖片不能顯示可檢視下面連結： 
https://www.jianshu.com/p/d636215d767f

###1、限制條件

一般不能是唯一鍵和主鍵，也不能是全表，程式碼如下：

```
if (used_index != MAX_KEY) //不能是唯一鍵(主鍵) 和 全表 
  { // Check if we are modifying a key that we are used to search with:
    used_key_is_modified= is_key_used(table, used_index, table->write_set);//透過寫點陣圖write_set 進行確認 查詢的條件和修改的條件相同
  }
```
  
###2、進入狀態

進入stage_searching_rows_for_update狀態

```
THD_STAGE_INFO(thd, stage_searching_rows_for_update);
        ha_rows tmp_limit= limit;

IO_CACHE *tempfile= (IO_CACHE*) my_malloc(key_memory_TABLE_sort_io_cache,
                                                  sizeof(IO_CACHE),
                                                  MYF(MY_FAE | MY_ZEROFILL));
```

###3、臨時檔案使用

建立MY開頭的臨時檔案，在tmp目錄下，掃描行加入到臨時檔案中，供後面實際的update操作使用，會進入實際的update操作會進入stage_updating狀態，如下：

```
       if (open_cached_file(tempfile, mysql_tmpdir,TEMP_PREFIX,
                             DISK_BUFFER_SIZE, MYF(MY_WME)))//開啟一個MY臨時檔案
        {
          my_free(tempfile);
          goto exit_without_my_ok;
        }

while (!(error=info.read_record(&info)) && !thd->killed)
        {
          thd->inc_examined_row_count(1);//掃描增加 
          bool skip_record= FALSE;
          if (qep_tab.skip_record(thd, &skip_record))
...
```

###4、測試總結：

```
mysql> show create table test0820;
+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                                                                                    |
+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test0820 | CREATE TABLE `test0820` (
  `id` int(11) NOT NULL,
  `name` varchar(20) DEFAULT NULL,
  `name1` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `idx_u_test` (`name1`),
  KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

- 主鍵ID更新不觸發
- 唯一鍵idx_u_test更新不觸發
- 普通索引name更新觸發

如果update執行計劃出現Using temporary 則會使用stage_searching_rows_for_update。

```

###5、stage_searching_rows_for_update狀態掃描資料已經加鎖，因此很容易測試這種情況

棧幀：

```
#0  0x00007ffff7bd368c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x0000000001b2f921 in os_event::wait (this=0x7ffee0e418e8) at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/include/os0event.h:156
#2  0x0000000001b2f269 in os_event::wait_low (this=0x7ffee0e418e8, reset_sig_count=1)
    at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/os/os0event.cc:131
#3  0x0000000001b2f692 in os_event_wait_low (event=0x7ffee0e418e8, reset_sig_count=0)
    at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/os/os0event.cc:328
#4  0x0000000001af0c4b in lock_wait_suspend_thread (thr=0x7ffee0e42ed0) at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0wait.cc:387
#5  0x0000000001bb6de8 in row_mysql_handle_errors (new_err=0x7fffec5eb7bc, trx=0x7fffd7804080, thr=0x7ffee0e42ed0, savept=0x0)
    at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/row/row0mysql.cc:1312
#6  0x0000000001bf9ed6 in row_search_mvcc (buf=0x7ffee097fb40 "\377", mode=PAGE_CUR_GE, prebuilt=0x7ffee0e42730, match_mode=1, direction=0)
    at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:6318
#7  0x0000000001a53113 in ha_innobase::index_read (this=0x7ffee0952030, buf=0x7ffee097fb40 "\377", key_ptr=0x7ffee0a2f6d0 "", key_len=63, find_flag=HA_READ_KEY_EXACT)
    at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/handler/ha_innodb.cc:9536
#8  0x0000000000f933c2 in handler::index_read_map (this=0x7ffee0952030, buf=0x7ffee097fb40 "\377", key=0x7ffee0a2f6d0 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT)
    at /mysqldata/percona-server-locks-detail-5.7.22/sql/handler.h:2942
#9  0x0000000000f83dac in handler::ha_index_read_map (this=0x7ffee0952030, buf=0x7ffee097fb40 "\377", key=0x7ffee0a2f6d0 "", keypart_map=1, 
    find_flag=HA_READ_KEY_EXACT) at /mysqldata/percona-server-locks-detail-5.7.22/sql/handler.cc:3248
#10 0x0000000000f8e844 in handler::read_range_first (this=0x7ffee0952030, start_key=0x7ffee0952118, end_key=0x7ffee0952138, eq_range_arg=true, sorted=true)
    at /mysqldata/percona-server-locks-detail-5.7.22/sql/handler.cc:7750
#11 0x0000000000f8c775 in handler::multi_range_read_next (this=0x7ffee0952030, range_info=0x7fffec5ec370)
    at /mysqldata/percona-server-locks-detail-5.7.22/sql/handler.cc:6817
#12 0x0000000000f8d68d in DsMrr_impl::dsmrr_next (this=0x7ffee09524a0, range_info=0x7fffec5ec370) at /mysqldata/percona-server-locks-detail-5.7.22/sql/handler.cc:7204
#13 0x0000000001a6689a in ha_innobase::multi_range_read_next (this=0x7ffee0952030, range_info=0x7fffec5ec370)
    at /mysqldata/percona-server-locks-detail-5.7.22/storage/innobase/handler/ha_innodb.cc:22211
#14 0x00000000017bdbd8 in QUICK_RANGE_SELECT::get_next (this=0x7ffee0e40250) at /mysqldata/percona-server-locks-detail-5.7.22/sql/opt_range.cc:11237
#15 0x00000000014e27f5 in rr_quick (info=0x7fffec5ec870) at /mysqldata/percona-server-locks-detail-5.7.22/sql/records.cc:399
#16 0x000000000168c103 in mysql_update (thd=0x7ffee0000c00, fields=..., values=..., limit=18446744073709551615, handle_duplicates=DUP_ERROR, 
    found_return=0x7fffec5ecbd8, updated_return=0x7fffec5ecbd0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_update.cc:691
#17 0x0000000001692e40 in Sql_cmd_update::try_single_table_update (this=0x7ffee0006bc0, thd=0x7ffee0000c00, switch_to_multitable=0x7fffec5ecc7f)
    at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_update.cc:2896
#18 0x000000000169338d in Sql_cmd_update::execute (this=0x7ffee0006bc0, thd=0x7ffee0000c00) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_update.cc:3023
#19 0x00000000015cc801 in mysql_execute_command (thd=0x7ffee0000c00, first_level=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:3756
#20 0x00000000015d2fde in mysql_parse (thd=0x7ffee0000c00, parser_state=0x7fffec5ee600) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:5901
#21 0x00000000015c6b72 in dispatch_command (thd=0x7ffee0000c00, com_data=0x7fffec5eed70, command=COM_QUERY)
    at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1490
#22 0x00000000015c58ff in do_command (thd=0x7ffee0000c00) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1021
#23 0x000000000170e578 in handle_connection (arg=0x6795460) at /mysqldata/percona-server-locks-detail-5.7.22/sql/conn_handler/connection_handler_per_thread.cc:312
#24 0x0000000001945538 in pfs_spawn_thread (arg=0x6947660) at /mysqldata/percona-server-locks-detail-5.7.22/storage/perfschema/pfs.cc:2190
#25 0x00007ffff7bcfaa1 in start_thread () from /lib64/libpthread.so.0
#26 0x00007ffff6b37c4d in clone () from /lib64/libc.so.6

```

|T1|T2|
|---|---|
|BEGIN; ||
|delete from test0820;||
|| update test0820 set name='100' where name='90'|

顯示如下：

```

```  
          
          
###6、疑問：

- 其他還有一些特殊情況，包含哪些？
- update執行計劃出現Using temporary是在哪裡做的？

隨筆MySQL：Searching rows for update狀態解析

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