MySQL如何創建和刪除臨時表

 

1.介紹:

MySQL臨時表,屬於session級別,當session退出時,臨時表被刪除。臨時表允許與其他表同名,並單獨維護在thd的結構體中;因此,不同的session可以創建同名的臨時表,並且隻操作自己擁有的臨時表;

 

 

創建臨時表的語法很簡單:

 

root@test 03:26:44>show create table tmp1\G

*************************** 1. row ***************************

       Table: tmp1

Create Table: CREATE TEMPORARY TABLE `tmp1` (

  `a` int(11) NOT NULL AUTO_INCREMENT,

  `b` int(11) DEFAULT NULL,

  `c` int(11) DEFAULT NULL,

  PRIMARY KEY (`a`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.00 sec)

 

當創建臨時表後,會在tmp文件夾下生成兩個文件:

#sql3e95_1a_0.frm

#sql3e95_1a_0.ibd

 

那麼MySQL本身究竟是如何創建和刪除臨時表的呢?

 

2.創建

執行SQL:

 

CREATE TEMPORARY TABLE `tmp1` (   `a` int(11) NOT NULL AUTO_INCREMENT,   `b` int(11) DEFAULT NULL,   `c` int(11) DEFAULT NULL,   PRIMARY KEY (`a`) );

 

1)斷點:ysql_execute_command

 

mysql_execute_command:

2205      switch (lex->sql_command) {

(gdb)

2532        if (!(lex->create_info.options & HA_LEX_CREATE_TMP_TABLE))

(gdb) p lex->create_info.options          ——–if語句裡為false

$2 = 1

          create_table_precheck——檢查是否具有創建表的權限,以及表名在全局鏈表上是否已存在(臨時表無需檢查)

         

          append_file_to_dir     ——Fix names if symlinked tables

 

           if (select_lex->item_list.elements)               ——————-當為create ….select這樣的語句時select_lex->item_list.elements為非0值,這裡我們隻考慮簡單的情況

                 if ((result= new select_create))

                           res= handle_select(thd, lex, result, 0);

           else

               (1)mysql_create_like_table           —————create table like…類似的語句

               (2)mysql_create_table                 —————主要分析這個函數

 

2)斷點:mysql_create_table

 

mysql_create_table

       mysql_create_table_no_lock

             check_engine

             file = get_new_handler

3842      set_table_default_charset(thd, create_info, (char*) db);

3844      if (mysql_prepare_create_table(thd, create_info, alter_info,

3854      path_length= build_tmptable_filename(thd, path, sizeof(path));   —–創建臨時表文件名:#sql{進程id}_{thread_id}_{當前線程的臨時表整數標識thd->tmp_table}               

3978      rea_create_table ——————————————————————–創建frm文件和ibd文件

3986      open_temporary_table————————————————————-打開臨時表

                            (1)構建table和table_share結構體

                            (2)將table結構體加入到thd->temporary_tables鏈表中

                               

4009      error= write_create_table_bin_log———————————————-寫入binlog

 

3.刪除臨時表

手動執行 drop table tmp1

 

mysql_execute_command

      case SQLCOM_DROP_TABLE:

            mysql_rm_table

                mysql_rm_table_part2

                          for (table= tables; table; table= table->next_local)

                                     drop_temporary_table—————————–從thd->temporary_tables上查找臨時表           

                                                           調用close_temporary_table來關閉、刪除臨時表文件,並從thd->temporary_tables上刪除相應節點

                                     if (!drop_temporary)——————————-當刪除的是非臨時表時,執行下面的邏輯

—————————————-

4. 當session退出時。

看看堆棧:

Breakpoint 16, rm_temporary_table (base=0xc8c560, path=0x1427c10 "/u01/mysql-5148.stock/tmp/#sql3e95_1d_0") at sql_base.cc:5634

5634    bool rm_temporary_table(handlerton *base, char *path)

(gdb)

5641      strmov(ext= strend(path), reg_ext);

(gdb) bt

#0  rm_temporary_table (base=0xc8c560, path=0x1427c10 "/u01/mysql-5148.stock/tmp/#sql3e95_1d_0") at sql_base.cc:5641

#1  0x00000000005f6eaa in close_temporary (table=0x1427030, free_share=true, delete_table=true) at sql_base.cc:1928

#2  0x00000000005f725f in close_temporary_tables (thd=0x14065f0) at sql_base.cc:1549

#3  0x0000000000592d9b in THD::cleanup (this=0x14065f0) at sql_class.cc:967

#4  0x00000000005a3579 in unlink_thd (thd=0xc8c560) at mysqld.cc:1858

#5  0x00000000005a35dc in one_thread_per_connection_end (thd=0xc8c560, put_in_cache=16) at mysqld.cc:1945

#6  0x00000000005ac208 in handle_one_connection (arg=0x14065f0) at sql_connect.cc:1141

#7  0x0000003e638064a7 in start_thread () from /lib64/libpthread.so.0

#8  0x0000003e630d3c2d in clone () from /lib64/libc.so.6

#9  0x0000000000000000 in ?? ()

 

在session結束的時候,會調用THD::cleanup來做臨時表的清理工作

 

摘自 記錄成長之路

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