MySQL 8.0 GDB源码调试表的打开

MySQL 8.0 表打开与缓存机制

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需要打断点的函数如下

get_table_share、open_table_from_share、Table_cache::add_used_table

[root@ecs-test-node2 mysql-8.0.42]# gdb -p $(pgrep -x mysqld)
..
(gdb) break get_table_share
Breakpoint 1 at 0x34fb18b: get_table_share. (3 locations)
(gdb) break open_table_from_share
Breakpoint 2 at 0x37b5f77: file /data/mysql-8.0.42/sql/table.cc, line 2876.
(gdb) break Table_cache::add_used_table
Breakpoint 3 at 0x35166d7: file /data/mysql-8.0.42/sql/table_cache.h, line 348.

在终端中开启一个mysql客户端会话，查询一张表的记录

lily [(none)]> use summer
Database changed
lily [summer]> select * from emp where id = 1;

大致看一下函数的调用过程

(gdb) c
Continuing.
[Switching to Thread 0x7fac54166700 (LWP 1463531)]

Thread 36 "connection" hit Breakpoint 1, get_table_share (thd=0x7fabfc008050, db=0x7fabfc021e00 "summer", 
    table_name=0x7fabfc020a40 "emp", key=0x7fabfc021ba7 "summer", key_length=11, open_view=true, 
    open_secondary=false) at /data/mysql-8.0.42/sql/sql_base.cc:686
686	  bool open_table_err = false;
(gdb) backtrace
#0  get_table_share (thd=0x7fabfc008050, db=0x7fabfc021e00 "summer", table_name=0x7fabfc020a40 "emp", 
    key=0x7fabfc021ba7 "summer", key_length=11, open_view=true, open_secondary=false)
    at /data/mysql-8.0.42/sql/sql_base.cc:686
#1  0x00000000034fbe02 in get_table_share_with_discover (thd=0x7fabfc008050, table_list=0x7fabfc0217b0, 
    key=0x7fabfc021ba7 "summer", key_length=11, open_secondary=false, error=0x7fac54162c9c)
    at /data/mysql-8.0.42/sql/sql_base.cc:903
#2  0x00000000035014e5 in open_table (thd=0x7fabfc008050, table_list=0x7fabfc0217b0, ot_ctx=0x7fac54162f70)
    at /data/mysql-8.0.42/sql/sql_base.cc:3221
#3  0x000000000350573c in open_and_process_table (thd=0x7fabfc008050, lex=0x7fabfc21ee60, tables=0x7fabfc0217b0, 
    counter=0x7fabfc21eeb8, prelocking_strategy=0x7fac54163098, has_prelocking_list=false, ot_ctx=0x7fac54162f70)
    at /data/mysql-8.0.42/sql/sql_base.cc:5053
#4  0x00000000035070b0 in open_tables (thd=0x7fabfc008050, start=0x7fac54163040, counter=0x7fabfc21eeb8, flags=0, 
    prelocking_strategy=0x7fac54163098) at /data/mysql-8.0.42/sql/sql_base.cc:5858
#5  0x0000000003508b26 in open_tables_for_query (thd=0x7fabfc008050, tables=0x7fabfc0217b0, flags=0)
    at /data/mysql-8.0.42/sql/sql_base.cc:6747
#6  0x00000000036aab43 in Sql_cmd_dml::prepare (this=0x7fabfc107168, thd=0x7fabfc008050)
    at /data/mysql-8.0.42/sql/sql_select.cc:541
#7  0x00000000036ab580 in Sql_cmd_dml::execute (this=0x7fabfc107168, thd=0x7fabfc008050)
    at /data/mysql-8.0.42/sql/sql_select.cc:710
#8  0x00000000036266d2 in mysql_execute_command (thd=0x7fabfc008050, first_level=true)
    at /data/mysql-8.0.42/sql/sql_parse.cc:4724
#9  0x0000000003628aae in dispatch_sql_command (thd=0x7fabfc008050, parser_state=0x7fac54164970)
    at /data/mysql-8.0.42/sql/sql_parse.cc:5385
#10 0x000000000361e683 in dispatch_command (thd=0x7fabfc008050, com_data=0x7fac54165a60, command=COM_QUERY)
    at /data/mysql-8.0.42/sql/sql_parse.cc:2055
#11 0x000000000361c5ca in do_command (thd=0x7fabfc008050) at /data/mysql-8.0.42/sql/sql_parse.cc:1440
#12 0x0000000003844598 in handle_connection (arg=0xcbc83d0)
    at /data/mysql-8.0.42/sql/conn_handler/connection_handler_per_thread.cc:303
#13 0x00000000058bf53d in pfs_spawn_thread (arg=0xce22c50) at /data/mysql-8.0.42/storage/perfschema/pfs.cc:3050
#14 0x00007fac7cd991ca in start_thread (arg=<optimized out>) at pthread_create.c:479
#15 0x00007fac7b2b88d3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

首先会建立连接

handle_connection(void * arg) (/data/mysql-8.0.42/sql/conn_handler/connection_handler_per_thread.cc:303)

之后会分发解析语句

mysql_execute_command(THD * thd, bool first_level) (/data/mysql-8.0.42/sql/sql_parse.cc:4724)
dispatch_sql_command(THD * thd, Parser_state * parser_state) (/data/mysql-8.0.42/sql/sql_parse.cc:5385)

之后会打开表

open_tables(THD * thd, Table_ref ** start, uint * counter, uint flags, Prelocking_strategy * prelocking_strategy) (/data/mysql-8.0.42/sql/sql_base.cc:5858)

打开表的主函数是open_tables

/**
  Open all tables in list
*/

bool open_tables(THD *thd, Table_ref **start, uint *counter, uint flags,
                 Prelocking_strategy *prelocking_strategy)

在打开一张表的时候，MySQL会尝试从table definition cache这个缓存中去获取表的定义。如果TABLE_SHARE不存在，则会创建一份新的缓存。

对应的函数如下

/**
  Get the TABLE_SHARE for a table.
*/

TABLE_SHARE *get_table_share(THD *thd, const char *db, const char *table_name,
                             const char *key, size_t key_length, bool open_view,
                             bool open_secondary)

在open_table函数中，会尝试获取TABLE_SHARE缓存。

  if (!(share = get_table_share_with_discover(
            thd, table_list, key, key_length,
            flags & MYSQL_OPEN_SECONDARY_ENGINE, &error))) {
    mysql_mutex_unlock(&LOCK_open);
    ..}

在上面的函数中，会最终调用get_table_share这个函数来获取缓存。

/**
  Get a table share. If it didn't exist, try creating it from engine

  For arguments and return values, see get_table_share()
*/

static TABLE_SHARE *get_table_share_with_discover(
    THD *thd, Table_ref *table_list, const char *key, size_t key_length,
    bool open_secondary, int *error)

{
  TABLE_SHARE *share;
  bool exists;
  DBUG_TRACE;

  share = get_table_share(thd, table_list->db, table_list->table_name, key,
                          key_length, true, open_secondary);
  /*
    If share is not NULL, we found an existing share.
  */

如果这缓存存在，会使用缓存中的TABLE_SHARE对象。

对应的函数如下

/**
  Open a table based on a TABLE_SHARE
*/

int open_table_from_share(THD *thd, TABLE_SHARE *share, const char *alias,
                          uint db_stat, uint prgflag, uint ha_open_flags,
                          TABLE *outparam, bool is_create_table,
                          const dd::Table *table_def_param)

如果是一张新表，则会把新的表对象添加到table cache中。

    /* Add new TABLE object to table cache for this connection. */
    Table_cache *tc = table_cache_manager.get_cache(thd);

    tc->lock();

    if (tc->add_used_table(thd, table)) {
      tc->unlock();
      goto err_lock;
    }
    tc->unlock();