show create table底层流程跟踪

create table t1(c1 int);
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dispatch_sql_command   # 对sql语句做词法/语法解析，得到实际要运行的sql命令
mysql_execute_command  # 根据lex->sql_command值调用对应方法执行查询操作
Sql_cmd_show_noplan::execute
Sql_cmd_show_create_table::execute_inner  # '执行'show create table指令
mysqld_show_create  # 由Sql_cmd_show_create_table::execute_inner调用，获取表创建信息
store_create_info   # 根据表属性拼接建表字串
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bool Sql_cmd_show_create_table::execute_inner(THD *thd) {
  // ... ... 注：为显示和说明方便，部分代码已被省略，可自行参阅源码读取更全面信息
  // 将指定表加入至session的table list，并初始化表的锁信息；相当于让session知道，本次查询
  // 将会用到这张表。
  if (lex->query_block->add_table_to_list(thd, m_table_ident, nullptr, 0) ==
      nullptr)
    return true;
  TABLE_LIST *tbl = lex->query_tables;
  // ... ...
  if (mysqld_show_create(thd, tbl)) return true;  //断点位置

  return false;
}
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bool mysqld_show_create(THD *thd, TABLE_LIST *table_list) {
  // ... ...
  // 打开指定表/视图，获取显示数据信息所需的元数据锁(MDL)
  bool open_error = open_tables(thd, &table_list, &counter,
                                  MYSQL_OPEN_FORCE_SHARED_HIGH_PRIO_MDL);

  // 'show create table'语句也可以显示view的创建信息，本方法根据对应table element
  // 的属性来确定调用view_store_create_info()还是store_create_info().
  if (table_list->is_view())
    view_store_create_info(thd, table_list, &buffer);
  else if (store_create_info(thd, table_list, &buffer, nullptr, false)) //断点位置
    goto exit;

  // 获取到表/视图创建信息后，本方法还会组一张虚拟表返回给客户端，客户端根据得到的表头和
  // 数据内容，完成信息的展示，其结果示例如：
  // +-------+----------------------------+
  // | Table | Create Table               |
  // +-------+----------------------------+
  // | t1    | CREATE TABLE `t1` `c1` ... |
  // +-------+----------------------------+
  if (table_list->is_view()) {
    field_list.push_back(new Item_empty_string("View", NAME_CHAR_LEN));
    field_list.push_back(new Item_empty_string(
        "Create View", max<uint>(buffer.length(), 1024U)));
    field_list.push_back(
        new Item_empty_string("character_set_client", MY_CS_NAME_SIZE));
    field_list.push_back(
        new Item_empty_string("collation_connection", MY_CS_NAME_SIZE));
  } else {
    field_list.push_back(new Item_empty_string("Table", NAME_CHAR_LEN));
    // 1024 is for not to confuse old clients
    field_list.push_back(new Item_empty_string(
        "Create Table", max<size_t>(buffer.length(), 1024U)));
  }
  // ... ... 
}
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bool store_create_info(THD *thd, TABLE_LIST *table_list, String *packet,
                       HA_CREATE_INFO *create_info_arg, bool show_database) {

  // ... ...
  // 根据指定表的属性，确定创建语句
  if (share->tmp_table)
    packet->append(STRING_WITH_LEN("CREATE TEMPORARY TABLE "));
  else
    packet->append(STRING_WITH_LEN("CREATE TABLE "));
  if (create_info_arg &&
      (create_info_arg->options & HA_LEX_CREATE_IF_NOT_EXISTS))
    packet->append(STRING_WITH_LEN("IF NOT EXISTS "));
  if (table_list->schema_table)
    alias = table_list->schema_table->table_name;
  else {
    if (lower_case_table_names == 2)
      alias = table->alias;
    else {
      alias = share->table_name.str;
    }
  }

  // ... ...
  // 表的列信息生成
  for (ptr = table->field; (field = *ptr); ptr++) {
    // ... ...
    if (ptr != table->field) packet->append(STRING_WITH_LEN(",\n"));

    packet->append(STRING_WITH_LEN("  "));
    append_identifier(thd, packet, field->field_name,
                      strlen(field->field_name));
    packet->append(' ');
    // check for surprises from the previous call to Field::sql_type()
    if (type.ptr() != tmp)
      type.set(tmp, sizeof(tmp), system_charset_info);
    else
      type.set_charset(system_charset_info);

  // ... ...
}
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create index idx_c1 on t1(idx);
alter table t1 add column c2 int;
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> show create table t1;
+-------+----------------------------------------------------------+
| Table | Create Table                                             |
+-------+----------------------------------------------------------+
| t1    | CREATE TABLE `t1` (                                      |
`c1` int DEFAULT NULL                                              |        
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+----------------------------------------------------------+
1 row in set (1 hour 25 min 45.87 sec)
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> show create table t1;
+-------+----------------------------------------------------------+
| Table | Create Table                                             |
+-------+----------------------------------------------------------+
| t1    | CREATE TABLE `t1` (                                      |
`c1` int DEFAULT NULL,                                             |       
`c2` int DEFAULT NULL,                                             |
KEY `idx_c1` (`c1`)                                                |
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------+----------------------------------------------------------+
1 row in set (0.00 sec)
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