原因

• 想通过show global status来完成对MySQL关键指标的采集，但是不知道这个命令频繁执行是否会造成性能压力，网上也没有搜到相关的资料。mysql_exporter工具也是使用的这个命令。

结论

取出值的时候可能对心痛有影响

成都创新互联公司坚持“要么做到，要么别承诺”的工作理念，服务领域包括：成都网站设计、网站建设、企业官网、英文网站、手机端网站、网站推广等服务，满足客户于互联网时代的平昌网站设计、移动媒体设计的需求，帮助企业找到有效的互联网解决方案。努力成为您成熟可靠的网络建设合作伙伴！

思路

查看5.7源码

vim mysql-server-mysql-5.7.21/sql/mysqld.cc

/// 从6801行开始定义了输出的性能指标结构体
/// 结构体定义可以简单理解为
/*

struct st_mysql_show_var {
  const char *name;
  char *value;
  enum enum_mysql_show_type type;
};
*/

/*
  Variables shown by SHOW STATUS in alphabetical order
*/

SHOW_VAR status_vars[]= {
  {"Aborted_clients",          (char*) &aborted_threads,                              SHOW_LONG,               SHOW_SCOPE_GLOBAL},
#ifndef EMBEDDED_LIBRARY
  {"Aborted_connects",         (char*) &show_aborted_connects,                        SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
#endif
#ifdef HAVE_REPLICATION
#ifndef DBUG_OFF
  {"Ongoing_anonymous_gtid_violating_transaction_count",(char*) &show_ongoing_anonymous_gtid_violating_transaction_count, SHOW_FUNC, SHOW_SCOPE_GLOBAL},
#endif//!DBUG_OFF
  {"Ongoing_anonymous_transaction_count",(char*) &show_ongoing_anonymous_transaction_count, SHOW_FUNC, SHOW_SCOPE_GLOBAL},
#ifndef DBUG_OFF
  {"Ongoing_automatic_gtid_violating_transaction_count",(char*) &show_ongoing_automatic_gtid_violating_transaction_count, SHOW_FUNC, SHOW_SCOPE_GLOBAL},
#endif//!DBUG_OFF
#endif//HAVE_REPLICATION
  {"Binlog_cache_disk_use",    (char*) &binlog_cache_disk_use,                        SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Binlog_cache_use",         (char*) &binlog_cache_use,                             SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Binlog_stmt_cache_disk_use",(char*) &binlog_stmt_cache_disk_use,                  SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Binlog_stmt_cache_use",    (char*) &binlog_stmt_cache_use,                        SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Bytes_received",           (char*) offsetof(STATUS_VAR, bytes_received),          SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Bytes_sent",               (char*) offsetof(STATUS_VAR, bytes_sent),              SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Com",                      (char*) com_status_vars,                               SHOW_ARRAY,              SHOW_SCOPE_ALL},
  {"Com_stmt_reprepare",       (char*) offsetof(STATUS_VAR, com_stmt_reprepare),      SHOW_LONG_STATUS,        SHOW_SCOPE_ALL},
  {"Compression",              (char*) &show_net_compression,                         SHOW_FUNC,               SHOW_SCOPE_SESSION},
  {"Connections",              (char*) &show_thread_id_count,                         SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
#ifndef EMBEDDED_LIBRARY
  {"Connection_errors_accept",   (char*) &show_connection_errors_accept,              SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
  {"Connection_errors_internal", (char*) &connection_errors_internal,                 SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Connection_errors_max_connections",   (char*) &show_connection_errors_max_connection, SHOW_FUNC,           SHOW_SCOPE_GLOBAL},
  {"Connection_errors_peer_address", (char*) &connection_errors_peer_addr,            SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Connection_errors_select",   (char*) &show_connection_errors_select,              SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
  {"Connection_errors_tcpwrap",  (char*) &show_connection_errors_tcpwrap,             SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
#endif
  {"Created_tmp_disk_tables",  (char*) offsetof(STATUS_VAR, created_tmp_disk_tables), SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Created_tmp_files",        (char*) &my_tmp_file_created,                          SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Created_tmp_tables",       (char*) offsetof(STATUS_VAR, created_tmp_tables),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Delayed_errors",           (char*) &delayed_insert_errors,                        SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Delayed_insert_threads",   (char*) &delayed_insert_threads,                       SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Delayed_writes",           (char*) &delayed_insert_writes,                        SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Flush_commands",           (char*) &refresh_version,                              SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Handler_commit",           (char*) offsetof(STATUS_VAR, ha_commit_count),         SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_delete",           (char*) offsetof(STATUS_VAR, ha_delete_count),         SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_discover",         (char*) offsetof(STATUS_VAR, ha_discover_count),       SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_external_lock",    (char*) offsetof(STATUS_VAR, ha_external_lock_count),  SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_mrr_init",         (char*) offsetof(STATUS_VAR, ha_multi_range_read_init_count), SHOW_LONGLONG_STATUS, SHOW_SCOPE_ALL},
  {"Handler_prepare",          (char*) offsetof(STATUS_VAR, ha_prepare_count),        SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_first",       (char*) offsetof(STATUS_VAR, ha_read_first_count),     SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_key",         (char*) offsetof(STATUS_VAR, ha_read_key_count),       SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_last",        (char*) offsetof(STATUS_VAR, ha_read_last_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_next",        (char*) offsetof(STATUS_VAR, ha_read_next_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_prev",        (char*) offsetof(STATUS_VAR, ha_read_prev_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_rnd",         (char*) offsetof(STATUS_VAR, ha_read_rnd_count),       SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_read_rnd_next",    (char*) offsetof(STATUS_VAR, ha_read_rnd_next_count),  SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_rollback",         (char*) offsetof(STATUS_VAR, ha_rollback_count),       SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_savepoint",        (char*) offsetof(STATUS_VAR, ha_savepoint_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_savepoint_rollback",(char*) offsetof(STATUS_VAR, ha_savepoint_rollback_count), SHOW_LONGLONG_STATUS, SHOW_SCOPE_ALL},
  {"Handler_update",           (char*) offsetof(STATUS_VAR, ha_update_count),         SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Handler_write",            (char*) offsetof(STATUS_VAR, ha_write_count),          SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Key_blocks_not_flushed",   (char*) offsetof(KEY_CACHE, global_blocks_changed),    SHOW_KEY_CACHE_LONG,     SHOW_SCOPE_GLOBAL},
  {"Key_blocks_unused",        (char*) offsetof(KEY_CACHE, blocks_unused),            SHOW_KEY_CACHE_LONG,     SHOW_SCOPE_GLOBAL},
  {"Key_blocks_used",          (char*) offsetof(KEY_CACHE, blocks_used),              SHOW_KEY_CACHE_LONG,     SHOW_SCOPE_GLOBAL},
  {"Key_read_requests",        (char*) offsetof(KEY_CACHE, global_cache_r_requests),  SHOW_KEY_CACHE_LONGLONG, SHOW_SCOPE_GLOBAL},
  {"Key_reads",                (char*) offsetof(KEY_CACHE, global_cache_read),        SHOW_KEY_CACHE_LONGLONG, SHOW_SCOPE_GLOBAL},
  {"Key_write_requests",       (char*) offsetof(KEY_CACHE, global_cache_w_requests),  SHOW_KEY_CACHE_LONGLONG, SHOW_SCOPE_GLOBAL},
  {"Key_writes",               (char*) offsetof(KEY_CACHE, global_cache_write),       SHOW_KEY_CACHE_LONGLONG, SHOW_SCOPE_GLOBAL},
  {"Last_query_cost",          (char*) offsetof(STATUS_VAR, last_query_cost),         SHOW_DOUBLE_STATUS,      SHOW_SCOPE_SESSION},
  {"Last_query_partial_plans", (char*) offsetof(STATUS_VAR, last_query_partial_plans),SHOW_LONGLONG_STATUS,    SHOW_SCOPE_SESSION},
#ifndef EMBEDDED_LIBRARY
  {"Locked_connects",          (char*) &locked_account_connection_count,              SHOW_LONG,               SHOW_SCOPE_GLOBAL},
#endif
  {"Max_execution_time_exceeded",   (char*) offsetof(STATUS_VAR, max_execution_time_exceeded),   SHOW_LONGLONG_STATUS, SHOW_SCOPE_ALL},
  {"Max_execution_time_set",        (char*) offsetof(STATUS_VAR, max_execution_time_set),        SHOW_LONGLONG_STATUS, SHOW_SCOPE_ALL},
  {"Max_execution_time_set_failed", (char*) offsetof(STATUS_VAR, max_execution_time_set_failed), SHOW_LONGLONG_STATUS, SHOW_SCOPE_ALL},
  {"Max_used_connections",     (char*) &Connection_handler_manager::max_used_connections,        SHOW_LONG,        SHOW_SCOPE_GLOBAL},
  {"Max_used_connections_time",(char*) &show_max_used_connections_time,               SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
  {"Not_flushed_delayed_rows", (char*) &delayed_rows_in_use,                          SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Open_files",               (char*) &my_file_opened,                               SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Open_streams",             (char*) &my_stream_opened,                             SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Open_table_definitions",   (char*) &show_table_definitions,                       SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
  {"Open_tables",              (char*) &show_open_tables,                             SHOW_FUNC,               SHOW_SCOPE_ALL},
  {"Opened_files",             (char*) &my_file_total_opened,                         SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Opened_tables",            (char*) offsetof(STATUS_VAR, opened_tables),           SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Opened_table_definitions", (char*) offsetof(STATUS_VAR, opened_shares),           SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Prepared_stmt_count",      (char*) &show_prepared_stmt_count,                     SHOW_FUNC,               SHOW_SCOPE_GLOBAL},
  {"Qcache_free_blocks",       (char*) &query_cache.free_memory_blocks,               SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Qcache_free_memory",       (char*) &query_cache.free_memory,                      SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Qcache_hits",              (char*) &query_cache.hits,                             SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Qcache_inserts",           (char*) &query_cache.inserts,                          SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Qcache_lowmem_prunes",     (char*) &query_cache.lowmem_prunes,                    SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Qcache_not_cached",        (char*) &query_cache.refused,                          SHOW_LONG,               SHOW_SCOPE_GLOBAL},
  {"Qcache_queries_in_cache",  (char*) &query_cache.queries_in_cache,                 SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Qcache_total_blocks",      (char*) &query_cache.total_blocks,                     SHOW_LONG_NOFLUSH,       SHOW_SCOPE_GLOBAL},
  {"Queries",                  (char*) &show_queries,                                 SHOW_FUNC,               SHOW_SCOPE_ALL},
  {"Questions",                (char*) offsetof(STATUS_VAR, questions),               SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Select_full_join",         (char*) offsetof(STATUS_VAR, select_full_join_count),  SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},
  {"Select_full_range_join",   (char*) offsetof(STATUS_VAR, select_full_range_join_count), SHOW_LONGLONG_STATUS, SHOW_SCOPE_ALL},
  {"Select_range",             (char*) offsetof(STATUS_VAR, select_range_count),       SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Select_range_check",       (char*) offsetof(STATUS_VAR, select_range_check_count), SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Select_scan",          (char*) offsetof(STATUS_VAR, select_scan_count),              SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Slave_open_temp_tables",   (char*) &show_slave_open_temp_tables,                   SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
#ifdef HAVE_REPLICATION
  {"Slave_retried_transactions",(char*) &show_slave_retried_trans,                     SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Slave_heartbeat_period",   (char*) &show_heartbeat_period,                         SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Slave_received_heartbeats",(char*) &show_slave_received_heartbeats,                SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Slave_last_heartbeat",     (char*) &show_slave_last_heartbeat,                     SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
#ifndef DBUG_OFF
  {"Slave_rows_last_search_algorithm_used",(char*) &show_slave_rows_last_search_algorithm_used, SHOW_FUNC,     SHOW_SCOPE_GLOBAL},
#endif
  {"Slave_running",            (char*) &show_slave_running,                            SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
#endif
#ifndef EMBEDDED_LIBRARY
  {"Slow_launch_threads",      (char*) &Per_thread_connection_handler::slow_launch_threads, SHOW_LONG,         SHOW_SCOPE_ALL},
#endif
  {"Slow_queries",             (char*) offsetof(STATUS_VAR, long_query_count),         SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Sort_merge_passes",        (char*) offsetof(STATUS_VAR, filesort_merge_passes),    SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Sort_range",               (char*) offsetof(STATUS_VAR, filesort_range_count),     SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Sort_rows",                (char*) offsetof(STATUS_VAR, filesort_rows),            SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Sort_scan",                (char*) offsetof(STATUS_VAR, filesort_scan_count),      SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
#ifdef HAVE_OPENSSL
#ifndef EMBEDDED_LIBRARY
  {"Ssl_accept_renegotiates",  (char*) &show_ssl_ctx_sess_accept_renegotiate,          SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_accepts",              (char*) &show_ssl_ctx_sess_accept,                      SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_callback_cache_hits",  (char*) &show_ssl_ctx_sess_cb_hits,                     SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_cipher",               (char*) &show_ssl_get_cipher,                           SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_cipher_list",          (char*) &show_ssl_get_cipher_list,                      SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_client_connects",      (char*) &show_ssl_ctx_sess_connect,                     SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_connect_renegotiates", (char*) &show_ssl_ctx_sess_connect_renegotiate,         SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_ctx_verify_depth",     (char*) &show_ssl_ctx_get_verify_depth,                 SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_ctx_verify_mode",      (char*) &show_ssl_ctx_get_verify_mode,                  SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_default_timeout",      (char*) &show_ssl_get_default_timeout,                  SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_finished_accepts",     (char*) &show_ssl_ctx_sess_accept_good,                 SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_finished_connects",    (char*) &show_ssl_ctx_sess_connect_good,                SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_session_cache_hits",   (char*) &show_ssl_ctx_sess_hits,                        SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_session_cache_misses", (char*) &show_ssl_ctx_sess_misses,                      SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_session_cache_mode",   (char*) &show_ssl_ctx_get_session_cache_mode,           SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_session_cache_overflows", (char*) &show_ssl_ctx_sess_cache_full,               SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_session_cache_size",   (char*) &show_ssl_ctx_sess_get_cache_size,              SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_session_cache_timeouts", (char*) &show_ssl_ctx_sess_timeouts,                  SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_sessions_reused",      (char*) &show_ssl_session_reused,                       SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_used_session_cache_entries",(char*) &show_ssl_ctx_sess_number,                 SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Ssl_verify_depth",         (char*) &show_ssl_get_verify_depth,                     SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_verify_mode",          (char*) &show_ssl_get_verify_mode,                      SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_version",              (char*) &show_ssl_get_version,                          SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_server_not_before",    (char*) &show_ssl_get_server_not_before,                SHOW_FUNC,              SHOW_SCOPE_ALL},
  {"Ssl_server_not_after",     (char*) &show_ssl_get_server_not_after,                 SHOW_FUNC,              SHOW_SCOPE_ALL},
#ifndef HAVE_YASSL
  {"Rsa_public_key",           (char*) &show_rsa_public_key,                           SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
#endif
#endif
#endif /* HAVE_OPENSSL */
  {"Table_locks_immediate",    (char*) &locks_immediate,                               SHOW_LONG,              SHOW_SCOPE_GLOBAL},
  {"Table_locks_waited",       (char*) &locks_waited,                                  SHOW_LONG,              SHOW_SCOPE_GLOBAL},
  {"Table_open_cache_hits",    (char*) offsetof(STATUS_VAR, table_open_cache_hits),    SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Table_open_cache_misses",  (char*) offsetof(STATUS_VAR, table_open_cache_misses),  SHOW_LONGLONG_STATUS,   SHOW_SCOPE_ALL},
  {"Table_open_cache_overflows",(char*) offsetof(STATUS_VAR, table_open_cache_overflows), SHOW_LONGLONG_STATUS,SHOW_SCOPE_ALL},
  {"Tc_log_max_pages_used",    (char*) &tc_log_max_pages_used,                         SHOW_LONG,              SHOW_SCOPE_GLOBAL},
  {"Tc_log_page_size",         (char*) &tc_log_page_size,                              SHOW_LONG_NOFLUSH,      SHOW_SCOPE_GLOBAL},
  {"Tc_log_page_waits",        (char*) &tc_log_page_waits,                             SHOW_LONG,              SHOW_SCOPE_GLOBAL},
#ifndef EMBEDDED_LIBRARY
  {"Threads_cached",           (char*) &Per_thread_connection_handler::blocked_pthread_count, SHOW_LONG_NOFLUSH, SHOW_SCOPE_GLOBAL},
#endif
  {"Threads_connected",        (char*) &Connection_handler_manager::connection_count,  SHOW_INT,               SHOW_SCOPE_GLOBAL},
  {"Threads_created",          (char*) &show_num_thread_created,                       SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Threads_running",          (char*) &show_num_thread_running,                       SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
  {"Uptime",                   (char*) &show_starttime,                                SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
#ifdef ENABLED_PROFILING
  {"Uptime_since_flush_status",(char*) &show_flushstatustime,                          SHOW_FUNC,              SHOW_SCOPE_GLOBAL},
#endif
  {NullS, NullS, SHOW_LONG, SHOW_SCOPE_ALL}
};

• 这里可以看出的是，其值是一个指向内存位置的指针，其数据来源在6800行以上的部分。有些是直接变量值传过来
• 在mysqld.h文件中定义了若干个需要加锁的系统变量

/*
  Server mutex locks and condition variables.
 */
extern mysql_mutex_t
       LOCK_item_func_sleep, LOCK_status,
       LOCK_uuid_generator,
       LOCK_crypt, LOCK_timezone,
       LOCK_slave_list, LOCK_manager,
       LOCK_global_system_variables, LOCK_user_conn, LOCK_log_throttle_qni,
       LOCK_prepared_stmt_count, LOCK_error_messages,
       LOCK_sql_slave_skip_counter, LOCK_slave_net_timeout,
       LOCK_offline_mode, LOCK_default_password_lifetime;
#ifdef HAVE_OPENSSL
extern char* des_key_file;
extern mysql_mutex_t LOCK_des_key_file;
#endif
extern mysql_mutex_t LOCK_server_started;
extern mysql_cond_t COND_server_started;
extern mysql_mutex_t LOCK_reset_gtid_table;
extern mysql_mutex_t LOCK_compress_gtid_table;
extern mysql_cond_t COND_compress_gtid_table;
extern mysql_rwlock_t LOCK_sys_init_connect, LOCK_sys_init_slave;
extern mysql_rwlock_t LOCK_system_variables_hash;
extern mysql_cond_t COND_manager;
extern int32 thread_running;
extern mysql_mutex_t LOCK_keyring_operations;

extern char *opt_ssl_ca, *opt_ssl_capath, *opt_ssl_cert, *opt_ssl_cipher,
            *opt_ssl_key, *opt_ssl_crl, *opt_ssl_crlpath, *opt_tls_version;

extern char *opt_disabled_storage_engines;

下面供参考

vim mysql-server-mysql-5.7.21/sql/sql_show_status.cc

/**
  Build a replacement query for SHOW STATUS.
  When the parser accepts the following syntax:
    SHOW GLOBAL STATUS
  the parsed tree built for this query is in fact:
  SELECT * FROM
           (SELECT VARIABLE_NAME as Variable_name, VARIABLE_VALUE as Value
            FROM performance_schema.global_status) global_status

  Likewise, the query:
    SHOW GLOBAL STATUS LIKE ""
  is built as:
  SELECT * FROM
           (SELECT VARIABLE_NAME as Variable_name, VARIABLE_VALUE as Value
            FROM performance_schema.global_status) global_status
            WHERE Variable_name LIKE ""

  Likewise, the query:
    SHOW GLOBAL STATUS where 
  is built as:
    SELECT * FROM
             (SELECT VARIABLE_NAME as Variable_name, VARIABLE_VALUE as Value
              FROM performance_schema.global_status) global_status
              WHERE 
**/

此处，代码说明执行show slave status命令，在内部会通过转化为查询p_s.global_status表的SQL形式。

vim mysql-server-mysql-5.7.21/sql/sql_show.cc

/*第2537行开始是status信息函数的实现*/
/*****************************************************************************
  Status functions
*****************************************************************************/
int add_status_vars (const SHOW_VAR *list) {.....}
void init_status_vars() {.............}
void reset_status_vars(){.......}
bool get_status_var(THD *thd, SHOW_VAR *list, const char * name, char * const value, enum_var_type var_type, size_t *length)  {......}
/// 这里的注释表明show 命令一次获取status+vars信息，然后通过下面这个命令清掉vars信息                                      
void remove_status_vars(SHOW_VAR *list) {.....}
const char* get_one_variable(THD *thd, const SHOW_VAR *variable,
                             enum_var_type value_type, SHOW_TYPE show_type,
                             system_status_var *status_var,
                             const CHARSET_INFO **charset, char *buff,
                             size_t *length) {...........}
/**
  @brief Returns the value of a system or a status variable.

  @param running_thd [in]     The handle of the current THD.
  @param target_thd  [in]     The handle of the remote THD.
  @param variable    [in]     Details of the variable.
  @param value_type  [in]     Variable type.
  @param show_type   [in]     Variable show type.
  @param charset     [out]    Character set of the value.
  @param buff        [in,out] Buffer to store the value.
                              (Needs to have enough memory
                              to hold the value of variable.)
  @param length      [out]    Length of the value.

  @return                     Pointer to the value buffer.
*/

const char* get_one_variable_ext(THD *running_thd, THD *target_thd,
                                 const SHOW_VAR *variable,
                                 enum_var_type value_type, SHOW_TYPE show_type,
                                 system_status_var *status_var,
                                 const CHARSET_INFO **charset, char *buff,
                                 size_t *length)
{....................}

查看5.5源码

5.5.62版本中并没有sql_show_status.cc文件，show global status应该是使用sql_show.cc来实现。
天色已晚，明日再看

总结

• 虽然没有看出如何查出对应的status值，单是对于global 信息来说，有些信息是需要对各个线程的指标进行相加的到的、
• 5.7可以使用查询表的方式查询指标信息，但是必须要一次查完，对多个的指标的分批查询会造成监控数据的不一致性，影响分析
• 关键在于对status_vars对象的查找，分析这个变量名，可以找出`show global status``命令的来龙去脉

参考链接

• 思考mysql初级系列

新闻名称：【MySQL】showglobalstatus命令
网站地址：http://scyingshan.cn/article/ggdiej.html