Mysql源代码阅读笔记（七）查询缓冲

  /*
    Warning.
    The purpose of query_cache_send_result_to_client() is to lookup the

    query in the query cache first, to avoid parsing and executing it.

    先在查询缓冲里查找查询，如果有则不需要解析和执行该查询

    So, the natural implementation would be to:
    - first, call query_cache_send_result_to_client,

    - second, if caching failed, initialise the lexical and syntactic parser.

    先执行query_cache_send_result_to_client函数，如果失败，则初始化词法和语法分析器。

    The problem is that the query cache depends on a clean initialization
    of (among others) lex->safe_to_cache_query and thd->server_status,

    which are reset respectively in

    - lex_start()

    - mysql_reset_thd_for_next_command()

    So, initializing the lexical analyser *before* using the query cache
    is required for the cache to work properly.
    FIXME: cleanup the dependencies in the code to simplify this.
  */