词法解析

LEX是词法分析的主要对象，其结构体在st_lex(sql_lex.h),在THD中，有一个字段thd->lex指向该结构体。

在mysqld启动时初始化LEX（在函数init_common_variables内调用lex_init()）,对sql/lex.h中的两个关键字数组symbols和sql_functions进行初始化

这两个关键字数组的结构体为ＳＹＭＢＯＬ：

   1:  struct st_sym_group;

   2:  

   3:  typedef struct st_symbol {

   4:    const char *name;

   5:    uint    tok;

   6:    uint length;

   7:    struct st_sym_group *group;

   8:  } SYMBOL;

   9:  

  10:  typedef struct st_lex_symbol

  11:  {

  12:    SYMBOL *symbol;

  13:    char   *str;

  14:    uint   length;

  15:  } LEX_SYMBOL;

  16:  

  17:  typedef struct st_sym_group {

  18:    const char *name;

  19:    const char *needed_define;

  20:  } SYM_GROUP;

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初始化主要是对ＳＹＭＢＯＬ的ｌｅｎｇｔｈ字段赋值。

当需要解析一个ＳＱＬ语句时，调用函数：

   1:  void lex_start(THD *thd)

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该函数将对lex对象进行设置(sql_lex.cc)。

在sql_lex.cc中定义了很多对lex操作的函数，如find_keyword、lex_end、lex_free等