create procedure aa()
begin
declare x varchar(10);
select count(*) from test where aaa = x ;
end

 #0  JOIN::exec (this=0x900e548) at sql_select.cc:2311
#1  0x08268373 in mysql_select (thd=0xb53d2fd0, rref_pointer_array=0x9005984, tables=0x9006040, wild_num=0, fields=@0x9005914, conds=0x90065f8, og_num=0, order=0x0, group=0x0, having=0x0, proc_param=0x0, select_options=2147765760, result=0x900c698, unit=0x90055ec, select_lex=0x9005880) at sql_select.cc:3067
#2  0x0826887d in handle_select (thd=0xb53d2fd0, lex=0x9005590, result=0x900c698, setup_tables_done_option=0) at sql_select.cc:310
#3  0x081e1af7 in execute_sqlcom_select (thd=0xb53d2fd0, all_tables=0x9006040) at sql_parse.cc:4943
#4  0x081e432f in mysql_execute_command (thd=0xb53d2fd0) at sql_parse.cc:2157
#5  0x0835c987 in sp_instr_stmt::exec_core (this=0x90066f0, thd=0xb53d2fd0, nextp=0xb54912d0) at sp_head.cc:2927
#6  0x0835cbd1 in sp_lex_keeper::reset_lex_and_exec_core (this=0x9006718, thd=0xb53d2fd0, nextp=0xb54912d0, open_tables=false, instr=0x90066f0) at sp_head.cc:2751
#7  0x08363962 in sp_instr_stmt::execute (this=0x90066f0, thd=0xb53d2fd0, nextp=0xb54912d0) at sp_head.cc:2864
#8  0x08360f89 in sp_head::execute (this=0x9004560, thd=0xb53d2fd0) at sp_head.cc:1248
#9  0x08361a53 in sp_head::execute_procedure (this=0x9004560, thd=0xb53d2fd0, args=0xb53d44bc) at sp_head.cc:1989
#10 0x081e966a in mysql_execute_command (thd=0xb53d2fd0) at sql_parse.cc:4401
#11 0x081ebbfa in mysql_parse (thd=0xb53d2fd0, inBuf=0x8fdfa58 "call aa()", length=9, found_semicolon=0xb5491f14) at sql_parse.cc:5958
#12 0x081ecae6 in dispatch_command (command=COM_QUERY, thd=0xb53d2fd0, packet=0xb53f35c9 "call aa()", packet_length=9) at sql_parse.cc:1049
#13 0x081eddaa in do_command (thd=0xb53d2fd0) at sql_parse.cc:731
#14 0x081dd5a7 in handle_one_connection (arg=0xb53d2fd0) at sql_connect.cc:1146
#15 0x4dfe92db in start_thread (arg=0xb5492790) at pthread_create.c:296
#16 0x006cf14e in clone () from /lib/libc.so.

case SQLCOM_CALL:
....
sp= sp_find_routine(thd, TYPE_ENUM_PROCEDURE, lex->spname,
&thd->sp_proc_cache, TRUE)) -------此处对存储过程进行解析
....
res= sp->execute_procedure(thd, &lex->value_list); -------执行存储过程
}
bool sp_head::execute_procedure(THD *thd, List<Item> *args){
....
//前面对参数进行一些处理
if (!err_status)
err_status= execute(thd);//具体执行存储过程

}
bool sp_head::execute(THD *thd){
      ...
      do
      {
            sp_instr *i;
            uint hip;
            i = get_instr(ip);// Returns NULL when we're done.
            err_status= i->execute(thd, &ip); //执行存储过程中的每一个语句，每个具体语句都解析为sp_instr的子类
      }while (!err_status && !thd->killed && !thd->is_fatal_error);
      //上面循环其实就是一个sql的执行引擎，不断读入执行，直到结束
}
int sp_instr_stmt::exec_core(THD *thd, uint *nextp)
{
      MYSQL_QUERY_EXEC_START(thd->query,
      thd->thread_id,
      (char *) (thd->db ? thd->db: ""),
      thd->security_ctx->priv_user,
      (char *) thd->security_ctx->host_or_ip,
      3);
      int res= mysql_execute_command(thd); //对每一个sql进行执行。这里就和普通的sql是一样了
      MYSQL_QUERY_EXEC_DONE(res);
      *nextp= m_ip+1; //将上面ip推进到下一条语句，让上面的循环执行下一个语句
      return res;
}

where aaa = x
|
|--> where Item_func_eq  -----表示这是个等于条件操作
|                                  / Item_field ----代表字段aaa
|-----> Item** args -----|
\ Item_sp_local--代表变量x

simple_ident:
ident
{
      THD *thd= YYTHD;
      LEX *lex= thd->lex;
      Lex_input_stream *lip= YYLIP;
      sp_variable_t *spv;
      sp_pcontext *spc = lex->spcont;
      if (spc && (spv = spc->find_variable(&$1))) //判断语句中是否有变量
      {
            /* We're compiling a stored procedure and found a variable */
            if (! lex->parsing_options.allows_variable)
            {
                  my_error(ER_VIEW_SELECT_VARIABLE, MYF(0));
                  MYSQL_YYABORT;
            }
            Item_splocal *splocal;
            splocal= new (thd->mem_root)
            Item_splocal($1,
            spv->offset, spv->type,
            lip->get_tok_start_prev() - lex->sphead->m_tmp_query,
            lip->get_tok_end() - lip->get_tok_start_prev());
            if (splocal == NULL)
            MYSQL_YYABORT;
            #ifndef DBUG_OFF
            splocal->m_sp= lex->sphead;
            #endif
            $$= splocal;
            lex->safe_to_cache_query=0;
      }
      else
      {
            SELECT_LEX *sel= Select;
            if ((sel->parsing_place != IN_HAVING) ||
            (sel->get_in_sum_expr() > 0))
            {
                  $$= new (thd->mem_root) Item_field(Lex->current_context(),
                  NullS, NullS, $1.str);
            }
            else
            {
                  $$= new (thd->mem_root) Item_ref(Lex->current_context(),
                  NullS, NullS, $1.str);
            }
            if ($$ == NULL)
            MYSQL_YYABORT;
      }
}
| simple_ident_q { $$= $1; }
;

--转自