先看下面的例子：
CREATE TABLE t1 (a INT, b VARCHAR(10));
INSERT INTO t1 VALUES (1,'11');
INSERT INTO t1 VALUES (2,'21');
MySQL> select * from t1;
+------+------+
| a    | b    |
+------+------+
|    1 | 11   |
|    2 | 21   |
+------+------+
2 rows in set (0.01 sec)
DELIMITER $$
CREATE PROCEDURE p1()
BEGIN
update t1 set b='aa' where a=1;
END $$
DELIMITER ;
MySQL> call p1; #這里第一次操作，因此會先執行update這句SQL的prepare再進行execute。
Query OK, 1 row affected (0.05 sec)

MySQL> select * from t1;
+------+------+
| a    | b    |
+------+------+
|    1 | aa   |
|    2 | 21   |
+------+------+
2 rows in set (0.01 sec)
MySQL> call p1; #這里第二次操作，直接執行update這句SQL的execute。
Query OK, 0 rows affected (13.78 sec)

#接著我們執行表結構的更新。
MySQL> alter table t1 add i int;
Query OK, 0 rows affected (0.41 sec)
Records: 0  Duplicates: 0  Warnings: 0

#然后再次執行sp，就會發現這次執行了這句SQL的prepare再進行execute。
MySQL> call p1;
Query OK, 0 rows affected (34.24 sec)

#首先看一下這個sp涉及的instr。
MySQL> show procedure code p1;
+-----+---------------------------------------+
| Pos | Instruction                           |
+-----+---------------------------------------+
|   0 | stmt "update t1 set b='aa' where a=1" |
+-----+---------------------------------------+
1 row in set (0.01 sec)
可以看到這個sp只涉及了sp_instr_stmt::execute的運行，因此跟蹤一下代碼找到sp_lex_instr::validate_lex_and_execute_core，可以發現這個函數里面有一個無限while循環，如果發現is_invalid()的話就重新執行parse動作，如果！is_invalid()就直接執行exec_core，這個跟上面的運行步驟就對的上了。但是表結構變更后在哪里被判定為rc=true的呢，那就從reset_lex_and_exec_core這個函數繼續跟蹤下去。
bool sp_lex_instr::validate_lex_and_execute_core(THD *thd, uint *nextp,
                                                 bool open_tables) {
  while (true) {
    if (is_invalid() || (m_lex->has_udf() && !m_first_execution)) {
      LEX *lex = parse_expr(thd, thd->sp_runtime_ctx->sp);
    }
    bool rc = reset_lex_and_exec_core(thd, nextp, open_tables);
    if (!rc) return false;
    thd->clear_error();
    invalidate();
  }
}
#跟蹤代碼發現有一個check_and_update_table_version函數是用來check表版本是否一致的
#打印堆棧看一下代碼調用過程：
Thread 51 "mysqld" hit Breakpoint 6, check_and_update_table_version (thd=0x7fff70001060, tables=0x7fff702c4e20, 
    table_share=0x7fff70297640) at /mysql/sql/sql_base.cc:3722
3722   if (!tables->is_table_ref_id_equal(table_share)) {
(gdb) bt
#0  check_and_update_table_version (thd=0x7fff70001060, tables=0x7fff702c4e20, table_share=0x7fff70297640)
    at /mysql/sql/sql_base.cc:3722
#1  0x0000000003340f71 in open_and_process_table (thd=0x7fff70001060, lex=0x7fff702c2650, tables=0x7fff702c4e20, 
    counter=0x7fff702c26a8, prelocking_strategy=0x7fffec2e7478, has_prelocking_list=false, ot_ctx=0x7fffec2e7368)
    at /MySQL/sql/sql_base.cc:5223
#2  0x000000000333f788 in open_tables (thd=0x7fff70001060, start=0x7fffec2e7488, counter=0x7fff702c26a8, flags=0, 
    prelocking_strategy=0x7fffec2e7478) at /mysql/sql/sql_base.cc:5968
#3  0x0000000003343c96 in open_tables_for_query (thd=0x7fff70001060, tables=0x7fff702c4e20, flags=0)
    at /MySQL/sql/sql_base.cc:6958
#4  0x0000000003514334 in Sql_cmd_dml::execute (this=0x7fff702c6138, thd=0x7fff70001060)
    at /MySQL/sql/sql_select.cc:543
#5  0x0000000003475097 in mysql_execute_command (thd=0x7fff70001060, first_level=false)
    at /MySQL/sql/sql_parse.cc:3832
#6  0x00000000033075c6 in sp_instr_stmt::exec_core (this=0x7fff70249a80, thd=0x7fff70001060, nextp=0x7fffec2eac38)
    at /MySQL/sql/sp_instr.cc:1008
#7  0x00000000033052ed in sp_lex_instr::reset_lex_and_exec_core (this=0x7fff70249a80, thd=0x7fff70001060, 
    nextp=0x7fffec2eac38, open_tables=false) at /mysql/sql/sp_instr.cc:457
#8  0x00000000033060a4 in sp_lex_instr::validate_lex_and_execute_core (this=0x7fff70249a80, thd=0x7fff70001060, 
    nextp=0x7fffec2eac38, open_tables=false) at /mysql/sql/sp_instr.cc:741
#9  0x0000000003306748 in sp_instr_stmt::execute (this=0x7fff70249a80, thd=0x7fff70001060, nextp=0x7fffec2eac38)
    at /MySQL/sql/sp_instr.cc:925
#10 0x00000000032f4d74 in sp_head::execute (this=0x7fff7018e7a0, thd=0x7fff70001060, merge_da_on_success=true)
    at /MySQL/sql/sp_head.cc:2272
#11 0x00000000032f80e1 in sp_head::execute_procedure (this=0x7fff7018e7a0, thd=0x7fff70001060, args=0x0)
    at /MySQL/sql/sp_head.cc:2977
#可以發現open_tables函數調用了這個函數，這個函數調用了ask_to_reprepare，
#在sp運行中這個ask_to_reprepare返回的是true。因此這里就解開了之前的問題，
#為何表版本更新了會return true然后重新進行parse操作。
static bool check_and_update_table_version(THD *thd, TABLE_LIST *tables,
                                           TABLE_SHARE *table_share) {
  if (!tables->is_table_ref_id_equal(table_share)) {
    /*
      Version of the table share is different from the
      previous execution of the prepared statement, and it is
      unacceptable for this SQLCOM.
    */
    if (ask_to_reprepare(thd)) return true;
    /* Always maintain the latest version and type */
    tables->set_table_ref_id(table_share);
  }
  return false;
}