读未提交的实现
对于读未提交隔离级别，此级别不会对记录加锁，有如下几种情况：
1
对系统表的数据操作，是数据引擎自己发出的数据查询操作，使用读未提交隔离级别，目的是不与其它事务因锁的存在而冲突。
2
在row_search_mvcc()、row_sel_get_clust_rec_for_mysql()等获取记录的函数中确保读未提交隔离级别下允许读到最新的记录。
那么，怎么才能读到最新的记录呢？以row_search_mvcc()为例， 我们来看代码实现方式：对于读未提交的隔离级别，代码什么也没有做，默认本函数的“PHASE 4”调用“rec = btr_pcur_get_rec(pcur);”获得的rec就是可用的。如果不是读未提交的隔离级别，才判断rec的可见性，rec不可见时才去找旧版本。所以，InnoDB以此方式，实现了读未提交隔离级别。与此方式类似的处理，还有row_sel_get_clust_rec_for_mysql()函数，读者可以参考阅读。
row_search_mvcc(...) //获取记录
{...
/* We are
ready to look at a possible new index entry in the result
set: the cursor is now placed on a user
record */
if (prebuilt->select_lock_type
!= LOCK_NONE) { //如果需要加锁，则加锁的语义来自上层
...
} else { //不需要加锁
/*
This is a non-locking consistent read: if necessary, fetch a previous version
of the record */
if
(trx->isolation_level == TRX_ISO_READ_UNCOMMITTED)
{ //如果是读未提交，则什么也不做
/* Do nothing: we let a non-locking SELECT read the latest version of
the record */
}
else if (index == clust_index) { //否则，才需要检查“rec”是否满足可见性，不满足则找出满足可见性要求的旧版本
//所以，读未提交什么也不做，就是在说当前的rec是符合的，能被读到的，根本不用到回滚段找出旧版本（所得即所“可见”）
/* Fetch a previous version of
the row if the current one is not visible in the snapshot;
if we have a very high force recovery level
set, we try to avoid crashes by skipping this lookup */
if (srv_force_recovery < 5
&& !lock_clust_rec_cons_read_sees(rec,
index, offsets, trx_get_read_view(trx))) {
rec_t* old_vers;
/* The following call returns ‘offsets‘ associated with ‘old_vers‘ */
err = row_sel_build_prev_vers_for_mysql(trx->read_view,
clust_index, prebuilt, rec, &offsets, &heap,
&old_vers,
need_vrow ? &vrow : NULL, &mtr);
if (err != DB_SUCCESS) {
goto lock_wait_or_error;
}
if (old_vers == NULL) {
/* The row did not exist yet in the read view */
goto next_rec;
}
rec = old_vers;
}
}
else {
/* We are looking into a non-clustered
index, and to get the right version of the record we
have to look also into the clustered index:
this is necessary, because we can only get the undo
information via the clustered index record.
*/
...
}
}
}
...}