Is active_index thread safe?

[Ron Hu]

Hi,

active_index is a class member variable in MySQL's handler class.  MySQL
query processor uses this variable to tell storage engine which index id it
is using for a given table.  Storage engine may override the index id used
for a given table used in a SQL statement by assigning a specific index id
to active_index.

I observed that an instance of handler object is created for each table
descriptor.  A table handler may be shared by two concurrent users if they
access same table at the same time.  Suppose a table t1 has two indexes idx1
and idx2.  User 1 needs to use idx1 while user 2 needs to use idx2.  If
these two users run their queries concurrently, how can MySQL and storage
engine guarantee the right active_index value is used for two concurrent
users?

Any pointer will be appreciated.

Thanks.

  -Ron