need and scope of transaction isolation - MySQL - php

Question

http://dev.mysql.com/doc/refman/5.6/en/set-transaction.html#isolevel_repeatable-read
"All consistent reads within the same transaction read the snapshot
established by the first read."
What does this snapshot contain? Only a snapshot of the rows read by the first read, of the complete table or even complete database?
Actually I thought only a snapshot of the rows read by the first read, but this confuses me:
TRANSACTION 1 is started at first, then 2. The result of the last "SELECT * FROM B;" in T1 is EXACTLY the same as if I had not executed T2 meanwhile (NEITHER the UPDATE nor the INSERT appear and that, although the read and write are on DIFFERENT tables)
TRANSACTION 1:
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
START TRANSACTION;
SELECT * FROM A WHERE a_id = 1;
SELECT SLEEP(8);
SELECT * FROM B;
COMMIT;
TRANSACTION 2
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
START TRANSACTION;
UPDATE B SET b_name = 'UPDATE_6' WHERE b_id = 2;
INSERT INTO B (b_name) VALUES('NEW_5');
COMMIT;
OUTPUTS of TRANSACTION 1
# 1st query
a_id a_name
1 a_1
# 3rd query
b_id b_name
1 b_1
2 b_2
3 b_3
In my web application a PHP script imports data from files in a MySQL database (InnoDB). It is made sure by the application, that there is just this 1 writing process at the same time. However there may be additionally multiple concurrent readers.
Now I wonder, whether I should and if yes how I can prevent the following:
in one repeatable-read transaction:
reader R1 reads from table T1
reader R1 does sth. else
reader R1 reads from table T2
If data in T1 and T2 belong together in any way, it could happen, that the reader reads in the 1st step some data and in the 3rd step the related data, that now might not be related anymore, because a writer has changed T1 AND T2 meanwhile. AFAIK repeatable-read only guarantees, that the same reads return the same data, but the 2nd read is not the same as the 1st one.
I hope, you know, what I mean, and I fear, that I got sth. totally wrong about this topic.
(a week ago I asked this question in MySQL forum without answers: http://forums.mysql.com/read.php?20,629710,629710#msg-629710)

Answer 1

The snapshot is for all the tables in database. The MySQL documentation states that explicitly multiple times here http://dev.mysql.com/doc/refman/5.6/en/innodb-consistent-read.html:
A consistent read means that InnoDB uses multi-versioning to present
to a query a snapshot of the database at a point in time.
and
Suppose that you are running in the default REPEATABLE READ isolation
level. When you issue a consistent read (that is, an ordinary SELECT
statement), InnoDB gives your transaction a timepoint according to
which your query sees the database.
and
The snapshot of the database state applies to SELECT statements within
a transaction, not necessarily to DML statements.