Locking that will delayed a SELECT

Hi:

Suppose I have a transaction (T1) which executes a
complicated stored procedure. While T1 is executing,
trasaction #2 (T2) begins to execute.

T1 take more time to execute that T2 in such a way
that T2 finished earlier than T1. The result is that
t2 returns set of data before it can be modified by
T1.

Given the above scenario. Is there a way such that
while T2 will only read that value updated by T1 (i.e.
T2 must wait until T1 is finished) ? What locks should
I used since a portion of T1 contains SELECT
statements? Should I used the "SERIALIZABLE
isolation".

Thank you in advance.

ludwig.

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