32.6. Logical Concurrency Control
Logical concurrency control is based on the idea that the machine can analyze the predicates in the queue of waiting queries and processes on a purely logical level, and then determine which of the statements can be allowed to operate on the database at the same time.
Clearly, all SELECT statements can operate at the same time, since they do not change the data. After that, it is tricky to determine which statements conflict with the others. For example, one pair of UPDATE statements on two separate tables might be allowed only in a certain order because of PRIMARY KEY and FOREIGN KEY constraints. Another of pair of UPDATE statements on the same tables might be disallowed because they modify the same rows and ...
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