HPlogo ALLBASE/SQL Reference Manual > Chapter 5 Concurrency Control through Locks and Isolation Levels

Scope and Duration of Locks

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In general, the length of a transaction affects concurrency. Long transactions hold locks longer, which increases the chances that another transaction is waiting for a lock. Short transactions are "in and out" quickly, which means they are less likely to interfere with other transactions.

The isolation level determines what kinds of locks are obtained in particular circumstances, and also how long these locks are held. Great differences can be found between isolation levels in the duration of locks. For example, a sequential scan that obtains share locks at the RR level holds them while the entire table is read, making updates impossible by others during that time. At the RU level, other users can update the table throughout an entire scan by another reader. Figure 5-6 Scope and Duration of Share Locks for Different Isolation shows the relative scope and duration of share locks obtained for a sequential scan by the RR, CS, and RC isolation levels on PUBLIC and PUBLICROW tables. RU is not shown, because it does not obtain any share locks on user data.

Figure 5-6 Scope and Duration of Share Locks for Different Isolation Levels

[Scope and Duration of Share
Locks for Different Isolation Levels]




What Determines Lock Types


Examples of Obtaining and Releasing Locks