HP 3000 Manuals

ALLBASE/SQL Reference Manual

Scope and Duration of Locks 

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
 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 

MPE/iX 5.5 Documentation