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5.2 TurboIMAGE locking: mode 1 user without an SQL predicate
Reading from a data set without a predicate selects all rows
NOTE that each of the above SELECT statements does not
contain a WHERE
clause, so all rows in each data set will be returned. The most efficient way
to lock all rows is to lock the entire data set.
Key points
- A MODE 1 user has the capability to both read and write TurboIMAGE data.
- When a
SELECT statement does not contain a WHERE clause, all
rows in the data set qualify. The isolation level of the transaction affects
the locks that are obtained on the data set:
- If the isolation level is RU (Read Uncommitted), locks are not
acquired when the transaction reads data. RU is the weakest isolation
level. An RU transaction can read rows that another transaction is actively
modifying, even though the other transaction may eventually decide to issue a
ROLLBACK WORK and cancel its changes.
- If the isolation level is RR (Repeatable Read), rows that are read
cannot be modified by another transaction until the RR transaction ends.
RR is the strongest isolation level. An RR transaction can read a row
(that it has already read once) for a second (or third, etc.) time, and the
information returned for the second read will be identical to the information
that was returned for the first read. Hence, reads are repeatable.
ALLBASE/SQL has five different types of locks - the most important are
S (share) and X (exclusive). S locks improve the
concurrency of RR transactions. When data is locked with an S lock, no other
transaction may modify it but other transactions are allowed to read it.
An X lock prevents access by any other user.
Today, TurboIMAGE only has one type of lock, which is equivalent to an X lock
in ALLBASE/SQL. The only way for TurboIMAGE to guarantee that the data read by
an RR transaction will not change is to lock it exclusively. Unfortunately,
this negatively affects the concurrency of other RR transactions, especially
those who simply need to read the data.
- The CS (Cursor Stability) and RC (Read Committed) isolation levels are
not really implemented in TurboIMAGE. Today, they simply act like RR.
- SQL isolation levels are described in more
detail later in this chapter.
Writing to a data set without a predicate selects all rows
NOTE that the above UPDATE statement does not contain a
WHERE clause, so all rows in the data set will be modified. The most
efficient way to lock all rows is to lock the entire data set.
Key points
- When an
UPDATE or DELETE statement does not
contain a WHERE clause, all
rows in the data set qualify. A lock is always obtained on the entire data
set.
Page last updated on November 29, 1995
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