HP 3000 Manuals HP 3000 Manuals
SET TRANSACTION
[REV BEG]
The SET TRANSACTION statement sets one or more transaction attributes for
a transaction. These attributes include: isolation level, priority,
user label, constraint checking mode, timeout rollback, user timeout,
termination level, and DML atomicity level.[REV END]
Scope
ISQL or Application Programs
SQL Syntax
{ {RR } }
{ {CS } }
{ {RC } }
{ {RU } }
{ISOLATION LEVEL {REPEATABLE READ } }
{ {SERIALIZABLE } }
{ {CURSOR STABILITY} }
{ {READ COMMITTED } }
{ {READ UNCOMMITTED} }
{ {:HostVariable1 } }
{ }
{PRIORITY {Priority } }
{ {:HostVariable2} }
{ }
{LABEL {'LabelString' } }
{ {:HostVariable3} }
{ }
{ConstraintType [,...] CONSTRAINTS {DEFERRED }}
[REV BEG]SET TRANSACTION { {IMMEDIATE}}
{ }
{DML ATOMICITY AT {STATEMENT} LEVEL }
{ {ROW } }
{ }
{ON {TIMEOUT } ROLLBACK {QUERY } }
{ {DEADLOCK} {TRANSACTION} }
{ }
{ {DEFAULT }}
{ {MAXIMUM }}
{ {TimeoutValue [{SECONDS}] }}
{USER TIMEOUT [TO] { [{MINUTES}] }}
{ { }}
{ {:HostVariable4[{SECONDS}]}}
{ { [{MINUTES}]}}
{ }
{ {SESSION } }
{TERMINATION AT {TRANSACTION} LEVEL }
{ {QUERY } }
{ {RESTRICTED } }
[,...]
[REV END]
Parameters
RR Repeatable Read. Means that the transaction uses
locking strategies to guarantee repeatable reads.
RR is the default isolation level.
CS Cursor Stability. Means that your transaction
uses locking strategies to assure cursor-level
stability only.
RC Read Committed. Means that your transaction uses
locking strategies to ensure that you retrieve
only rows that have been committed by some
transaction.
RU Read Uncommitted. Means that the transaction
reads data without obtaining additional locks.
REPEATABLE READ Same as RR.
SERIALIZABLE Same as RR.
CURSOR STABILITY Same as CS.
READ COMMITTED Same as RC.
READ UNCOMMITTED Same as RU.
HostVariable1 [REV BEG]
is a string host variable containing one of the
isolation level specifications above.[REV END]
Priority is an integer from 0 to 255 specifying the
priority of the transaction. Priority 127 is the
default. ALLBASE/SQL uses the priority to
resolve a deadlock. The transaction with the
largest priority number is aborted to remove the
deadlock.
For example, if a priority-0 transaction and a
priority-1 transaction are deadlocked, the
priority-1 transaction is aborted. If two
transactions involved in a deadlock have the same
priority, the deadlock is resolved by aborting
the newer transaction (the last transaction
begun, either implicitly or with a BEGIN WORK
statement).[REV BEG]
HostVariable2 is an integer host variable containing the
priority specification.[REV END]
LabelString is a user defined character string of up to 8
characters. The default is a blank string.
The label is visible in the SYSTEM.TRANSACTION
pseudo-table and also in SQLMON. Transaction
labels can be useful for troubleshooting and
performance tuning. Each transaction in an
application program can be marked uniquely,
allowing the DBA to easily identify the
transaction being executed by any user at any
moment.
Labels for a new transaction can be specified
with the BEGIN WORK, SET TRANSACTION, and SET
SESSION statements. SET TRANSACTION can also be
used to change the existing label of an active
transaction. If a transaction consists of
multiple queries and unique labels are set
between each query, a DBA can identify the
actual query being executed by an active
transaction.[REV BEG]
HostVariable3 is a string host variable containing the
LabelString.[REV END]
ConstraintType identifies the types of constraints that are
affected by the DEFERRED and IMMEDIATE options.
Each ConstraintType can be one of the following:
UNIQUE
REFERENTIAL
CHECK
DEFERRED specifies that constraint errors are not checked
until the constraint checking mode is reset to
IMMEDIATE or the current transaction ends.
IMMEDIATE specifies that constraint errors are checked when
a statement executes. This is the default.
STATEMENT specifies that error checking occurs at the
statement level. This is the default.
ROW specifies that error checking occurs at the row
level.
[REV BEG]
QUERY sets the action for timeouts or deadlocks to
rollback the statement or query.
TRANSACTION sets the action for timeouts or deadlocks to
rollback the transaction.
DEFAULT specifies to use the default timeout duration for
the DBE specified in the START DBE statement.
MAXIMUM specifies to use the maximum timeout duration for
the DBE specified in the START DBE statement.
TimeoutValue specifies the timeout duration to use in seconds
or minutes.
:HostVariable4 is an integer host variable specifying the
timeout duration to use in seconds or minutes.
SESSION specifies self-termination at the session level,
and allows external termination at the session
level only.
TRANSACTION specifies self-termination at the transaction
level, and allows external termination at the
session or transaction level.
QUERY specifies self-termination at the query level,
and allows external termination at the session,
transaction, or query level.
RESTRICTED specifies no self-termination, and allows
external termination at the session level only.
This is the default.[REV END]
Description
* Detailed information about isolation levels is presented in the
"Concurrency Control through Locks and Isolation Levels" chapter.
* You can issue the SET TRANSACTION statement at any point in an
application or ISQL session. If the SET TRANSACTION statement is
issued outside of an active transaction, its attribute(s) apply to
the next transaction. If issued within a transaction, its
attribute(s) apply to the current transaction.
* Within a transaction, any attribute specified in a SET TRANSACTION
statement remains in effect until the transaction terminates or
until reset by another statement issued within the transaction.
See the "Using ALLBASE/SQL" chapter, "Scoping of Transaction and
Session Attributes" section for information about statements used
to set transaction attributes.
* When using RC or RU, you should verify the existence of a row
before you issue an UPDATE statement. In application programs
that employ cursors, you can use the REFETCH statement prior to
updating. REFETCH is not available in ISQL. Therefore, you should
use caution in employing RC and RU in ISQL if you are doing
updates.
* Within a transaction, different isolation levels can be set for
different DML statements. For example, a cursor opened following
a SET TRANSACTION statement is opened with the specified isolation
level, but any cursor opened prior to this SET TRANSACTION
statement maintains the isolation level with which it was opened.
* As with the SET CONSTRAINTS statement, the SET TRANSACTION
statement allows you to set the UNIQUE, REFERENTIAL or CHECK
constraint error checking mode. If the constraint checking mode
is deferred, checking of constraints is deferred until the end of
a transaction or until the constraint mode is set back to
immediate. If the constraint mode is immediate, integrity
constraints are checked following processing of each SQL statement
(if statement level atomicity is in effect) or each row (if row
level atomicity is in effect). Refer to the SET DML ATOMICITY
statement in this chapter for further information on statement and
row level error checking. The following paragraph assumes that
statement level atomicity is in effect.
When constraint checking is deferred, a COMMIT WORK, or SET
CONSTRAINTS IMMEDIATE statement executes if zero constraint
violations exist at that time, otherwise a constraint error is
reported. When constraint checking is immediate (the default),
zero constraint violations must exist when an SQL statement
executes, otherwise a constraint error is reported and the
statement is rolled back. The SET CONSTRAINTS statement in this
chapter gives further detail about constraint checking.
* As with the SET DML ATOMICITY statement, the SET TRANSACTION
statement allows you to set the general error checking level in
data manipulation statements. General error checking refers to
any errors, for example, arithmetic overflows or constraint
violation errors.
Setting ROW LEVEL atomicity guarantees that internal savepoints
are not generated. For example, if an error occurs on the nth row
of a bulk statement such as LOAD, BULK INSERT, or Type2 INSERT,
the row is not processed, statement execution terminates, and any
previously processed rows are not rolled back. In contrast,
STATEMENT LEVEL atomicity guarantees that the entire statement is
rolled back if it does not execute without error. STATEMENT LEVEL
atomicity is the default. Refer to the SET DML ATOMICITY
statement in this chapter for further information on statement and
row level error checking.
* All transaction attributes are sensitive to savepoints. That is,
if you establish a savepoint, then change the transaction
attribute(s) by issuing a SET TRANSACTION statement, and then roll
back to the savepoint, the transaction attribute(s) set after the
savepoint are undone.[REV BEG]
* When ON TIMEOUT ROLLBACK or ON DEADLOCK ROLLBACK is set to
TRANSACTION, the whole transaction is aborted as a result of a
timeout or deadlock.
* When ON TIMEOUT ROLLBACK or ON DEADLOCK ROLLBACK is set to QUERY,
only the SQL statement which has timed out will be rolled back.
This means rolling back results of statements that modify the
database and closing cursor for the cursor-related statements.
(Cursor-related statements change the cursor position, and are not
statements like UPDATE or DELETE WHERE CURRENT.)[REV END]
* In general, if a transaction with KEEP cursor(s) is committed, the
new transaction started on behalf of the user inherits the most
recent transaction attributes of the old transaction. However,
the KEEP cursor(s) inherit the isolation level attribute of the
old transaction at the time the cursor(s) were opened. For
example:
BEGIN WORK RC
:
OPEN C1 KEEP CURSOR ...
:
SET TRANSACTION ISOLATION LEVEL CS
:
OPEN C2 KEEP CURSOR ...
:
SET TRANSACTION ISOLATION LEVEL RU
:
COMMIT WORK
:
OPEN C3
:
In the above example, the new transaction started on behalf of the
user after the COMMIT WORK has isolation level RU; cursor C1 has
isolation RC; cursor C2 has isolation level CS; and cursor C3 has
isolation level RU.
* The SET TRANSACTION statement is not allowed within a stored
procedure.
Authorization
You do not need authorization to use the SET TRANSACTION statement.
Example
Declare multiple cursors
DECLARE C1 CURSOR FOR SELECT BranchNo FROM Branches
WHERE TellerNo > :TellerNo
DECLARE C2 CURSOR FOR SELECT BranchNo FROM Tellers
WHERE BranchNo = :HostBranchNo FOR UPDATE OF Credit
DECLARE C3 CURSOR FOR SELECT * FROM PurchDB.Parts
Set the isolation level to RC.
SET TRANSACTION ISOLATION LEVEL RC, PRIORITY 100, LABEL 'xact1'
:
Implicit BEGIN WORK with transaction isolation level RC.
OPEN C1
FETCH C1 INTO :HostBranchNo1
:
Change isolation level to CS.
SET TRANSACTION ISOLATION LEVEL CS
OPEN C2
FETCH C2 INTO :HostBranchNo2
UPDATE Tellers SET Credit = Credit * 0.005
WHERE CURRENT OF C2
CLOSE C2 Close cursor C2.
CLOSE C1
Close cursor C1.
Change the transaction isolation level back to RC.
SET TRANSACTION ISOLATION LEVEL RC
OPEN C3
FETCH C3 INTO :PartsBuffer
:
End the transaction. Transaction attributes return to those set at the
session level or to the session default.
COMMIT WORK