HP 3000 Manuals HP 3000 Manuals
CHECKPOINT
The CHECKPOINT statement causes an ALLBASE/SQL system checkpoint to be
taken.
Scope
ISQL or Application Programs
SQL Syntax
[:HostVariable ]
CHECKPOINT [:LocalVariable ]
[:ProcedureParameter]
Parameters
HostVariable identifies an output host variable used to
communicate the amount of log space available for
use. The host variable is an integer.
LocalVariable contains a value in a procedure.
ProcedureParameter contains a value that is passed into or out of a
procedure.
Description
* Specifying a host variable with CHECKPOINT statement in an
application allows you to determine how much free space is
available in the log file.
* The LocalVariable parameter is used in the stored procedure for
obtaining free log space.
* When you can use the host variable in a CHECKPOINT statement in an
application program or procedure, the host variable can be omitted
if you don't need to know the number of free blocks available.
* When you enter a CHECKPOINT statement interactively in ISQL, you
cannot specify a host variable. Returned information is displayed
on the screen.
* Checkpoint processing is as follows:
* Contents of the log buffers are written to the log
files(s).
* Data buffers containing changed pages are written to
DBEFiles.
* A checkpoint record containing a list of the transactions
currently in progress is written in the log.
* When nonarchive logging is in effect, space containing log
records written prior to the beginning of the oldest
incomplete transaction is made available for reuse. When
archive logging is in effect, however, this step is skipped
and no log file space is recovered by checkpoints.
* For a brief interval while a checkpoint is being taken, SQL
statements that modify the DBEnvironment continue to be
accepted but their processing is temporarily suspended.
This suspension occurs for the amount of time needed to
write the log buffers and changed pages to permanent
storage. Retrieval from the DBEnvironment is not suspended
during a checkpoint.
* Contents of the log buffer are also written to the log file(s)
when a COMMIT WORK is executed.
* When you submit a START DBE statement, ALLBASE/SQL processes all
log records created since the last checkpoint record. Therefore
taking a checkpoint just before stopping the DBE reduces the
amount of time that is needed when a DBEnvironment is started up.
* ALLBASE/SQL automatically takes a checkpoint when the log file is
full, when the data buffer is full, and when the STOP DBE and
COMMIT ARCHIVE statements are processed. When the START DBE
statement is processed, ALLBASE/SQL writes a checkpoint record.
* Submitting a CHECKPOINT statement allows you to determine how much
free space is available in the log file.
Authorization
You must have DBA authority to use this statement.
Example
A stored procedure retrieves the number of free blocks of log space
available. Create a stored procedure with an output parameter.
EXEC SQL create procedure cp (freeblock integer OUTPUT) as
begin
checkpoint :freeblock;
end;
Pass the host variable as an output parameter to procedure.
EXEC SQL execute procedure cp (hstfblk output);
writeln('free log space available', hstfblk);
if hstfblk <= TOOLOW then
writeln('Add new log files ');
A log block is a 512-byte allocation of storage. When you submit the
CHECKPOINT statement interactively, ISQL displays the amount of log space
available for use.
isql=> CHECKPOINT;
Number of free log blocks is 240
isql=>
ISQL assigns and displays the free log space.
A program retrieves the number of free blocks of log space available. In
a Pascal application program, declare a host variable.
EXEC SQL begin declare section;
hstfblk : integer;
EXEC SQL end declare section;
Submit a checkpoint with host variable to obtain free log space
available.
EXEC SQL checkpoint :hstfblk;
writeln('free log space: ',hstfblk);
if hstfblk <= TOOLOW then
writeln('Add new log files ');