Starts the database cleaner and sets the maximum length of time for which it can run.

Syntax

sa_clean_database( [ duration ] )

Arguments

Remarks

The database cleaner is an internal task that runs on a default schedule. You can use this system procedure to force the database cleaner to run immediately and to specify how long the cleaner can run each time it is invoked.

Some database tasks, such as processing snapshot isolation transactions, index maintenance, and deleting rows, can execute more efficiently if some portions of the request are deferred to a later time. These deferrable activities typically involve cleanup by removing deleted, historical, and otherwise unnecessary entries from database pages, or reorganizing database pages for more efficient access.

Postponing some of these activities not only allows the current request to finish more quickly, it potentially allows cleanup to occur when the database server is less active. These unnecessary entries are identified so that they are not visible to other transactions; however, they do take up space on a page, and must be removed at some point.

The database cleaner performs any deferred cleanup activities. It is scheduled to run every 20 seconds. When it is invoked, the database cycles sequentially through the database's dbspaces, examining and cleaning each cleanable page before moving on to the next one. When invoked automatically by the database server, the database cleaner is a self-tuning process. The amount of work that the database cleaner performs, and the duration for which it executes, depend on several factors, including the fraction of outstanding cleanable pages in a dbspace, the current amount of activity in the database server, and the amount of time that the database cleaner has already spent cleaning. If, after running for 0.5 seconds, the cleaner detects active requests in the server, it stops and reschedules itself to execute at its regular interval. The database cleaner attempts to process pages when there are no other requests executing in the server, and therefore takes advantage of periods of server inactivity.

Database cleaner statistics are available through four database properties:

The difference between the values of CleanablePagesAdded and CleanablePagesCleaned indicates how many database pages still require cleaning.

You can use the sa_clean_database system procedure to configure the database cleaner to run until all the pages in a database are cleaned, or to specify a maximum duration for the database cleaner to run.

To further customize the behavior of the database cleaner, you can set up an event that starts the database cleaner if the number of pages or rows that need to be cleaned exceed a specified threshold. See CREATE EVENT statement.

Permissions

DBA authority required

Side effects

None

See also

• CREATE EVENT statement
• CleanablePagesAdded, CleanablePagesCleaned, CleanableRowsAdded, and CleanableRowsCleaned properties: Database properties

Example

The following example sets the duration of the database cleaner to 10 seconds:

CALL sa_clean_database( 10 );

The following example creates a scheduled event that runs daily to allow the database cleaner to run until all pages in the database are cleaned:

CREATE EVENT DailyDatabaseCleanup
SCHEDULE
   START TIME '6:00 pm'
   ON ( 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' )
   HANDLER
      BEGIN
         CALL sa_clean_database( );
      END;

The following example forces the database cleaner to run when 20% or more of the pages in the database need to be cleaned:

CREATE EVENT PERIODIC_CLEANER
SCHEDULE
BETWEEN '9:00 am' and '5:00 pm'
EVERY 1 HOURS
HANDLER
BEGIN
     DECLARE @num_db_pages INTEGER;
     DECLARE @num_dirty_pages INTEGER;

     -- Get the number of database pages
     SELECT (SUM( DB_EXTENDED_PROPERTY( 'FileSize', t.dbspace_id ) - 
                     DB_EXTENDED_PROPERTY( 'FreePages', t.dbspace_id ) ))
     INTO @num_db_pages
     FROM (SELECT dbspace_id FROM SYSDBSPACE) AS t;

     -- Get the number of dirty pages to be cleaned
     SELECT (DB_PROPERTY( 'CleanablePagesAdded' ) - 
                     DB_PROPERTY( 'CleanablePagesCleaned' ))
     INTO @num_dirty_pages;

     -- Check whether the number of dirty pages exceeds 20% of
     -- the size of the database
     IF @num_dirty_pages > @num_db_pages * 0.20 THEN
       -- Start cleaning the database for a maximum of 60 seconds
       CALL sa_clean_database( 60 );
     END IF;
END;