Different Types of Checkpoints in Oracle

What is a Checkpoint?

• A synchronization event at a specific point in time
• Causes some or all dirty block images to be written to the database thereby guaranteeing that blocks dirtied prior to that point in time get written
• Brings administration up to date
• Several types of checkpoint exist

Types of Checkpoints?

• Full Checkpoint
• Thread Checkpoint
• File Checkpoint
• Object “Checkpoint”
• Parallel Query Checkpoint
• Incremental Checkpoint
• Log Switch Checkpoint

Full Checkpoint

Writes block images to the database for all dirty buffers from all instances
• Statistics updated:
– DBWR checkpoints
– DBWR checkpoint buffers written
– DBWR thread checkpoint buffers written
• Caused by:
– Alter system checkpoint [global]
– Alter database close
– Shutdown
• Controlfile and datafile headers are updated
– CHECKPOINT_CHANGE#

Thread Checkpoint

Writes block images to the database for all dirty buffers from one instance
• Statistics updated:
– DBWR checkpoints
– DBWR checkpoint buffers written
– DBWR thread checkpoint buffers written
• Caused by:
– Alter system checkpoint local
• Controlfile and datafile headers are updated
– CHECKPOINT_CHANGE#

File Checkpoint

Writes block images to the database for all dirty buffers for all files of a tablespace from all instances
• Statistics updated:
– DBWR tablespace checkpoint buffers written
– DBWR checkpoint buffers written
– DBWR checkpoints
• Caused by:
– Alter tablespace XXX offline
– Alter tablespace XXX begin backup
– Alter tablespace XXX read only
• Controlfile and datafile headers are updated
– CHECKPOINT_CHANGE#

Parallel Query Checkpoint

Writes block images to the database for all dirty buffers belonging to objects accessed by the
query from all instances
• Statistics updated:
– DBWR checkpoint buffers written
– DBWR checkpoints
• Caused by:
– Parallel Query
– Parallel Query component of PDML or PDDL
– Mandatory for consistency

Object “Checkpoint”

Writes block images to the database for all dirty buffers belonging to an object from all instances
• Statistics updated:
– DBWR object drop buffers written
– DBWR checkpoints
• Caused by:
– Drop table XXX
– Drop table XXX purge
– Truncate table XXX
• Mandatory for media recovery purposes

Incremental Checkpoint

Writes the contents of “some” dirty buffers to the database from CKPT-Q
• Block images written in SCN order
• Checkpoint RBA updated in SGA
• Statistics updated:
– DBWR checkpoint buffers written
• Controlfile is updated every 3 seconds by CKPT
– Checkpoint progress record

Log Switch Checkpoint

Writes the contents of “some” dirty buffers to the database
• Statistics updated:
– DBWR checkpoints
– DBWR checkpoint buffers written
– background checkpoints started
– background checkpoints completed
• Controlfile and datafile headers are updated
– CHECKPOINT_CHANGE#

What is “some” above?

Every 3 seconds CKPT calculates the checkpoint target RBA based on:
The most current RBA
-  log_checkpoint_timeout
– log_checkpoint_interval
– fast_start_mttr_target
– fast_start_io_target
– 90% of the size of the smallest online redo log file
• All buffers dirtied prior to the time corresponding to the target RBA are written to the database

Useful views:-

Useful checkpoint administration views:
– V$INSTANCE_RECOVERY
– V$SYSSTAT
– V$DATABASE
– V$INSTANCE_LOG_GROUP
– V$THREAD
– V$DATAFILE
– V$DATAFILE_HEADER

Temporary Tables

Creation of Global Temporary Tables

The data in a global temporary table is private, such that data inserted by a session can only be accessed by that session. The session-specific rows in a global temporary table can be preserved for the whole session, or just for the current transaction. The ON COMMIT DELETE ROWS clause indicates that the data should be deleted at the end of the transaction.

CREATE GLOBAL TEMPORARY TABLE my_temp_table (

  column1  NUMBER,

  column2  NUMBER

) ON COMMIT DELETE ROWS;

In contrast, the ON COMMIT PRESERVE ROWS clause indicates that rows should be preserved until the end of the session.

CREATE GLOBAL TEMPORARY TABLE my_temp_table (

  column1  NUMBER,

  column2  NUMBER

) ON COMMIT PRESERVE ROWS;

Miscellaneous Features

• If the TRUNCATE statement is issued against a temporary table, only the session specific data is truncated. There is no affect on the data of other sessions.
• Data in temporary tables is stored in temp segments in the temp tablespace.
• Data in temporary tables is automatically deleted at the end of the database session, even if it ends abnormally.
• Indexes can be created on temporary tables. The content of the index and the scope of the index is the same as the database session.
• Views can be created against temporary tables and combinations of temporary and permanent tables.
• Temporary tables can have triggers associated with them.
• Export and Import utilities can be used to transfer the table definitions, but no data rows are processed.
• Statistics on temporary tables are common to all sessions. Oracle 12c allows session specific statistics.
• There are a number of restrictions related to temporary tables but these are version specific.