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Checkpoints, savepoints, and transaction rollback

Adaptive Server IQ relies on three transaction-related commands that help you recover a stable set of data in the event of system or media failure. These commands set checkpoints, set and release savepoints, and roll back transactions.

Checkpoints

A checkpoint marks a significant point in a transaction, when Adaptive Server IQ writes to disk certain information it tracks internally. It uses this information in the event you need to recover your database.

Adaptive Server IQ uses checkpoints differently than OLTP databases such as Adaptive Server Anywhere. OLTP databases tend to have short transactions, that affect only a small number of rows. It would be very expensive for them to write entire pages to disk. Instead, OLTP databases generally write to disk at checkpoints, and write only the changed data rows.

As discussed in Chapter 1, “Overview of Adaptive Server IQ System Administration”, Adaptive Server IQ is an OLAP database. A single OLAP transaction can change thousands or millions of rows of data. For this reason, Adaptive Server IQ does not wait for a checkpoint to occur to perform physical writes. It writes updated data pages to disk after each transaction commits. For an OLAP database, it is much more effective to write full pages of data to disk than to write small amounts of data at arbitrary checkpoints.

Checkpoints aid in recovery

In order to recover from a system or media failure, Adaptive Server IQ must be able to restore the database to a point where it is internally consistent. IQ uses checkpoints to generate reference points and other information that it needs to recover databases. The information that IQ writes to disk at each checkpoint is essential to the recovery process.

When checkpoints occur

Most Adaptive Server IQ checkpoints occur automatically. You can also set explicit checkpoints, although you do not need to do so.

A checkpoint occurs at the following times:

•When a transaction issues a CHECKPOINT command.

•When the CHECKPOINT_TIME is exceeded.

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