MariaDB starting with 10.3.4

Support for system-versioned tables was added in MariaDB 10.3.4.

System-versioned tables is a feature first introduced in the SQL:2011 standard. System-versioned tables store the history of all changes, not only the data which is valid at the current moment in time. This allows data analysis for any point in time, auditing of changes and comparison of data from different points in time. Typical uses cases are:

  • Forensic analysis & legal requirements to store data for N years.
  • Data analytics (retrospective, trends etc.), e.g. to get your staff information as of one year ago.
  • Point-in-time recovery - recover a table state as of particular point in time.

Creating a System-Versioned Table

The CREATE TABLE syntax has been extended to permit creating a system-versioned table. To be system-versioned, according to SQL:2011, a table must have two generated columns, a period, and a special table option clause:

CREATE TABLE t(
   x INT,
   start_timestamp TIMESTAMP(6) GENERATED ALWAYS AS ROW START,
   end_timestamp TIMESTAMP(6) GENERATED ALWAYS AS ROW END,
   PERIOD FOR SYSTEM_TIME(start_timestamp, end_timestamp)
) WITH SYSTEM VERSIONING;

In MariaDB one can also use a simplified syntax:

CREATE TABLE t (
   x INT
) WITH SYSTEM VERSIONING;

In the latter case no extra columns will be created and they won't clutter the output of, say, SELECT * FROM t. The versioning information will still be stored, and it can be accessed via the pseudo-columns ROW_START and ROW_END:

SELECT x, ROW_START, ROW_END FROM t;

Adding or Removing System Versioning To/From a Table

An existing table can be altered to enable system versioning for it.

CREATE TABLE t(
  x INT
);
ALTER TABLE t ADD SYSTEM VERSIONING;
SHOW CREATE TABLE t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE `t` (
  `x` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 WITH SYSTEM VERSIONING

Similarly, system versioning can be removed from a table:

ALTER TABLE t DROP SYSTEM VERSIONING;
SHOW CREATE TABLE t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE `t` (
  `x` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1

One can also add system versioning with all columns created explicitly:

ALTER TABLE t ADD COLUMN ts TIMESTAMP(6) GENERATED ALWAYS AS ROW START,
              ADD COLUMN te TIMESTAMP(6) GENERATED ALWAYS AS ROW END,
              ADD PERIOD FOR SYSTEM_TIME(ts, te),
              ADD SYSTEM VERSIONING;
SHOW CREATE TABLE t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE `t` (
  `x` int(11) DEFAULT NULL,
  `ts` timestamp(6) GENERATED ALWAYS AS ROW START,
  `te` timestamp(6) GENERATED ALWAYS AS ROW END,
  PERIOD FOR SYSTEM_TIME (`ts`, `te`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 WITH SYSTEM VERSIONING

Querying Historical Data

SELECT

To query the historical data one uses the clause FOR SYSTEM_TIME directly after the table name (before the table alias, if any). SQL:2011 provides three syntactic extensions:

  • AS OF is used to see the table as it was at a specific point in time in the past:
SELECT * FROM t FOR SYSTEM_TIME AS OF TIMESTAMP'2016-10-09 08:07:06'
  • BETWEEN start AND end will show all rows that were visible at any point between two specified points in time. It works inclusively, a row visible exactly at start or exactly at end will be shown too.
SELECT * FROM t FOR SYSTEM_TIME BETWEEN (NOW() - INTERVAL 1 YEAR) AND NOW()
  • FROM start TO end will also show all rows that were visible at any point between two specified points in time, including start, but excluding end.
SELECT * FROM t FOR SYSTEM_TIME FROM '2016-01-01 00:00:00' TO '2017-01-01 00:00:00'

Additionally MariaDB implements a non-standard extension:

  • ALL will show all rows, historical and current.
SELECT * FROM t FOR SYSTEM_TIME ALL

If the FOR SYSTEM_TIME clause is not used, the table will show the current data, as if one had specified FOR SYSTEM_TIME AS OF CURRENT_TIMESTAMP.

Views and Subqueries

When a system-versioned tables is used in a view or in a subquery in the from clause, FOR SYSTEM_TIME can be used directly in the view or subquery body, or (non-standard) applied to the whole view when it's being used in a SELECT:

CREATE VIEW v1 AS SELECT * FROM t FOR SYSTEM_TIME AS OF TIMESTAMP'2016-10-09 08:07:06'

Or

CREATE VIEW v1 AS SELECT * FROM t;
SELECT * FROM v1 FOR SYSTEM_TIME AS OF TIMESTAMP'2016-10-09 08:07:06';

Transaction-Precise History in InnoDB

A point in time when a row was inserted or deleted does not necessarily mean that a change became visible at the same moment. With transactional tables, a row might have been inserted in a long transaction, and became visible hours after it was inserted.

For some applications — for example, when doing data analytics on one-year-old data — this distinction does not matter much. For others — forensic analysis — it might be crucial.

MariaDB supports transaction-precise history (only for the InnoDB storage engine) that allows seeing the data exactly as it would've been seen by a new connection doing a SELECT at the specified point in time — rows inserted before that point, but committed after will not be shown.

To use transaction-precise history, InnoDB needs to remember not timestamps, but transaction identifier per row. This is done by creating generated columns as BIGINT UNSIGNED, not TIMESTAMP(6):

CREATE TABLE t(
   x INT,
   start_trxid BIGINT UNSIGNED GENERATED ALWAYS AS ROW START,
   end_trxid BIGINT UNSIGNED GENERATED ALWAYS AS ROW END,
   PERIOD FOR SYSTEM_TIME(start_trxid, end_trxid)
) WITH SYSTEM VERSIONING;

These columns must be specified explicitly, but they can be made INVISIBLE to avoid cluttering SELECT * output.

When one uses transaction-precise history, one can optionally use transaction identifiers in the FOR SYSTEM_TIME clause:

SELECT * FROM t FOR SYSTEM_TIME AS OF TRANSACTION 12345;

This will show the data, exactly as it was seen by the transaction with the identifier 12345.

Storing the History Separately

When the history is stored together with the current data, it increases the size of the table, so current data queries — table scans and index searches — will take more time, because they will need to skip over historical data. If most queries on that table use only current data, it might make sense to store the history separately, to reduce the overhead from versioning.

This is done by partitioning the table by SYSTEM_TIME. Because of partition pruning optimization, all current data queries will only access one partition, the one that stores current data.

This example shows how to create such a partitioned table:

CREATE TABLE t (x INT) WITH SYSTEM VERSIONING
  PARTITION BY SYSTEM_TIME (
    PARTITION p_hist HISTORY,
    PARTITION p_cur CURRENT
  );

In this example all history will be stored in the partition p_hist while all current data will be in the partition p_cur. The table must have exactly one current partition and at least one historical partition.

Partitioning by SYSTEM_TIME also supports automatic partition rotation. One can rotate historical partitions by time or by size. This example shows how to rotate partitions by size:

CREATE TABLE t (x INT) WITH SYSTEM VERSIONING
  PARTITION BY SYSTEM_TIME LIMIT 100000 (
    PARTITION p0 HISTORY,
    PARTITION p1 HISTORY,
    PARTITION pcur CURRENT
  );

MariaDB will start writing history rows into the partition p0, and when it'll reach the size of 100000 rows, MariaDB will switch to the partition p1. There are only two historical partitions, so when p1 overflows, MariaDB will issue a warning, but will continue writing into it.

Similarly, one can rotate partitions by time:

CREATE TABLE t (x INT) WITH SYSTEM VERSIONING
  PARTITION BY SYSTEM_TIME INTERVAL 1 WEEK (
    PARTITION p0 HISTORY,
    PARTITION p1 HISTORY,
    PARTITION p2 HISTORY,
    PARTITION pcur CURRENT
  );

This will initially write the history to p0, in a week it'll switch to p1, and then to p2.

One can combine partitioning by SYSTEM_TIME and subpartitions:

CREATE TABLE t (x INT) WITH SYSTEM VERSIONING
  PARTITION BY SYSTEM_TIME
    SUBPARTITION BY KEY (x)
    SUBPARTITIONS 4 (
    PARTITION ph HISTORY,
    PARTITION pc CURRENT
  );

Removing Old History

Because it stores all the history, a system-versioned table might grow very large over time. There are many options to trim down the space and remove the old history.

One can completely drop the versioning from the table and add it back again, this will delete all the history:

ALTER TABLE t DROP SYSTEM VERSIONING;
ALTER TABLE t ADD SYSTEM VERSIONING;

It might be a rather time-consuming operation, though, as the table will need to be rebuilt, possibly twice (depending on the storage engine).

Another option would be to use partitioning and drop some of historical partitions:

ALTER TABLE t DROP PARTITION p0;

Note, that one cannot drop a current partition or the only historical partition.

And the third option; one can use a variant of the DELETE statement to prune the history:

DELETE HISTORY FROM t;

or only old history up to a specific point in time:

DELETE HISTORY FROM t BEFORE SYSTEM_TIME '2016-10-09 08:07:06';

or to a specific transaction (with BEFORE SYSTEM_TIME TRANSACTION xxx).

To protect the integrity of the history, this statement requires a special DELETE HISTORY privilege.

Excluding Columns From Versioning

Another MariaDB extension allows to version only a subset of columns in a table. This is useful, for example, if you have a table with user information that should be versioned, but one column is, let's say, a login counter that is incremented often and is not interesting to version. Such a column can be excluded from versioning by declaring it WITHOUT VERSIONING

CREATE TABLE t (
   x INT,
   y INT WITHOUT SYSTEM VERSIONING
) WITH SYSTEM VERSIONING;

A column can also be declared WITH VERSIONING, that will automatically make the table versioned. The statement below is equivalent to the one above:

CREATE TABLE t (
   x INT WITH SYSTEM VERSIONING,
   y INT
);

System Variables

There are a number of system variables related to system-versioned tables:

system_versioning_alter_history

  • Description: SQL:2011 does not allow ALTER TABLE on system-versioned tables. When this variable is set to ERROR, an attempt to alter a system-versioned table will result in an error. When this variable is set to KEEP, ALTER TABLE will be allowed, but the history will become incorrect — querying historical data will show the new table structure. This mode is still useful, for example, when adding new columns to a table.
  • Commandline: --system-versioning-alter-history=value
  • Scope: Global, Session
  • Dynamic: Yes
  • Type: Enum
  • Default Value: ERROR
  • Valid Values: ERROR, KEEP
  • Introduced: MariaDB 10.3.4

system_versioning_asof

  • Description: If set to a specific timestamp value, an implicit FOR SYSTEM_TIME AS OF clause will be applied to all queries. This is useful if one wants to do many queries for history at the specific point in time. Set it to DEFAULT to restore the default behavior.
  • Commandline: None
  • Scope: Global, Session
  • Dynamic: Yes
  • Type: Varchar
  • Default Value: DEFAULT
  • Introduced: MariaDB 10.3.4

system_versioning_innodb_algorithm_simple

  • Description: Not fully implemented yet.
  • Commandline: --system-versioning-innodb-algorithm-simple[={0|1}]
  • Scope: Global, Session
  • Dynamic: Yes
  • Type: Boolean
  • Default Value: ON
  • Introduced: MariaDB 10.3.4

Limitations

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