Dolt databases allow you to query the data at any point in the commit history. There are several ways to do so.
Please note: when querying history, the unit of snapshot is the dolt commit. SQL transaction commits do not create a Dolt commit by default.
Querying past snapshots with AS OF
Dolt SQL supports a variant of syntax to query non-HEAD revisions of a database via the AS OF clause:
SELECT * FROM myTable AS OF 'kfvpgcf8pkd6blnkvv8e0kle8j6lug7a';
SELECT * FROM myTable AS OF 'myBranch';
SELECT * FROM myTable AS OF 'HEAD^2';
SELECT * FROM myTable AS OF TIMESTAMP('2020-01-01');
SELECT * FROM myTable AS OF 'myBranch' JOIN myTable AS OF 'yourBranch' AS foo;
The AS OF expression must name a valid Dolt reference, such as a commit hash, branch name, or other reference. Timestamp / date values are also supported. Each table in a query can use a different AS OF clause.
In addition to this AS OF syntax for SELECT statements, Dolt also supports various extensions to the standard MySQL syntax to examine the schemas of snapshots:
SHOW TABLES AS OF 'kfvpgcf8pkd6blnkvv8e0kle8j6lug7a';
SHOW CREATE TABLE myTable AS OF 'myBranch';
DESCRIBE myTable AS OF 'HEAD~';
Note that AS OF always names a revision at a specific Dolt commit. Changes on a branch's that have not been committed to that head via select dolt_commit() or similar are not visible via this syntax.
Specifying a revision in the database name
You can connect to any commit in the database history by including its commit hash in the name of the database, like this:
The database will be read-only in this case. You can do the same thing on an existing connection with a USE statement.
USE mydb/ia1ibijq8hq1llr7u85uivsi5lh3310p
Or specify the commit hash directly in the query. This is equivalent to AS OF, but works in some queries where the AS OF syntax is not supported.
show create table `mydb/ia1ibijq8hq1llr7u85uivsi5lh3310p`.myTable;
Querying history using Dolt system tables
For every table in the database, Dolt also provides a set of system tables that you can query to see past values of rows, diffs between revisions, and more.
the dolt_history tables provide a row for every revision of a row in a table.
SELECT * FROM dolt_history_mytable
WHERE state = 'Virginia'
ORDER BY 'commit_date'
+----------+------------+----------+-------------+-----------+---------------------------------+
| state | population | capital | commit_hash | committer | commit_date |
+----------+------------+----------+-------------+-----------+---------------------------------+
| Virginia | 691937 | NULL | ... | billybob | 1790-01-09 00:00:00.0 +0000 UTC |
| Virginia | 807557 | Richmond | ... | billybob | 1800-01-01 00:00:00.0 +0000 UTC |
| Virginia | 877683 | NULL | ... | billybob | 1810-01-01 00:00:00.0 +0000 UTC |
+----------+------------+----------+-------------+-----------+---------------------------------+
To query how rows changed between two commits, use the dolt_commit_diff and dolt_diff tables.
SELECT * FROM dolt_commit_diff_mytable
WHERE to_commit = HASHOF('HEAD')
AND from_commit = HASHOF('HEAD~')
ORDER BY state, to_commit_date;
Querying historical view data
Database views are an edge case for historical queries. When you have a database view whose definition has changed, querying it with AS OF will use the current definition of the view, but use rows from the tables as they existed at the revision provided.
