Over the years many people have asked for “timetravel” or “AS OF”-queries in PostgreSQL. Oracle has provided this kind of functionality for quite some time already. However, in the PostgreSQL world “AS OF timestamp” is not directly available. The question now is: How can we implement this vital functionality in user land and mimic Oracle functionality?
Implementing “AS OF” and timetravel in user land
Let us suppose we want to version a simple table consisting of just three columns: id, some_data1 and some_data2. To do this we first have to install the btree_gist module, which adds some valuable operators we will need to manage time travel. The table storing the data will need an additional column to handle the validity of a row. Fortunately, PostgreSQL supports “range types”, which allow to store ranges in an easy and efficient way. Here is how it works:
CREATE EXTENSION IF NOT EXISTS btree_gist; CREATE TABLE t_object ( id int8, valid tstzrange, some_data1 text, some_data2 text, EXCLUDE USING gist (id WITH =, valid WITH &&) );
Mind the last line here: “EXLUDE USING gist” will ensure that if the “id” is identical the period (“valid”) must not overlap. The idea is to ensure that the same “id” only has one entry at a time. PostgreSQL will automatically create a Gist index on that column. The feature is called “exclusion constraint”. If you are looking for more information about this feature consider checking out the official documentation (https://www.postgresql.org/docs/current/ddl-constraints.html).
If you want to filter on some_data1 and some_data2 consider creating indexes. Remember, missing indexes are in many cases the root cause of bad performance:
CREATE INDEX idx_some_index1 ON t_object (some_data1); CREATE INDEX idx_some_index2 ON t_object (some_data2);
By creating a view, it should be super easy to extract data from the underlying tables:
CREATE VIEW t_object_recent AS SELECT id, some_data1, some_data2 FROM t_object WHERE current_timestamp <@ valid; SELECT * FROM t_object_recent;
For the sake of simplicity I have created a view, which returns the most up to date state of the data. However, it should also be possible to select an old version of the data. To make it easy for application developers I decided to introduce a new GUC (= runtime variable), which allows users to set the desired point in time. Here is how it works:
SET timerobot.as_of_time = '2018-01-10 00:00:00';
Then you can create a second view, which returns the old data:
CREATE VIEW t_object_historic AS SELECT id, some_data1, some_data2 FROM t_object WHERE current_setting('timerobot.as_of_time')::timestamptz <@ valid; SELECT * FROM t_object_historic;
It is of course also possible to do that with just one view. However, the code is easier to read if two views are used (for the purpose of this blog post). Feel free to adjust the code to your needs.
If you are running an application you usually don’t care what is going on behind the scenes – you simply want to modify a table and things should take care of themselves in an easy way. Therefore, it makes sense to add a trigger to your t_object_current table, which takes care of versioning. Here is an example:
CREATE FUNCTION version_trigger() RETURNS trigger AS $$ BEGIN IF TG_OP = 'UPDATE' THEN IF NEW.id <> OLD.id THEN RAISE EXCEPTION 'the ID must not be changed'; END IF; UPDATE t_object SET valid = tstzrange(lower(valid), current_timestamp) WHERE id = NEW.id AND current_timestamp <@ valid; IF NOT FOUND THEN RETURN NULL; END IF; END IF; IF TG_OP IN ('INSERT', 'UPDATE') THEN INSERT INTO t_object (id, valid, some_data1, some_data2) VALUES (NEW.id, tstzrange(current_timestamp, TIMESTAMPTZ 'infinity'), NEW.some_data1, NEW.some_data2); RETURN NEW; END IF; IF TG_OP = 'DELETE' THEN UPDATE t_object SET valid = tstzrange(lower(valid), current_timestamp) WHERE id = OLD.id AND current_timestamp <@ valid; IF FOUND THEN RETURN OLD; ELSE RETURN NULL; END IF; END IF; END; $$ LANGUAGE plpgsql; CREATE TRIGGER object_trig INSTEAD OF INSERT OR UPDATE OR DELETE ON t_object_recent FOR EACH ROW EXECUTE PROCEDURE version_trigger();
The trigger will take care that INSERT, UPDATE, and DELETE is properly taken care of.
Finally: Timetravel made easy
It is obvious that versioning does have an impact on performance. You should also keep in mind that UPDATE and DELETE are more expensive than previously. However, the advantage is that things are really easy from an application point of view. Implementing time travel can be done quite generically and most applications might not have to be changed at all. What is true, however, is that foreign keys will need some special attention and might be easy to implement in general. It depends on your applications whether this kind of restriction is in general a problem or not.