[ Database Content Versioning ]
I am interested in keeping a running history of every change which has happened on some tables in my database, thus being able to reconstruct historical states of the database for analysis purposes.
I am using Postgres, and this MVCC thing just seems like I should be able to exploit it for this purpose but I cannot find any documentation to support this. Can I do it? Is there a better way?
Any input is appreciated!
I have marked Denis' response as the answer, because he did in fact answer whether MVCC is what I want which was the question. However, the strategy I have settled on is detailed below in case anyone finds it useful:
The Postgres feature that does what I want: online backup/point in time recovery.
http://www.postgresql.org/docs/8.1/static/backup-online.html explains how to use this feature but essentially you can set this "write ahead log" to archive mode, take a snapshot of the database (say, before it goes live), then continually archive the WAL. You can then use log replay to recall the state of the database at any time, with the side benefit of having a warm standby if you choose (by continually replaying the new WALs on your standby server).
Perhaps this method is not as elegant as other ways of keeping a history, since you need to actually build the database for every point in time you wish to query, however it looks extremely easy to set up and loses zero information. That means when I have the time to improve my handling of historical data, I'll have everything and will therefore be able to transform my clunky system to a more elegant system.
One key fact that makes this so perfect is that my "valid time" is the same as my "transaction time" for the specific application- if this were not the case I would only be capturing "transaction time".
Before I found out about the WAL, I was considering just taking daily snapshots or something but the large size requirement and data loss involved did not sit well with me.
For a quick way to get up and running without compromising my data retention from the outset, this seems like the perfect solution.
PostgreSQL used to have just this feature, and called it "Time Travel". See the old documentation.
There's somewhat similar functionality in the spi contrib module that you might want to check out.
Composite type audit trigger
What I usually do instead is to use triggers to log changes along with timestamps to archival tables, and query against those. If the table structure isn't going to change you can use something like:
CREATE TABLE sometable_history(
command_tag text not null check (command_tag IN ('INSERT','DELETE','UPDATE','TRUNCATE')),
change_time timestamp with time zone
and your versioning trigger can just
insert into sometable_history(TG_OP,NEW,current_timestamp) (with a different
NEW is not defined).
hstore audit trigger
That gets painful if the schema changes to add new
NOT NULL columns though. If you expect to do anything like that consider using a
hstore to archive the columns, instead of a composite type. I've already added an implementation of that on the PostgreSQL wiki already.
If you want to avoid impact on your master database (growing tables, etc), you can alternately use continuous archiving and point-in-time recovery to log WAL files that can, using a
recovery.conf, be replayed to any moment in time. Note that WAL files are big and they include not only the tuples you changed, but
VACUUM activity and other details. You'll want to run them through clearxlogtail since they can have garbage data on the end if they're partial segments from an archive timeout, then you'll want to compress them heavily for long term storage.
I am using Postgres, and this MVCC thing just seems like I should be able to exploit it for this purpose but I cannot find any documentation to support this. Can I do it?
Not really. There are tools to see dead rows, because auto-vacuuming is so that will eventually be reclaimed.
Is there a better way?
If I get your question right, you're looking into logging slowly changing dimensions.
You might find this recent related thread interesting:
I'm not aware of any tools/products that are built for that purpose.
While this may not be exactly what you're asking for, you can configure Postgresql to log ddl changes. Setting the log_line_prefix parameter (try including %d, %m, and %u) and setting the log_statement parameter to ddl should give you a reasonable history of who made what ddl changes and when.
Having said that, I don't believe logging ddl to be foolproof. For example, consider a situation where:
- Multiple schemas have a table with the same name,
- one of the tables is altered, and
- the ddl doesn't fully qualify the table name (relying on the search path to get it right),
- then it may not be possible to know from the log which table was actually altered.
Another option might be to log ddl as above but then have a watcher program perform a pg_dump of the database schema whenever a ddl entry get's logged. You could even compare the new dump with the previous dump and extract just the objects that were changed.
you must try nextep . this software create a history of database. It use a modified eclipse IDE