DELETE FROM important WHERE overly broad condition;

Kevin Way — Tue, 13 Mar 2007 23:20:00 -0400

A client had an interesting problem. They had a critical PostgreSQL database, and they accidentally ran an overly broad delete on a table.

And their newest backup was two weeks old.

Could we help? Of course we could help.

The procedure used was as follows:

1) Stop the database.

2) Backup the database files to a secure location.

3) Make a second copy of the database files, to a new, empty postgresql instance.

4) Now if you have a dump prior to the error, and xlogs for the entire time period, you can do a Point-In-Time Recovery, with a properly configured recovery.conf. Unfortunately we didn’t have that. We had a two-week old dump and 6 hours of xlogs.

5) Try to determine an approximate transaction ID of the mistake. We did this by looking at timestamps of files on the newest pg_clog file, finding one that happened before the mistake, and then multiplying it’s name by 1048576.

6) Then we used that xid with pg_resetxlog, on the secondary database that we had created. It worked, and we had a view of the data. We then played a bit, to find an xid that was acceptably close to the point of failure.

7) From there, we just did a straight SQL COPY, to get the data from the db server we had just setup onto the original database, and to get it loaded.

It wasn’t the most elegant solution, but it worked beautifully, and the client was pleased.

Here’s to hoping you never need this information!

blog@insidesystems.net: DELETE FROM important WHERE overly broad condition;

DELETE FROM important WHERE overly broad condition;