Hi,
I am using PostgreSQL 8.1.4 for an embedded application. For some reason, vacuum is not able to identify rows that are candidates for removal (i.e., mark space as available).
Background Info:
I observed some performance problems – our database seemed to be using an unusually high amount of cpu. Further investigation of the problem revealed a very bloated database; the database was around 300M when it should have been about 150M. A number of the database files were quite large, however, the tables that they stored information for were very small. For example, we had one table that had only 46 rows, but was using up more than 17M of disk space. We had a number of other tables that were similarly large.
We run auto vacuum and I can see from the logs that it is running quite frequently. When I run vacuum full from the psql, I can see that space is not being recovered. I have run vacuum full with the verbose flag set, I can see that messages that indicate the existence of “dead row versions that cannot be removed yet.
<--- CUT FROM VACUUM OUTPUT --->
CPU 0.00s/0.00u sec elapsed 0.18 sec.
INFO: "ibportreceivestatsca": found 0 removable, 88017 nonremovable row versions in 4001 pages
DETAIL: 87957 dead row versions cannot be removed yet.
There were 1 unused item pointers.
<--- CUT FROM VACUUM OUTPUT --->
If I shutdown our application and run a vacuum full, the space is recovered and the database size goes down to 150M.
So, my best guess is that something in our application is preventing vacuum from removing dead rows. What could cause this? Would it be caused by a long-living transaction? What is the best way to track the problem down...right now, I am looking through pg_stat_activity and pg_locks to find processes that are “in transaction” and what locks they are holding.
Has anyone had a similar problem? If so, how did you resolve it?
Thanks
Ike
