I have a mobile application sync process. The transaction does a lot of modification on the database. Since this is done on mobile I need to issue a VACUUM to compact the database.
I am wondering when should I issue a VACUUM
I am currently looking for SQLite, but if it's different for other engines, let me know in the answers (PostgreSQL, MySQL, Oracle, SQLServer)
Vacuum is like defrag, it's good to do if youve recently deleted a lot of stuff, or maybe after youve inserted a lot of stuff, but by no means should you do it in every transaction. It's slower than almost any other database command and is more of a maintenance task.
We sometimes add/remove the majority of our db file, so then a vacuum would be a good idea, but I still would not consider it a part of the same transaction that did the work.
How frequently is the transaction run?
It's really a daily sort of process not a query by query process, but if you use it without full then it can be used in a transaction since it doesn't acquire a lock.
If your going to do it then it should be outside the transaction, since it is independent of the transactions data integrity.
I would say outside of the transaction. Certainly in PostgreSQL, VACUUM is designed to remove the "dead" tuples (i.e. the old row when a record has been changed or deleted.)
If you're running VACUUM in a transaction that has modified records, these dead rows won't have been marked for deletion.
Depending on which type of VACUUM you're doing, it may also require a table lock which will block if there are other transactions running, so you could potentially end up in a deadlock situation (transaction 1 is blocked waiting for a table lock to do its VACUUM, transaction 2 gets blocked waiting for a row to be released that transaction 1 has locked.)
I'd also recommend that this isn't done in an application (perhaps as a scheduled task) as it can take a while to complete and can negatively affect speed of other queries.
As for SQL Server, there is no VACUUM - what you're looking for is shrink. You can turn on auto shrink in 2005 which will automatically reclaim space when it the server decides, or issue a DBCC statement to shrink the database and log file, but this depends on your backup routine and strategy on a per-database level.
Want it or not when using PostgreSQL you can't run VACUUM in transaction as stated in the manual:
VACUUM cannot be executed inside a transaction block.