First, if you haven't done the first two that vyegorov suggests do it now. If you have already vacuumed, it is not clear what sort of damage you may have already done in this case. This error should not be possible on new versions of PostgreSQL due to a number of checks, and had you upgraded some time ago, this wouldn't have happened. I highly recommend trying to stay on supported branches in the future.
I want to take a moment and describe what likely caused the problem and what it means. The prognosis IMO is not good and recovering the data, if it is even possible, is likely to be expensive and time consuming. I sincerely hope you have a good backup from before the transaction wraparound. If not, ouch....
What Went Wrong
PostgreSQL uses something called MVCC, which means that old versions of rows are kept around until they are clearly no longer used. MVCC as practiced by PostgreSQL stamps each row with a minimum transaction and a maximum transaction, and uses these for visibility management. When a transaction rolls back those rows entered are no longer visible and those rows deleted remain visible. Transaction ID's are 32-bit integers.
Periodically you are supposed to vacuum PostgreSQL instances. This, among other things, manages MVCC so that fewer transaction id's need to be checked, manages free space in tables, and can reset the transaction id sequence.
When the transaction ID wraps around, bad things happen but they boil down to the fact that PostgreSQL can no longer be sure what rows are visible and what rows are not. Note thais applies not only to rows in your own tables but in the system catalogs as well. This is a very, very bad thing. The best approach, if you can do it, is to restore from a backup before the wraparound occurred. This is why databases and tables are not showing up and why vacuuming full duplicates them.
Why Upgrading Would have Fixed This
Modern versions of PostgreSQL come with autovacuum automatically enabled which runs vacuum processes in the background to help prevent this sort of problem. More recent versions also will refuse to start new transactions for non-superusers once wraparound is approaching. This gives you a chance to detect and correct the problem before you suffer possibly catastrophic data loss.
PostgreSQL 7.4 hasn't been supported in nearly three years. I don't know when the last time anything was vacuumed but it must have been billions of transactions ago. This is not good.
Maybe it's just index bloat. VACUUM FULL does not help with index bloat, on the contrary, as said in the doc for 8.4:
The FULL option is not recommended for routine use, but might be
useful in special cases. An example is when you have deleted or
updated most of the rows in a table and would like the table to
physically shrink to occupy less disk space and allow faster table
scans. VACUUM FULL will usually shrink the table more than a plain
VACUUM would. The FULL option does not shrink indexes; a periodic
REINDEX is still recommended. In fact, it is often faster to drop all
indexes, VACUUM FULL, and recreate the indexes.
(in more recent versions, this advice has disappeared because VACUUM FULL has been reimplemented differently).
See Routine reindeing and the REINDEX command.
The easiest way to reindex is to connect to the database with the db user that owns it and issue:
REINDEX database test_db;
Ideally, it should be done immediately after VACUUM FULL and the database should shrink to its lowest possible size at this point in time.
Best Answer
The comments so far are roughly correct, but to give an authoritative answer from looking at src/backend/tablecmds.c:
If you're only performing
ALTER TABLE ... SET TABLESPACE ... ;, thenATExecSetTableSpace()will be invoked to handle theSET TABLESPACE, and it usescopy_relation_data()to perform a WAL-logged block-by-block copy of the table. However, if you were to specify additional actions to theALTER TABLEcommand which require table rewriting, then the new copy of the table should be built (and compacted) in the new tablespace viaATRewriteTable().