To return space to the OS, use VACUUM FULL
. While being at it, I suppose you run VACUUM FULL ANALYZE
. I quote the manual:
FULL
Selects "full" vacuum, which can reclaim more space, but takes much
longer and exclusively locks the table. This method also requires
extra disk space, since it writes a new copy of the table and doesn't
release the old copy until the operation is complete. Usually this
should only be used when a significant amount of space needs to be
reclaimed from within the table.
Bold emphasis mine.
CLUSTER
achieves that, too, as a collateral effect.
Plain VACUUM
does not normally achieve your goal ("one or more pages at the end of a table entirely free"). It does not reorder rows and only prunes empty pages from the physical end of the file when the opportunity arises - like your quote from the manual instructs.
You can get empty pages at the end of the physical file when you INSERT
a batch of rows and DELETE
them before other tuples get appended. Or it can happen by coincidence if enough rows are deleted.
There are also special settings that might prevent VACUUM FULL
from reclaiming space. See:
Prepare empty pages at the end of a table for testing
The system column ctid
represents the physical position of a row. You need to understand that column:
We can work with that and prepare a table by deleting all rows from the last page:
DELETE FROM tbl t
USING (
SELECT (split_part(ctid::text, ',', 1) || ',0)')::tid AS min_tid
, (split_part(ctid::text, ',', 1) || ',65535)')::tid AS max_tid
FROM tbl
ORDER BY ctid DESC
LIMIT 1
) d
WHERE t.ctid BETWEEN d.min_tid AND d.max_tid;
Now, the last page is empty. This ignores concurrent writes. Either you are the only one writing to that table or you need to to take a write lock to avoid interference.
The query is optimized to identify qualifying rows quickly. The second number of a tid
is the tuple index stored as unsigned int2
, and 65535
is the maximum for that type (2^16 - 1
), so that's the safe upper bound.
SQL Fiddle (reusing a simple table from a different case.)
Tools to measure row / table size:
Disk full
You need wiggle room on disk for any of these operations. There is also the community tool pg_repack
as replacement for VACUUM FULL
/ CLUSTER
. It avoids exclusive locks but needs free space to work with as well. The manual:
Requires free disk space twice as large as the target table(s) and indexes.
As a last resort, you can run a dump/restore cycle. That removes all bloat from tables and indexes, too. Closely related question:
The answer over there is pretty radical. If your situation allows for it (no foreign keys or other references preventing row deletions), and no concurrent access to the table), you can just:
Dump the table to disk connecting from a remote computer with plenty of disk space (-a
for --data-only
):
From remote shell, dump table data:
pg_dump -h <host_name> -p <port> -t mytbl -a mydb > db_mytbl.sql
In a pg session, TRUNCATE
the table:
-- drop all indexes and constraints here for best performance
TRUNCATE mytbl;
From remote shell, restore to same table:
psql -h <host_name> -p <port> mydb -f db_mytbl.sql
-- recreate all indexes and constraints here
It is now free of any dead rows or bloat.
But maybe you can have that simpler?
Can you make enough space on disk by deleting (moving) unrelated files?
Can you VACUUM FULL
smaller tables first, one by one, thereby freeing up enough disk space?
Can you run REINDEX TABLE
or REINDEX INDEX
to free disk space from bloated indexes?
Whatever you do, don't be rash. If in doubt, backup everything to a secure location first.
Here's a short concise answer.
Vacuum full takes out an exclusive lock and rebuilds the table so that it has no empty blocks (we'll pretend fill factor is 100% for now).
Vacuum freeze marks a table's contents with a very special transaction timestamp that tells postgres that it does not need to be vacuumed, ever. Next update this frozen id will disappear.
For instance, the template0 database is frozen because it never changes (by default you cannot connect to it.)
Every so often the autovacuum daemon will check a database and its tables to see what needs to be vacuumed. If a table is vacuum freeze'd and then never updated, the autovacuum daemon will simply pass it by. Also the "wrap around" protection in postgresql will never kick in on that table either.
tl;dr freezing marks a table as not needing any autovac maintenance. The next update will unfreeze it.
Best Answer
So
VACUUM FULL
is going to be a problem since it takes an exclusive lock on each table it processes. Consider the community toolpg_repack
instead which achieves the same without exclusive locks.Related:
None of this affects the size of backups, since those do not include dead rows to begin with.