I need to run VACUUM FULL with no available disk space

Since you don't have enough space to run a vacumm or rebuild, you can always rebuild your postgresql databases by restoring them. Restoring the databases, tables, indexes will free up space and defragment. Afterwards, you can setup automated maintenance to vacumm your databases on a regular basis.

1 Backup all of the databases on your postgresql server

You will want to backup all of your databases to a partition that has enough space. If you were on Linux, you can use gzip to further compress the backup to save space

su - postgres
pg_dumpall | gzip -9 > /some/partition/all.dbs.out.gz

2 Backup your configuration files

cp /path/to/postgresql/data_directory/*.conf /some/partition/

3 Stop Postgresql

pg_ctl -D /path/to/postgresql/data_directory stop

4 erase the contents of the data directory

rm -Rf /path/to/postgresql/data_directory/*

5 Run initdb to reinitalize your data directory

initdb -D /path/to/postgresql/data_directory

6 Restore configuration files

cp /some/partition/*.conf /path/to/postgresql/data_directory/*.conf 

7 Start Postgresql

pg_ctl -D /path/to/postgresql/data_directory start

8 Restore the dump of all the databases you made

gunzip /some/partition/all.dbs.out.gz
psql -f /some/partition/all.dbs.out

NOTE: I have tested this on 9.1. I have no 9.0 server lying around here. I am preeeettty sure though it will work on 9.0 though.

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CAUTION (As noted in the comments by @erny):

Note that high CPU load due to I/O operations may be expected.

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You can do this with pretty much no down-time by using a temporary tablespace. The down-time will be in the form of exclusive locks. But only on the table you are vacuuming. So all that will happen is that client queries will simply wait for the lock to be acquired if they access the table in question. You don't need to close existing connections.

One thing to be aware of though, is that moving the table and the vacuum full will themselves need to wait for an exclusive lock first!

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First, you obviously need some additional storage. As Stéphane mentions in the comments, this needs to be at least twice as big as the table in question as VACUUM FULL does a full copy. If you are lucky and can dynamically add a disk to the machine, do that. In the worst case you can just attach an USB disk (risky and slow though)!

Next, mount the new device and make it available as tablespace:

CREATE TABLESPACE tempspace LOCATION '/path/to/new/folder';

You can list the tablespaces easily using:

\db

Double-check the current tablespace of your table (you need to know where to move it back to):

SELECT tablespace FROM pg_tables WHERE tablename = 'mytable';

If it's NULL, it will be in the default tablespace:

SHOW default_tablespace;

If that is NULL as well, it will likely be pg_default (check the official docs in case it's changed).

Now move the table over:

ALTER TABLE mytable SET TABLESPACE tempspace;
COMMIT;  -- if autocommit is off

Vacuum it:

VACUUM FULL mytable;

Move it back:

-- assuming you are using the defaults, the tablespace will be "pg_default".
-- Otherwise use the value from the SELECT we did earlier.
ALTER TABLE mytable SET TABLESPACE pg_default;
COMMIT;  -- if autocommit is off

Remove the temporary space:

DROP TABLESPACE tempspace;

Quick and dirty:

• Stop Postgres
• Move the main database directory to another disk where there's enough room for vacuuming
• In the original location of main, add a symlink to the new location
• Vacuum
• Delete the symlink and move the main directory back to its original location
• Start Postgres

E.g.,:

$ service postgresql stop $ mv /var/lib/postgresql/9.5/main /mnt/bigdisk $ ln -sr /mnt/bigdisk/main /var/lib/postgresql/9.5 $ vacuumdb --all --full $ rm /var/lib/postgresql/9.5/main $ mv /mnt/bigdisk/main /var/lib/postgresql/9.5 $ service postgresql start