Обсуждение: Difference between Vacuum and Vacuum full

Hi,

I have recently had to change our nightly jobs from running vacuum
full, as it has caused problems for us. Upon doing more reading on
this topic, I understand that vacuum full needs explicit locks on the
entire db and explicit locking conflicts with all other locks.

But this has bought me to the question of what exactly is the
difference between vacuum and vacuum full. If both give back free
space to the disk, then why have vacuum full.

Thank you.
Radhika

--
It is all a matter of perspective. You choose your view by choosing
where to stand. --Larry Wall

On 10/2/07, Radhika S <radhika.sambamurti@gmail.com> wrote:
> Hi,
>
> I have recently had to change our nightly jobs from running vacuum
> full, as it has caused problems for us. Upon doing more reading on
> this topic, I understand that vacuum full needs explicit locks on the
> entire db and explicit locking conflicts with all other locks.
>
> But this has bought me to the question of what exactly is the
> difference between vacuum and vacuum full. If both give back free
> space to the disk, then why have vacuum full.

Vacuum analyzes the tables and indexes, and marks deleted entries as
free and available and puts and entry into the free space map for
them.  The next time that table or index is updated, instead of
appending the new tuple to the end it can be placed in the middle of
the table / index.  this allows the database to reuse "empty" space in
the database.  Also, if there are dead tuples on the very end of the
table or index, it can truncate the end of the file and free that
space up.

Vaccum full basically re-writes the whole file minus all the dead
tuples, which requires it to lock the table while it is doing so.

Generally speaking, regular vacuum is preferable.  Vacuum full should
only be used to recover lost space due to too infrequent regular
vacuums or too small of a free space map.

vacuum full is much more invasive and should be avoided unless
absolutely necessary.

On 10/2/07, Radhika S <radhika.sambamurti@gmail.com> wrote:
> ... why have vacuum full...

See:
http://www.postgresql.org/docs/8.2/static/routine-vacuuming.html

On Tue, 2 Oct 2007 21:45:37 -0400
"Radhika S" <radhika.sambamurti@gmail.com> wrote:
> But this has bought me to the question of what exactly is the
> difference between vacuum and vacuum full. If both give back free
> space to the disk, then why have vacuum full.

Not quite.  "VACUUM FULL" returns space to the system.  "VACUUM" only
frees the space for use by the database.  In most cases a simple VACUUM
is all you need since you are going to just be asking for the space
back anyway eventually as your database grows.

--
D'Arcy J.M. Cain <darcy@druid.net>         |  Democracy is three wolves
http://www.druid.net/darcy/                |  and a sheep voting on
+1 416 425 1212     (DoD#0082)    (eNTP)   |  what's for dinner.

Thank you much for such a precise explanation. That was very helpful.

Regards,
Radhika




--
It is all a matter of perspective. You choose your view by choosing where to stand. --Larry Wall