Re: Vacuum-full very slow

On Wed, Apr 25, 2007 at 09:36:35AM -0700, Steve Crawford wrote:
> Hmmm, why would cluster be faster?

Basically, vacuum full moves tuples from the end to the beginning of a
table so it can compact the table. In the process it needs to update
all the indexes too. So you save heap space but it tends to fragment
your index. Lots of disk writes also.

OTOH, cluster simply scans the table, sorts it, writes it out then
rebuilds the indexes. If you've removed a lot of tuples, empirically
it's faster.

VACUUM FULL is discouraged these days, simply becuase it isn't actually
as efficient as you might expect. Better to make sure it doesn't grow
big in the first place, and use CLUSTER to rebuild the table if you
really need to.

Hope this helps,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.