Re: Compression

On Thursday, April 14, 2011 5:51:21 pm Yang Zhang wrote:

> >
> > adrian(dot)klaver(at)gmail(dot)com
>
> Already know about TOAST. I could've been clearer, but that's not the
> same as the block-/page-level compression I was referring to.

I am obviously missing something. The TOAST mechanism is designed to keep tuple
data below the default 8KB page size. In fact it kicks in at a lower level than
that:

"The TOAST code is triggered only when a row value to be stored in a table is
wider than TOAST_TUPLE_THRESHOLD bytes (normally 2 kB). The TOAST code will
compress and/or move field values out-of-line until the row value is shorter than
TOAST_TUPLE_TARGET bytes (also normally 2 kB) or no more gains can be had.
During an UPDATE operation, values of unchanged fields are normally preserved as-
is; so an UPDATE of a row with out-of-line values incurs no TOAST costs if none
of the out-of-line values change.'

Granted no all data types are TOASTable. Are you looking for something more
aggressive than that?

--
Adrian Klaver
adrian(dot)klaver(at)gmail(dot)com