[postgis-devel] Validity flag

[Sandro Santilli]

On Mon, Mar 07, 2016 at 03:00:48PM +0100, Hugo Mercier wrote:
> On 06/03/2016 18:15, Paul Ramsey wrote:
> > It's a quick read at the front, rather than a full traversal to the end.
> > You can't know where the end is without parsing the stuff in the middle.
> 
> Can't we ?
> Since it's a varlena, we have access to the total size of the
> GSERIALIZED right ? So it seems to be a simple pointer arithmetic. Or am
> I missing something ?

I was thinking the same. We can just jump to the end (and proceed
backward, if another "extend" flag is found).

What I'm not sure about is the implication of the double slice
in terms of performance. At the bare minimum for a big geometry
you'd be reading two blocks rather than one (the header
and the footer, if available), in the worst scenario PostgreSQL
would need to decompress the whole data in order to get the
final slice (but the same could be true for the initial slice).

But all of this really only matters for the cases in which you
have a function that does _not_ need to deserialize the geometry,
and those kind of functions are not very frequently used.

--strk;