[postgis-devel] Bit setting to compress PostGIS geometries

[Sandro Santilli]

On Mon, Feb 19, 2018 at 09:20:16AM -0500, Daniel Baston wrote:

> We have one extra bit in the SRID, no? (There are 21 bits set aside for
> SRID, but 2^20 > SRID_MAXIMUM). So we could combine this extra bit with the
> 3 spare bits after the SRID and represent all meaningful values of
> precision (1-15).

Let's not consume all bits w/out leaving the door open for future
expansion please.

> Or we could store precision only in the 3 bits after the SRID, and not
> allow all values of precision (there's little value in 1, 2, 3, 4, 12, 13,
> and 14 in my opinion). We could also steal a bit from the flags ("readonly"
> does not appear to be used).

"readonly" is used internally by liblwgeom for memory management.

> This all assumes that we want an implementation of precision based around
> significant digits. That's convenient for this use case, but I'd guess many
> users expect precision in the sense of grid spacing, as is used (I think)
> with the topology functions.

Yes, and GEOS fixed precision model. And SnapToGrid.

--strk;