[postgis-devel] Float vs Double Box

[Paul Ramsey]

On Wed, Nov 9, 2011 at 10:47 AM, Sandro Santilli <strk at keybit.net> wrote:
> On Wed, Nov 09, 2011 at 09:43:14AM -0800, Paul Ramsey wrote:
>> So, we've had this argument before, and Mark wanted double boxes for
>> simplicity, and I wanted float boxes to keep index sizes down.
>>
>> And I realized on the way into work that actually we could have both,
>> in that we could serialized double boxes into the geometry objects,
>> but build the index out of float boxes, since the *key* used by the
>> index is actually a different structure from the geometries
>> themselves.
>>
>> My objection to doubles in the serialization was more concerned with
>> index size than object size, since for small objects (points,
>> two-vertex lines) we can just omit the boxes entirely. My objection to
>> doubles for the index stands, 50% smaller is a big win for an index,
>> but we can actually have float based indexes and double based
>> serialization.
>>
>> Anyhow, that's the good news.
>>
>> The bad news it it's another dump/restore change that touches quite a
>> pile of critical code, and we're supposed to be feature freezing in a
>> couple weeks.
>
> It's not a new feature, and people would need to dump/reload anyway
> when coming from postgis < 2.0. I'm more concerned about alignment.

Well, if we don't do it on this round, it's a dump/reload potentially,
though as you note we could use version flags...

> Will your coordinates still be aligned by changing that ?

Yes, we'll still be aligned.

> Also, are we caching 2d boxes always, even for 3d or 4d geoms ?

We're always caching a box with the dimension of the underlying
object, though for indexes we're building with 2d boxes or n-d boxes
depending on the gist ops chosen.

> Could be another nice thing to change.
> BTW, did you leave a few zeroed-out bit to use for next time we need
> to do such changes ? It would allow us to do w/out a dump/reload.

Ensuring the remaining flags are zeroed is still an open ticket, but
there is a ticket.

p.

> --strk;
>
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