[postgis-users] help optimize distance calculation

[Paul Ramsey]

On Wed, Jan 28, 2009 at 10:41 AM, Sky <shlyon+postgis at gmail.com> wrote:

> Again, the query doesn't seem to use the ao.the_geom index... correct?
> Here is the query I used:
>
>  explain analyse select ao.gid, min(distance(f.the_geom,ao.the_geom))
> from features f, ao ao group by ao.gid limit 400000
>
> This query should use the grid_inx_geo... yeah?

No, this query has not got a spatial constraint, so the spatial
indexes have nothing to work against.

First, is the "max mindistance" concept going to be useful to you? If
not, just discard the index idea -- you're going to have to brute
force it, which is what you're already doing. For the "max
mindistance" to help, it has to be a small fraction of your grid
extent.  If I knew that, in my grid, I had a candidate point at the
center of each quadrant of the grid, I could use a "max mindistance"
of grid_width / 4 and know that I would pick up one candidate for
every cell.

Or, perhaps there are distances you just don't care about, they are
too big to be meaningful to your analysis. That's another upper
threshold.

In all events, the distance has to be small enough that applying it at
a grid cell reduces the amount of candidate area to be tested
substantially relative to the total grid extent.

Best,

P

>
> Thanks again for your assistance Paul,
> Sky
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