[Tile-serving] [osm2pgsql] Use GeoHash ordering instead of way ordering (#242)

[Paul Norman]

> ST_Geohash uses a loop and I don't understand why less iterations could take more time

`ST_GeoHash` computation speed differences are not likely to be the most significant factor in an ORDER BY, given that it is big enough to go out to disk.

> I did a quick test yesterday on a small extract (to limit other side effects like I/O)

You really need to make sure that you're hitting disk. An in-memory sort is a different beast from a disk-based sort.

> 6 chars ST_Geohash were faster than the 8 chars one in my test

Numbers?

> My test was not the same, I simply created a Geohash based index later used by CLUSTER.
> 
> Maybe a side effect on the CREATE TABLE / SELECT / ORDER BY due to much more similar values ?

I wouldn't be surprised if the CLUSTER case had difference performance characteristics, but without details I can't really comment.

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