Paris Code Sprint, PostGIS Recap

07 Mar 2016

At the best of times, I find it hard to generate a lot of sympathy for my work-from-home lifestyle as an international coder-of-mystery. However, the last few weeks have been especially difficult, as I try to explain my week-long business trip to Paris, France to participate in an annual OSGeo Code Sprint.

Yes, really, I “had” to go to Paris for my work. Please, stop sobbing. Oh, that was light jealous retching? Sorry about that.

Anyhow, my (lovely, wonderful, superterrific) employer, CartoDB was an event sponsor, and sent me and my co-worker Paul Norman to the event, which we attended with about 40 other hackers on PDAL, GDAL, PostGIS, MapServer, QGIS, Proj4, PgPointCloud etc.

Paul Norman got set up to do PostGIS development and crunched through a number of feature enhancements. The feature enhancement ideas were courtesy of Remi Cura, who brought in some great power-user ideas for making the functions more useful. As developers, it is frequently hard to distinguish between features that are interesting to us and features that are using to others so having feedback from folks like Remi is invaluable.

The Oslandia team was there in force, naturally, as they were the organizers. Because they work a lot in the 3D/CGAL space, they were interested in making CGAL faster, which meant they were interested in some “expanded object header” experiments I did last month. Basically the EOH code allows you to return an unserialized reference to a geometry on return from a function, instead of a flat serialiation, so that calls that look like ST_Function(ST_Function(ST_Function())) don’t end up with a chain of three serialize/deserialize steps in them. When the deserialize step is expensive (as it in for their 3D objects) the benefit of this approach is actually measureable. For most other cases it’s not.

(The exception is in things like mutators, called from within PL/PgSQL, for example doing array appends or insertions in a tight loop. Tom Lane wrote up this enhancement of PgSQL with examples for array manipulation and did find big improvements for that narrow use case. So we could make things like ST_SetPoint() called within PL/PgSQL much faster with this approach, but for most other operations probably the overhead of allocating our objects isn’t all that high to make it worthwhile.)

There was also a team from Dalibo and 2nd Quadrant. They worked on a binding for geometry to the BRIN indexes (9.5+). I was pretty sceptical, since BRIN indexes require useful ordering, and spatial data is not necessarily well ordered, unlike something like time, for example. However, they got a prototype working, and showed the usual good BRIN properties: indexes were extremely small and extremely cheap to build. For narrow range queries, they were about 5x slower than GIST-rtree, however, the differences were on the order of 25ms vs 5ms, so not completely unusable. They managed this result with presorted data, and with some data in its “natural” order, which worked because the “natural” order of GIS data is often fairly spatially autocorrelated.

I personally thought I would work on merging the back-log of GitHub pull-requests that have built up on the PostGIS git mirror, and did manage to merge several, both new ones from Remi’s group and some old ones. I merged in my ST_ClusterKMeans() clustering function, and Dan Baston merged in his ST_ClusterDBSCAN() one, so PostGIS 2.3 will have a couple of new clustering implementations.

However, in the end I spent probably 70% of my time on a blocker in 2.2, which was related to upgrade. Because the bug manifests during upgrade, when there are two copies of the postgis.so library floating in memory, and because it only showed up on particular Ubuntu platforms, it was hard to debug: but in the end we found the problem and put in a fix, so we are once again able to do upgrades on all platforms.

The other projects also got lots done, and there are more write-ups at the event feedback page. Thanks to Olivier Courtin from Oslandia for taking on the heavy weight of organizing such an amazing event!