Empty value stops processing of layer data

[tomass]

It is useful to add some casing to polygon geometries (for example to water features). But as OpenStreetMap water polygons are made of parts (for various reasons), it could give unwanted artefacts as can be seen in this example where smaller river flows into the larger one:

The usual solution is to aggregate geometries with st_union when creating vector tile (aggregating water geometries beforehand for whole region could create huge geometries so it is not a good solution). For example:

select
  gid,
  st_union(geom) AS geom
from
  planet_osm_polygon
where
  geom && !BBOX!

When a tile has no geometries in a BBOX, this produces a null value for geom (because st_union is an aggregate function). Such null value stops Tegola processing of layer data with following output to console:

2018/06/09 22:15:25 map.go:193: err fetching tile (z: 10, x: 584, y: 323) features: unable to decode layer (water10) geometry field (geom) into wkb where (gid = 1): EOF

Now this would not be a problem (if we ignore error message), but if there is another part in a query:

select ... st_union(geom) AS geom ...
union all
select somethingelse AS geom from sometable

then geometries of the second part of such query are not placed into the vector tile basically leaving layer empty in particular tile.
I guess Tegola should handle such null values as "no geometry" and continue with processing query results.

[ARolek]

@tomass thanks for the detailed report. I think your proposed solution should work fine. I will add it to our roadmap.

[ARolek]

This has been addressed in the v0.7.0 branch.

[tomass]

Can you reopen this one or should I create a new issue as originally I did not mention usage of st_intersection which is causing "second part" of this problem?
(more info here: #430)

[ARolek]

@tomass re-opening.

[ARolek]

This fix has been merged into v0.9.x branch. I'm working on cutting a release soon.