Torsten | library of C++ functions that support applications of Stan | Machine Learning library

the <-> operator in postgresql is a custom operator defined by the PostGIS extension, see it's documentation. Basically:

The <-> operator returns the 2D distance between two geometries. Used in the "ORDER BY" clause provides index-assisted nearest-neighbor result sets. For PostgreSQL below 9.5 only gives centroid distance of bounding boxes and for PostgreSQL 9.5+, does true KNN distance search giving true distance between geometries, and distance sphere for geographies.

The main issue is that in DB you return a. If you look at the condition of the loop just above that return, you'll see that this loop exits when a consists only of zeroes. So it is no wonder you only get zeroes in the output.

In the C-code you referred to, DB does not return anything. It prints. So if you want to make this a function that returns the result, you should collect the output in a variable at the same spot as where the C-code prints. This could be a JavaScript string, and then the function should return that string:

You just want this:

So I was wondering why the import of the 4 other serializers triggered the error and then spotted it: