[Tile-serving] [osm2pgsql-dev/osm2pgsql] [flex output] Get the first node and last node of a linestring using lua, and insert into as a point in the database (Discussion #2091)

[Julien Fastré]

First, thanks for developing flex, this is a pleasure to use it.

This is our use case: we are developing a spatial analysis which will compute the distance between different points on the same level (a platform in a station). We would like to know, for instance, the distance between the steps and a `railway=platform_marker` node.

For this, we summary every object on the train's platform on a point geometry.

This is very easy with elevator, which, for instance, are already a point.

For steps, we are currently using the centroid of the linestring. But, for accuracy, we would like to take the step where it does reach the platform level. Steps are a linestring. So, for this, we have to analyze the first and the last node of the linestring, and the content of the `level=*` key to know if it is going up or down. This will create two points in the table: one with the step's start and the corresponding level, and another with the step's end and the corresponding level. Later in the database, our queries will do a processing to compare the platform's level with the  step's one.

So, it would be nice to be able to extract the first and last from a linestring, and be able to create a "geometry" from that.

Thanks.

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