Allow using mvt_postgis provider for PostGIS version lower than 3.0

[blinkzz]

It seems like the only thing that stops using it for lower versions of PostGIS extension, that ST_AsMVT fifth argument (feature_id_name) was added in 3.0, but there is literally nothing bad in opting it out and use ST_AsMVT without it. I tried editing postgis.go in my local fork and it was successfull and was working perfectly fine at this AWS RDS instance with this configuration:

• PostgreSQL 10.15
• PostGIS extension 2.5

Here is what my edit looks like:

sqls = append(sqls, fmt.Sprintf(
    `(SELECT ST_AsMVT(q,'%s',%d,'%s') AS data FROM (%s) AS q)`,
    layers[i].MVTName,
    tegola.DefaultExtent,
    l.GeomFieldName(),
    sql,
))

My experience in Go rather low by now, so i am asking for maintainers to look upon this and maybe consider making feature_id_name optional, this way Tegola will support older versions of PostGIS with much faster MVT generation approach

[ARolek]

@blinkzz you are correct, the feature_id parameter is the key detail forcing PostGIS 3.0. I'm trying to remember why we ended up making this decision. I recall a technical limitation, but I would need to do some digging. @gdey can you recall why we require feature_id for the MVT Provider?

[gdey]

I don't recall. I tried to look to see what may have been the reason. I think it had to be with part of the rendering engine requiring the FeatureID, and if you don't specify it in AS_MVT, it will not add that and an empty field. I could be mistaken on this.

[blinkzz]

Well, looks like Mapbox GL JS is doing fine job handling it right now, also promoteId can be used in source specification too to force using field other than gid

[blinkzz]

Seems like this issue was resolved in #744