postgis loader issues metadata queries but not data query

[d1manson]

Hopefully this is the right place to ask.

When I do something like the following, with logging turned on for my DB, I see that the request is made with limit 0 at the end, which obviously means it returns empty.

var postgis = new mapnik.Datasource({
  type: 'postgis',
  host: ... etc,
  table: 'some_geometry_table',
  geometry_field: 'geom',
  srid: 4326,
  extent: "-180,-85.0511,180,85.0511",
  estimate_extent: false,
  row_limit:  10 // !! this doesn't seem to do anything
});

var map = new mapnik.Map(256, 256);
var layer = new mapnik.Layer('some_layer');
layer.datasource = postgis; 
map.add_layer(layer);
map.render(new mapnik.VectorTile(z, x, y), {}, (err, vtile) => {
    if (err) next(err);
    var data = vtile.getDataSync({});
    var file = path + z + "," + x + "," + y + ".pbf"
    console.log(data);
    console.log("written: " + file);
    fs.writeFileSync(file, data);
    next(null);
  });
});

Statement as seen in postgres logs:

SELECT * FROM some_geometry_table LIMIT 0

It doesn't seem to matter what I put in row_limit, or if I remove it, I always get limit 0. Looking at the mapnik src it seems this is the correct name for the param, but I have no idea how this all works.

UPDATE (As mentioned below): the limit-0 query is for metadata. There should be another query that follows it, but I don't see one.

Now posted as a question on SO.

[talaj]

I think those limit 0 queries in the log are fine. Their purpose is to collect some metadata during initialization. See here.

[lightmare]

SELECT * FROM some_geometry_table LIMIT 0

This is a query mapnik uses at datasource creation time to get column names and types. Later, when mapnik actually pulls data for rendering, it never does SELECT *, it always names the columns explicitly, so you should also see SELECT ...something... LIMIT your_row_limit in the log.

[d1manson]

I don't see anything else being queried on the table after that first line with the limit-0.

[d1manson]

Any ideas what I'm doing wrong? Note I have now posted this as a question on SO. Thanks