Use z_order for roads instead of priority table

[pnorman]

This is getting a start on #2046. There should be no rendering changes.

The prio join added extra filtering, but this isn't needed, and was in fact undesirable. The features to be rendered are specified in the MSS, and the data volume returned to Mapnik isn't noticeably impacted because the huge majority of lines matching the WHERE clause also match the features being rendered.

[kocio-pl]

The code is shorter with this change, which is great, but are there any other interesting implications?

Should it be used also in this code?

[pnorman]

The code is shorter with this change, which is great, but are there any other interesting implications?

I'm not sure what you mean, but I'm not aware of any problems.

Should it be used also in this code?

Yes.

[kocio-pl]

I'm not sure what you mean, but I'm not aware of any problems.

I wonder for example if z-order really gives the same result as priority? If I understand correctly, z-order means that we use layer=* tag, while prio does not use it. Am I right or it works in some other way?

[pnorman]

I wonder for example if z-order really gives the same result as priority? If I understand correctly, z-order means that we use layer=* tag, while prio does not use it. Am I right or it works in some other way?

layernotnull comes first in the sort, so that's used before either. z_order does not depend on layer.

[pnorman]

Just to note, if there are any changes in rendered output that someone spots, I'd be interested, because I'm not expecting any.

[matthijsmelissen]

I tested this and reviewed the code, all seems fine. About time we finally go through with this!

[matthijsmelissen]

Can we make this change in a separate PR for clarity?

[pnorman]

I've done it in a separate commit, which should be clear enough

[matthijsmelissen]

Ok, fine.