shp2pgsql: a switch to drop M from 4d imports

[strk]

Annoyingly enough the only way for a shapefile to hold XYZ geometries is by having an M too. This in turn forces you to drop the M after import, if you know that the M really has no meaning.

Now, given the pain it is to convert a column's type (drop 2 constraints, update, re-add the constraints, update geometry columns), we might be nice and provide a switch for shp2pgsql to do that at import time directly.

[pramsey]

Suppressing M looks to be not too hard, has only taken me about 10 minutes to get the core changed. But I'm thinking that once one can support M one might also thing suppressing Z is necessary too. So, another flag or a modal option? (Output 2d, output 3dm, output 3dz)

[strk]

I'm for a modal option, we have so many switches already!

What about a -t switch which could grow in time ? Mnemonic: type. Semantic: you can specify a pattern to either _force_ or _filter_ on.

Do we also want to be able to _add_ an M or Z when missing ?

PS: is this really a 2.0 item?

[pramsey]

Not necessarily a 2.0 item, but you wanted it and it's easy to implement, so why not? I should have enough moments this week to get it in.

[strk]

lol, I forgot it was _me_ requesting it ! :)

Well I'm surely not in rush, especially since 2.0 will make it somewhat easier to do the drop later (no constraints to deal with).

The -t idea could be generally useful though. If we can get to design a comfortable syntax for it.

[pramsey]

I was thinking something ugly like '-t z' or '-t zm' ... hm, except that doesn't provide an option for forcing 2d. The programmers answer is a bitmask integer, 0-3. But that's not nice. tokens? 2D, 3DZ, 3DM, 4D?

[strk]

The tokens one seems fine to me, I was thinking if we could include -S functionality in this new -t switch, something like -t Point, -t MultiLinestringZ, -t PolygonM, -t LineStringZM ...

Adding the type would give you your ZM flags, but would require user to _know_ the type of the shapefile in advance, unless we have a way to split between type and dimension, like: -t Polygon:, -t Polygon:ZM

[pramsey]

Code in trunk at r9204

[pramsey]

Docs in r9205