Add VIEWS support

[ajnavarro]

[erizocosmico]

A few questions:

• Do we want view to be persistent across database runs?
• Should views show in show tables?
• Should we modify the Database interface so that all MUST be able to create views? Right now Database interface does not have anything for that, only Alterable interface, but that does not work for views. Another option that would not require changing database interface would be to globally register views in the catalog, but when we implement database qualification that would need to change, so I think the other approach does not require much more work and is more future-proof.

[erizocosmico]

Moved to backlog again, it's not a priority right now, WIP is here

[smola]

@erizocosmico

Do we want view to be persistent across database runs?

Ideally yes. But maybe it is ok to have a first iteration without persistence.

Should views show in show tables?

I would say that we should try to match MySQL behaviour here. And it seems it is possible to use SHOW TABLES to show vews. I'm not sure about syntax details though.

Should we modify the Database interface so that all MUST be able to create views?

Views should probably use a global registry, since they could span multiple databases.