sql: asynchronous ALTER TABLE name [ADD|DROP] COLUMN ...

[petermattis]

Lots of work to make this happen. Adding a non-unique column should be a straightforward change to the TableDescriptor. Adding a unique column would also require adding a unique index which will require the infrastructure for the multi-step process of:

• Adding the index to the TableDescriptor but marking it as write-only.
• Waiting for the TableDescriptor change to propagate to all of the nodes.
• Backfilling the index with some sort of mapreduce over the table.
• Marking the index as read-write in the TableDescriptor.

Dropping a column (assuming it is not referenced by an index) will also be a multi-step process:

• Marking the column as deleting in the TableDescriptor.
• Waiting for the TableDescriptor change to propagate to all of the nodes.
• Deleting the column with some sort of mapreduce over the table.
• Removing the column from the TableDescriptor.

[petermattis]

If you'd like, I can do the grammar changes required for this.

[tamird]

The remaining work here is tracked by #2845.