Materialize: Add copy option to strip constraints from DDL

[teejae]

Signed-off-by: Toliver Jue toliver@planetscale.com

Add a copy:drop_constraint copy type to Materialize

[sougou]

This is a good idea. The one problem with our DDL parser is that it doesn't handle all constructs. So, many constructs will fail if we went this route.

Due to this, I tried a different approach for modifying the DDL, which is based on knowing how mysql generates the create table string. I'm wondering if we can capture more use cases. Essentially, I massage the statement using string matches. Here's how I did it: https://github.com/vitessio/vitess/blob/master/go/vt/wrangler/materializer.go#L305-L331.

If this is too much trouble we can go with your approach knowing that it won't work for all cases.

[teejae]

@sougou yea, i was potentially worried about it failing for the general case. however @systay mentioned that this happened to work for constraints in particular.

separately, i had formatted the copy:xxx string such that we could eventually go with many options, like copy:drop_constraint,drop_index or whatever, and be backwards compatible api-wise.

one benefit with this PR's approach is that it should be robust to failure, and fail-fast, by not being able to parse certain constructs. so i'm inclined to have a relatively limited option first, knowing we can go with the more generalized approaches later. this has limited blast radius in event it's "done wrong".