fix compute for Teradata

[But2ene]

This PR was created to share some fix for Teradata used at work. It should fix #147

Indexes must be passed at table creation.

Happy to correct code style / design choices / etc...

[mgirlich]

@hadley Any idea how you want to handle this? Returning "select 1" in sql_table_index.Teradata() doesn't feel right. It should probably error as it doesn't work anyway.

[hadley]

Actually I don't see why this is a problem since indexes and unique_indexes default to NULL, so sql_table_index will be never be called. I think we need more details here. (It also seems unrelated to #147).

[hadley]

@But2ene if this is still troubling you, I'd recommend starting by filing an issue with reprex so we can look into the underlying cause in more detail.