Use old_relation for renaming in default materializations

[jtcohen6]

Describe the bug

In the default view and table materializations, dbt should rename old_relation to backup_relation, instead of renaming target_relation to backup_relation. The issue here is that, if old_relation.type != target_relation.type, dbt will run the wrong alter ... rename statement.

I think this is as simple as switching these to

  {% if old_relation is not none %}
    {{ adapter.rename_relation(old_relation, backup_relation) }}
  {% endif %}

Steps To Reproduce

• Create + run a view model
• Change config to table + rerun

This does not break on any of the core adapters:

• Redshift and Postgres use alter ... rename, but they support alter table for both tables + views
• Snowflake and BigQuery don't use alter ... rename, they instead use create or replace

It does break on Presto, which distinguishes between alter view and alter table. dbt will attempt to run alter table (because target_relation.type is table) against an existing view and return an error.

Expected behavior

dbt should use the type of the old relation when it attempts to rename it.

The output of dbt --version:

dbt --version
installed version: 0.15.2
   latest version: 0.15.2

Up to date!

[drewbanin]

Thanks for the report @jtcohen6. We can get this fixed in a future release, but I'd advise addressing this in a materialization override in the presto plugin for the time being!