Snowflake table materialization can drop the wrong type of backup table

[drewbanin]

Issue

Issue description

dbt's Snowflake table materialization can errantly assume that an existing backup relation is of the wrong type. The materialization assumes that the backup_relation is of the same type as the old_relation, which is not necessarily true:

https://github.com/fishtown-analytics/dbt/blob/371e8b438ab0939f000e3844162c50644f19d6d8/dbt/include/global_project/macros/materializations/table/snowflake_table.sql#L17

Results

dbt issued a drop table ... statement whereas it should have issued a drop view ... statement for the backup relation.

System information

The output of dbt --version:

>= 0.10.2

Steps to reproduce

The database needs to be in a very weird state to trigger this bug. There needs to be a {identifier}__dbt_backup relation with a different type than the {identifier} relation.

schema.test_relation <view>
schema.test_relation__dbt_backup <table>

Then run a model like:

-- models/test_relation.sql

{{ config(materialized='table') }}

select 1 as id

^ Importantly, this needs to be materialized as a table. The view materialization does not suffer from this bug, as we do not use a backup relation there. See also: #1101