(fixes #1292) Check for relation type in is_incremental()

If the target relation is a non-table (probably a view) then dbt
should return False from the is_incremental() macro. The
materialization will drop this relation before running the model
code as a `create table as` statement, so the incremental filter
would likely be invalid.

core/dbt/include/global_project/macros/etc/is_incremental.sql

@@ -5,6 +5,6 @@
         {{ return(False) }}
     {% else %}
         {% set relation = adapter.get_relation(this.database, this.schema, this.table) %}
-        {{ return(relation is not none and not flags.FULL_REFRESH) }}
+        {{ return(relation is not none and relation.type == 'table' and not flags.FULL_REFRESH) }}
     {% endif %}
 {% endmacro %}

test/integration/035_changing_relation_type_test/models/model.sql

@@ -1,5 +1,9 @@
 
 
-{{ config(materialized=var('materialized'), sql_where='TRUE') }}
+{{ config(materialized=var('materialized')) }}
 
 select '{{ var("materialized") }}' as materialization
+
+{% if var('materialized') == 'incremental' and is_incremental() %}
+    where 'abc' != (select max(materialization) from {{ this }})
+{% endif %}