Adding a schema check before privilege dependent ddl in Snowflake (#215)

macros/plugins/snowflake/create_external_schema.sql

@@ -0,0 +1,32 @@
+{% macro snowflake__create_external_schema(source_node) %}
+
+    {% set schema_exists_query %}
+        show terse schemas like '{{ source_node.schema }}' in database {{ source_node.database }} limit 1;
+    {% endset %}
+    {% if execute %}
+        {% set schema_exists = run_query(schema_exists_query)|length > 0 %}
+    {% else %}
+        {% set schema_exists = false %}
+    {% endif %}    
+
+    {% if schema_exists %}
+        {% set ddl %}
+            select 'Schema {{ source_node.schema }} exists' from dual;
+        {% endset %}
+    {% else %}
+        {% set fqn %}
+            {% if source_node.database %}
+                {{ source_node.database }}.{{ source_node.schema }}
+            {% else %}
+                {{ source_node.schema }}
+            {% endif %}
+        {% endset %}
+
+        {% set ddl %}
+            create schema if not exists {{ fqn }};
+        {% endset %}
+    {% endif %}
+
+    {% do return(ddl) %}
+
+{% endmacro %}