4. Append-only merge if no unique_key

README.md

@@ -192,7 +192,8 @@ where date_day::date >= '2019-01-01'
 group by 1
 ```
 
-The `merge` strategy is only supported when using file_format `delta` (supported in Databricks). It also requires you to specify a `unique key` to match existing records.
+The `merge` strategy is only supported when using file_format `delta` (supported in Databricks). If a `unique key` is specified, the statement will match existing records and overwrite them with new values. If a `unique key` config is not specified, dbt will simply
+append new data to the model, without overwriting any existing data. (For atomic replacement of an entire Delta table, use the `'table'` materialization instead.)
 
 ```
 {{ config(

dbt/include/spark/macros/materializations/incremental.sql

@@ -61,23 +61,21 @@
   {% do return(strategy) %}
 {% endmacro %}
 
-{% macro dbt_spark_validate_merge(file_format) %}
-  {% set invalid_file_format_msg -%}
-    You can only choose the 'merge' incremental_strategy when file_format is set to 'delta'
-  {%- endset %}
-
-  {% if file_format != 'delta' %}
-    {% do exceptions.raise_compiler_error(invalid_file_format_msg) %}
-  {% endif %}
-
-{% endmacro %}
-
 
 {% macro spark__get_merge_sql(target, source, unique_key, dest_columns, predicates=none) %}
   {# ignore dest_columns - we will just use `*` #}
+
+  {% set merge_condition %}
+    {% if unique_key %}
+        on DBT_INTERNAL_SOURCE.{{ unique_key }} = DBT_INTERNAL_DEST.{{ unique_key }}
+    {% else %}
+        on false
+    {% endif %}
+  {% endset %}
+
     merge into {{ target }} as DBT_INTERNAL_DEST
       using {{ source.include(schema=false) }} as DBT_INTERNAL_SOURCE
-      on DBT_INTERNAL_SOURCE.{{ unique_key }} = DBT_INTERNAL_DEST.{{ unique_key }}
+      {{ merge_condition }}
       when matched then update set *
       when not matched then insert *
 {% endmacro %}
@@ -97,21 +95,17 @@
 
 {% materialization incremental, adapter='spark' -%}
   {#-- Validate early so we don't run SQL if the file_format is invalid --#}
-  {% set file_format = dbt_spark_validate_get_file_format() -%}
+  {%- set file_format = dbt_spark_validate_get_file_format() -%}
   {#-- Validate early so we don't run SQL if the strategy is invalid --#}
-  {% set strategy = dbt_spark_validate_get_incremental_strategy(file_format) -%}
+  {%- set strategy = dbt_spark_validate_get_incremental_strategy(file_format) -%}
+  {%- set unique_key = config.get('unique_key', none) -%}
 
   {%- set full_refresh_mode = (flags.FULL_REFRESH == True) -%}
 
   {% set target_relation = this %}
   {% set existing_relation = load_relation(this) %}
   {% set tmp_relation = make_temp_relation(this) %}
 
-  {% if strategy == 'merge' %}
-    {%- set unique_key = config.require('unique_key') -%}
-    {% do dbt_spark_validate_merge(file_format) %}
-  {% endif %}
-
   {% if strategy == 'insert_overwrite' and config.get('partition_by') %}
     set spark.sql.sources.partitionOverwriteMode = DYNAMIC
   {% endif %}