From 25fc718abbc6775446f3c8717166d2ff0cc993a6 Mon Sep 17 00:00:00 2001
From: Axell <68310020+axellpadilla@users.noreply.github.com>
Date: Mon, 29 Sep 2025 23:36:50 +0000
Subject: [PATCH] Add behavior flags property and optimize column alteration
 logic in SQLServerAdapter

---
 dbt/adapters/sqlserver/sqlserver_adapter.py   | 21 ++++++-
 .../sqlserver/macros/adapter/columns.sql      | 55 +++++++++++--------
 2 files changed, 51 insertions(+), 25 deletions(-)

diff --git a/dbt/adapters/sqlserver/sqlserver_adapter.py b/dbt/adapters/sqlserver/sqlserver_adapter.py
index 6f05c5017..9665b2474 100644
--- a/dbt/adapters/sqlserver/sqlserver_adapter.py
+++ b/dbt/adapters/sqlserver/sqlserver_adapter.py
@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import List, Optional
 
 import dbt.exceptions
 from dbt.adapters.base.impl import ConstraintSupport
@@ -65,3 +65,22 @@ def valid_incremental_strategies(self):
         Not used to validate custom strategies defined by end users.
         """
         return ["append", "delete+insert", "merge", "microbatch"]
+
+    @property
+    def _behavior_flags(self) -> List[dict]:
+        """Adapter-specific behavior flags. These are merged with project overrides
+        by the BaseAdapter.behavior machinery.
+        """
+        return [
+            {
+                "name": "sqlserver__prefer_single_alter_column",
+                "default": False,
+                "source": "dbt-sqlserver",
+                "description": (
+                    "If true, prefer running a single "
+                    "ALTER ... ALTER COLUMN for type expansions on tables. When false, "
+                    "fall back to add/copy/drop/rename flow."
+                ),
+                "docs_url": None,
+            },
+        ]
diff --git a/dbt/include/sqlserver/macros/adapter/columns.sql b/dbt/include/sqlserver/macros/adapter/columns.sql
index a98750e7e..5a35a1779 100644
--- a/dbt/include/sqlserver/macros/adapter/columns.sql
+++ b/dbt/include/sqlserver/macros/adapter/columns.sql
@@ -24,27 +24,34 @@
 {% endmacro %}
 
 {% macro sqlserver__alter_column_type(relation, column_name, new_column_type) %}
-
-    {%- set tmp_column = column_name + "__dbt_alter" -%}
-    {% set alter_column_type %}
-        alter {{ relation.type }} {{ relation }} add "{{ tmp_column }}" {{ new_column_type }};
-    {%- endset %}
-
-    {% set update_column %}
-        update {{ relation }} set "{{ tmp_column }}" = "{{ column_name }}";
-    {%- endset %}
-
-    {% set drop_column %}
-        alter {{ relation.type }} {{ relation }} drop column "{{ column_name }}";
-    {%- endset %}
-
-    {% set rename_column %}
-        exec sp_rename '{{ relation | replace('"', '') }}.{{ tmp_column }}', '{{ column_name }}', 'column'
-    {%- endset %}
-
-    {% do run_query(alter_column_type) %}
-    {% do run_query(update_column) %}
-    {% do run_query(drop_column) %}
-    {% do run_query(rename_column) %}
-
-{% endmacro %}
+    {% if flags.sqlserver__prefer_single_alter_column and relation.type == 'table' %}
+        {% set alter_column_type_sql %}
+            alter {{ relation.type }} {{ relation }} alter column "{{ column_name }}" {{ new_column_type }};
+        {%- endset %}
+
+        {% do run_query(alter_column_type_sql) %}
+    {% else %}
+        {%- set tmp_column = column_name + "__dbt_alter" -%}
+
+        {% set alter_column_type %}
+            alter {{ relation.type }} {{ relation }} add "{{ tmp_column }}" {{ new_column_type }};
+        {%- endset %}
+
+        {% set update_column %}
+            update {{ relation }} set "{{ tmp_column }}" = "{{ column_name }}";
+        {%- endset %}
+
+        {% set drop_column %}
+            alter {{ relation.type }} {{ relation }} drop column "{{ column_name }}";
+        {%- endset %}
+
+        {% set rename_column %}
+            exec sp_rename '{{ relation | replace('"', '') }}.{{ tmp_column }}', '{{ column_name }}', 'column'
+        {%- endset %}
+
+        {% do run_query(alter_column_type) %}
+        {% do run_query(update_column) %}
+        {% do run_query(drop_column) %}
+        {% do run_query(rename_column) %}
+    {% endif %}
+ {% endmacro %}