Prefer unique constraints over indexes on SQL Server

Unique constraint and index do not differ in SQL Server today. The
problem is if we create an index and try to drop a constraint, the
drop fails.

Currently if the unique is added when the table is created, we add
constraint. If the table is altered with a unique constraint we create
an index instead. Dropping this does not work.

The PR makes altering the table with an `@unique` to use constraints
instead of an index.

Closes: prisma/prisma#14051

migration-engine/connectors/sql-migration-connector/src/sql_renderer/mssql_renderer.rs

@@ -125,12 +125,6 @@ impl SqlRenderer for MssqlFlavour {
 
     fn render_create_index(&self, index: IndexWalker<'_>) -> String {
         let mssql_schema_ext: &MssqlSchemaExt = index.schema.downcast_connector_data().unwrap_or_default();
-        let index_type = match index.index_type() {
-            IndexType::Unique => "UNIQUE ",
-            IndexType::Normal => "",
-            IndexType::Fulltext => unreachable!("Fulltext index on SQL Server"),
-        };
-
         let index_name = self.quote(index.name());
         let table_reference = self.quote_with_schema(index.table().name()).to_string();
 
@@ -153,7 +147,17 @@ impl SqlRenderer for MssqlFlavour {
 
         let columns = columns.join(", ");
 
-        format!("CREATE {index_type}{clustering}INDEX {index_name} ON {table_reference}({columns})",)
+        match index.index_type() {
+            IndexType::Unique => {
+                let constraint_name = self.quote(index.name());
+
+                format!("ALTER TABLE {table_reference} ADD CONSTRAINT {constraint_name} UNIQUE {clustering}({columns})")
+            }
+            IndexType::Normal => {
+                format!("CREATE {clustering}INDEX {index_name} ON {table_reference}({columns})",)
+            }
+            IndexType::Fulltext => todo!("SQL Server full-text indices..."),
+        }
     }
 
     fn render_create_table_as(&self, table: TableWalker<'_>, table_name: &str) -> String {

migration-engine/migration-engine-tests/tests/migrations/indexes/mssql.rs

@@ -1,4 +1,4 @@
-use indoc::formatdoc;
+use indoc::{formatdoc, indoc};
 use migration_engine_tests::test_api::*;
 
 #[test_connector(tags(Mssql))]
@@ -184,3 +184,82 @@ fn clustered_to_non_clustered_index(api: TestApi) {
         table.assert_index_on_columns(&["og"], |index| index.assert_non_clustered())
     });
 }
+
+#[test_connector(tags(Mssql))]
+fn creating_and_dropping_unique_constraint_works(api: TestApi) {
+    let dm1 = indoc! {r#"
+        model Logbook {
+          id         Int      @id
+          categoryId String?  @db.UniqueIdentifier
+          date       DateTime @db.Date()
+        }
+    "#};
+
+    api.schema_push_w_datasource(dm1).send().assert_green();
+
+    api.assert_schema()
+        .assert_table("Logbook", |cat| cat.assert_indexes_count(0));
+
+    let dm1 = indoc! {r#"
+        model Logbook {
+          id         Int      @id
+          categoryId String?  @db.UniqueIdentifier
+          date       DateTime @db.Date()
+
+          @@unique([categoryId, date])
+        }
+    "#};
+
+    api.schema_push_w_datasource(dm1).force(true).send();
+
+    api.assert_schema().assert_table("Logbook", |cat| {
+        cat.assert_indexes_count(1)
+            .assert_index_on_columns(&["categoryId", "date"], |idx| idx.assert_is_unique())
+    });
+
+    let dm2 = indoc! {r#"
+        model Logbook {
+          id         Int      @id
+          categoryId String?  @db.UniqueIdentifier
+          date       DateTime @db.Date()
+        }
+    "#};
+
+    api.schema_push_w_datasource(dm2).send().assert_green();
+
+    api.assert_schema()
+        .assert_table("Logbook", |cat| cat.assert_indexes_count(0));
+}
+
+#[test_connector(tags(Mssql))]
+fn dropping_unique_constraint_works(api: TestApi) {
+    let dm1 = indoc! {r#"
+        model Logbook {
+          id         Int      @id
+          categoryId String?  @db.UniqueIdentifier
+          date       DateTime @db.Date()
+
+          @@unique([categoryId, date])
+        }
+    "#};
+
+    api.schema_push_w_datasource(dm1).force(true).send();
+
+    api.assert_schema().assert_table("Logbook", |cat| {
+        cat.assert_indexes_count(1)
+            .assert_index_on_columns(&["categoryId", "date"], |idx| idx.assert_is_unique())
+    });
+
+    let dm2 = indoc! {r#"
+        model Logbook {
+          id         Int      @id
+          categoryId String?  @db.UniqueIdentifier
+          date       DateTime @db.Date()
+        }
+    "#};
+
+    api.schema_push_w_datasource(dm2).send().assert_green();
+
+    api.assert_schema()
+        .assert_table("Logbook", |cat| cat.assert_indexes_count(0));
+}