Filtered indexes for SQL Server (#7033)

* Enabled filtered indexes for SQL Server

* [ci skip] Added filtered indexes to docs/migrations.md

changelog.md

 # Future
+- [ADDED] Filtered Indexes support for SQL Server [#7016](https://github.com/sequelize/sequelize/issues/7016)
 - [FIXED] Set `timestamps` and `paranoid` options from through model on `belongsToMany` association
 - [FIXED] Properly apply paranoid condition when `groupedLimit.on` association is `paranoid`
 - [FIXED] Throw MSSQL MERGE Statement foreignKey violations as ForeignKeyConstraintError [#7011](https://github.com/sequelize/sequelize/pull/7011)

docs/docs/migrations.md

@@ -271,6 +271,7 @@ queryInterface.addIndex('Person', ['firstname', 'lastname'])
 // - parser: For FULLTEXT columns set your parser
 // - indexType: Set a type for the index, e.g. BTREE. See the documentation of the used dialect
 // - logging: A function that receives the sql query, e.g. console.log
+// - where: A hash of attributes to limit your index(Filtered Indexes - MSSQL & PostgreSQL only)
 queryInterface.addIndex(
   'Person',
   ['firstname', 'lastname'],
@@ -279,6 +280,18 @@ queryInterface.addIndex(
     indicesType: 'UNIQUE'
   }
 )
+
+queryInterface.addIndex(
+  'Person',
+  ['firstname', 'lastname'],
+  {
+    where: {
+      lastname: {
+        $ne: null
+      }
+    }
+  }
+)
 ```
 
 ### removeIndex(tableName, indexNameOrAttributes, options)

lib/dialects/mssql/index.js

@@ -44,7 +44,8 @@ MssqlDialect.prototype.supports = _.merge(_.cloneDeep(Abstract.prototype.support
     length: false,
     parser: false,
     type: true,
-    using: false
+    using: false,
+    where: true
   },
   NUMERIC: true,
   tmpTableTrigger: true

test/unit/sql/index.test.js

@@ -87,7 +87,7 @@ suite(Support.getTestDialectTeaser('SQL'), function() {
       });
     }
 
-    if (current.dialect.supports.index.using === 2) {
+    if (current.dialect.supports.index.where) {
       test('WHERE', function () {
         expectsql(sql.addIndexQuery('table', {
           fields: ['type'],
@@ -95,7 +95,8 @@ suite(Support.getTestDialectTeaser('SQL'), function() {
             type: 'public'
           }
         }), {
-          postgres: 'CREATE INDEX "table_type" ON "table" ("type") WHERE "type" = \'public\''
+          postgres: 'CREATE INDEX "table_type" ON "table" ("type") WHERE "type" = \'public\'',
+          mssql: 'CREATE INDEX [table_type] ON [table] ([type]) WHERE [type] = N\'public\''
         });
 
         expectsql(sql.addIndexQuery('table', {
@@ -109,7 +110,20 @@ suite(Support.getTestDialectTeaser('SQL'), function() {
             }
           }
         }), {
-          postgres: 'CREATE INDEX "table_type" ON "table" ("type") WHERE ("type" = \'group\' OR "type" = \'private\')'
+          postgres: 'CREATE INDEX "table_type" ON "table" ("type") WHERE ("type" = \'group\' OR "type" = \'private\')',
+          mssql: 'CREATE INDEX [table_type] ON [table] ([type]) WHERE ([type] = N\'group\' OR [type] = N\'private\')'
+        });
+
+        expectsql(sql.addIndexQuery('table', {
+          fields: ['type'],
+          where: {
+            type: {
+              $ne: null
+            }
+          }
+        }), {
+          postgres: 'CREATE INDEX "table_type" ON "table" ("type") WHERE "type" IS NOT NULL',
+          mssql: 'CREATE INDEX [table_type] ON [table] ([type]) WHERE [type] IS NOT NULL'
         });
       });
     }