diff --git a/app/server/lib/ActiveDoc.ts b/app/server/lib/ActiveDoc.ts
index 07859c8e..95e64c70 100644
--- a/app/server/lib/ActiveDoc.ts
+++ b/app/server/lib/ActiveDoc.ts
@@ -87,7 +87,7 @@ import {
   makeExceptionalDocSession,
   OptDocSession
 } from './DocSession';
-import {DocStorage} from './DocStorage';
+import {createAttachmentsIndex, DocStorage} from './DocStorage';
 import {expandQuery} from './ExpandedQuery';
 import {GranularAccess, GranularAccessForBundle} from './GranularAccess';
 import {OnDemandActions} from './OnDemandActions';
@@ -449,6 +449,9 @@ export class ActiveDoc extends EventEmitter {
     const initBundle = await this._rawPyCall('apply_user_actions', [["InitNewDoc", timezone, locale]]);
     await this.docStorage.execTransaction(() =>
       this.docStorage.applyStoredActions(getEnvContent(initBundle.stored)));
+    // DocStorage can't create this index in the initial schema
+    // because the table _grist_Attachments doesn't exist at that point - it's created by InitNewDoc.
+    await createAttachmentsIndex(this.docStorage);
 
     await this._initDoc(docSession);
     await this._tableMetadataLoader.clean();
diff --git a/app/server/lib/DocStorage.ts b/app/server/lib/DocStorage.ts
index c30890af..2b8793a6 100644
--- a/app/server/lib/DocStorage.ts
+++ b/app/server/lib/DocStorage.ts
@@ -381,11 +381,11 @@ export class DocStorage implements ISQLiteDB, OnDemandStorage {
         // Migration to add an index to _grist_Attachments.fileIdent for fast joining against _gristsys_Files.ident.
         const tables = await db.all(`SELECT * FROM sqlite_master WHERE type='table' AND name='_grist_Attachments'`);
         if (!tables.length) {
-          // _grist_Attachments is created in the first Python migration.
-          // For the sake of migration tests on ancient documents, just skip this migration if the table doesn't exist.
+          // _grist_Attachments is created in the first Python migration so doesn't exist here for new documents.
+          // createAttachmentsIndex is called separately by ActiveDoc for that.
           return;
         }
-        await db.exec(`CREATE INDEX _grist_Attachments_fileIdent ON _grist_Attachments(fileIdent)`);
+        await createAttachmentsIndex(db);
       },
 
     ]
@@ -661,7 +661,8 @@ export class DocStorage implements ISQLiteDB, OnDemandStorage {
 
   /**
    * Creates a new SQLite database. Will throw an error if the database already exists.
-   * After a database is created it should be initialized by applying the InitNewDoc action.
+   * After a database is created it should be initialized by applying the InitNewDoc action
+   * or by executing the initialDocSql.
    */
   public createFile(): Promise<void> {
     // It turns out to be important to return a bluebird promise, a lot of code outside
@@ -1698,3 +1699,10 @@ export interface IndexColumns {
 export interface IndexInfo extends IndexColumns {
   indexId: string;     // name of index
 }
+
+/**
+ * Creates an index that allows fast SQL JOIN between _grist_Attachments.fileIdent and _gristsys_Files.ident.
+ */
+export async function createAttachmentsIndex(db: ISQLiteDB) {
+  await db.exec(`CREATE INDEX _grist_Attachments_fileIdent ON _grist_Attachments(fileIdent)`);
+}
diff --git a/app/server/lib/SQLiteDB.ts b/app/server/lib/SQLiteDB.ts
index d73db0e5..a6c29f6e 100644
--- a/app/server/lib/SQLiteDB.ts
+++ b/app/server/lib/SQLiteDB.ts
@@ -148,7 +148,7 @@ export interface ISQLiteDB {
  *    SQLiteDB.openDB(): Opens a DB, and initialize or migrate it to correct schema.
  *    db.execTransaction(cb): Runs a callback in the context of a new DB transaction.
  */
-export class SQLiteDB {
+export class SQLiteDB implements ISQLiteDB {
   /**
    * Opens a database or creates a new one, according to OpenMode enum. The schemaInfo specifies
    * how to initialize a new database, and how to migrate an existing one from an older version.
diff --git a/app/server/lib/initialDocSql.ts b/app/server/lib/initialDocSql.ts
index 89aa02f5..8f0e9d1d 100644
--- a/app/server/lib/initialDocSql.ts
+++ b/app/server/lib/initialDocSql.ts
@@ -34,6 +34,7 @@ INSERT INTO _grist_ACLPrincipals VALUES(3,'group','','','Editors','');
 INSERT INTO _grist_ACLPrincipals VALUES(4,'group','','','Viewers','');
 CREATE TABLE IF NOT EXISTS "_grist_ACLMemberships" (id INTEGER PRIMARY KEY, "parent" INTEGER DEFAULT 0, "child" INTEGER DEFAULT 0);
 CREATE TABLE IF NOT EXISTS "_grist_Filters" (id INTEGER PRIMARY KEY, "viewSectionRef" INTEGER DEFAULT 0, "colRef" INTEGER DEFAULT 0, "filter" TEXT DEFAULT '');
+CREATE INDEX _grist_Attachments_fileIdent ON _grist_Attachments(fileIdent);
 COMMIT;
 `;
 
@@ -86,5 +87,6 @@ INSERT INTO _grist_ACLPrincipals VALUES(4,'group','','','Viewers','');
 CREATE TABLE IF NOT EXISTS "_grist_ACLMemberships" (id INTEGER PRIMARY KEY, "parent" INTEGER DEFAULT 0, "child" INTEGER DEFAULT 0);
 CREATE TABLE IF NOT EXISTS "_grist_Filters" (id INTEGER PRIMARY KEY, "viewSectionRef" INTEGER DEFAULT 0, "colRef" INTEGER DEFAULT 0, "filter" TEXT DEFAULT '');
 CREATE TABLE IF NOT EXISTS "Table1" (id INTEGER PRIMARY KEY, "manualSort" NUMERIC DEFAULT 1e999, "A" BLOB DEFAULT NULL, "B" BLOB DEFAULT NULL, "C" BLOB DEFAULT NULL);
+CREATE INDEX _grist_Attachments_fileIdent ON _grist_Attachments(fileIdent);
 COMMIT;
 `;