diff --git a/packages/common/infra/src/modules/doc/constants.ts b/packages/common/infra/src/modules/doc/constants.ts
index cde7a6b465..31a526fa85 100644
--- a/packages/common/infra/src/modules/doc/constants.ts
+++ b/packages/common/infra/src/modules/doc/constants.ts
@@ -2,10 +2,13 @@ import type { DocCustomPropertyInfo } from '../db';
 
 /**
  * default built-in custom property, user can update and delete them
+ *
+ * 'id' and 'type' is request, 'index' is a manually maintained incremental key.
  */
 export const BUILT_IN_CUSTOM_PROPERTY_TYPE = [
   {
     id: 'tags',
     type: 'tags',
+    index: 'a0000001',
   },
 ] as DocCustomPropertyInfo[];
diff --git a/packages/common/infra/src/utils/__tests__/fractional-indexing.spec.ts b/packages/common/infra/src/utils/__tests__/fractional-indexing.spec.ts
index b5ed42853c..aa5d7e9b4c 100644
--- a/packages/common/infra/src/utils/__tests__/fractional-indexing.spec.ts
+++ b/packages/common/infra/src/utils/__tests__/fractional-indexing.spec.ts
@@ -39,3 +39,21 @@ test('fractional-indexing', () => {
     }
   }
 });
+
+test('no postfix', () => {
+  expect(
+    generateFractionalIndexingKeyBetween('a0', null).startsWith('a1')
+  ).toBe(true);
+  expect(
+    generateFractionalIndexingKeyBetween('a01', null).startsWith('a1')
+  ).toBe(true);
+  expect(
+    generateFractionalIndexingKeyBetween('a0001', null).startsWith('a1')
+  ).toBe(true);
+  expect(
+    generateFractionalIndexingKeyBetween(null, 'a0').startsWith('Zz')
+  ).toBe(true);
+  expect(
+    generateFractionalIndexingKeyBetween('a0', 'a01').startsWith('a00V')
+  ).toBe(true);
+});
diff --git a/packages/common/infra/src/utils/fractional-indexing.ts b/packages/common/infra/src/utils/fractional-indexing.ts
index 7e0cf0d919..a5adaab78d 100644
--- a/packages/common/infra/src/utils/fractional-indexing.ts
+++ b/packages/common/infra/src/utils/fractional-indexing.ts
@@ -150,17 +150,19 @@ export function createFractionalIndexingSortableHelper<
  * the key always has a random suffix, so there is no need to worry about collision.
  *
  * make sure a and b are generated by this function.
+ *
+ * @param customPostfix custom postfix for the key, only letters and numbers are allowed
  */
 export function generateFractionalIndexingKeyBetween(
   a: string | null,
   b: string | null
 ) {
   const randomSize = 32;
-  function randomString(length: number = randomSize) {
-    const values = new Uint8Array(length);
-    crypto.getRandomValues(values);
+  function postfix(length: number = randomSize) {
     const chars =
       '123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
+    const values = new Uint8Array(length);
+    crypto.getRandomValues(values);
     let result = '';
     for (let i = 0; i < length; i++) {
       result += chars.charAt(values[i] % chars.length);
@@ -193,20 +195,20 @@ export function generateFractionalIndexingKeyBetween(
 
   if (aSubkey === null && bSubkey === null) {
     // generate a new key
-    return generateKeyBetween(null, null) + '0' + randomString();
+    return generateKeyBetween(null, null) + '0' + postfix();
   } else if (aSubkey === null && bSubkey !== null) {
     // generate a key before b
-    return generateKeyBetween(null, bSubkey) + '0' + randomString();
+    return generateKeyBetween(null, bSubkey) + '0' + postfix();
   } else if (bSubkey === null && aSubkey !== null) {
     // generate a key after a
-    return generateKeyBetween(aSubkey, null) + '0' + randomString();
+    return generateKeyBetween(aSubkey, null) + '0' + postfix();
   } else if (aSubkey !== null && bSubkey !== null) {
     // generate a key between a and b
     if (aSubkey === bSubkey && a !== null && b !== null) {
       // conflict, if the subkeys are the same, generate a key between fullkeys
-      return generateKeyBetween(a, b) + '0' + randomString();
+      return generateKeyBetween(a, b) + '0' + postfix();
     } else {
-      return generateKeyBetween(aSubkey, bSubkey) + '0' + randomString();
+      return generateKeyBetween(aSubkey, bSubkey) + '0' + postfix();
     }
   }
   throw new Error('Never reach here');