[Discover] Improve performance for unmapped fields (#159113)

Enhanced Discover performance for scenarios with numerous unmapped fields. While investigating an edge case involving 4k unmapped fields, an issue was identified in our nested field detection code, resulting in significant processing time. This commit optimizes the code by addressing redundant RegEx execution, resulting in a performance boost for the specific scenario.

packages/kbn-discover-utils/src/utils/nested_fields.test.ts

@@ -0,0 +1,40 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import type { DataView } from '@kbn/data-views-plugin/common';
+import { isNestedFieldParent } from './nested_fields';
+describe('isNestedFieldParent', () => {
+  it('correctly identifies nested parent fields', () => {
+    const nestedField = {
+      name: 'nested.field',
+      type: 'keyword',
+      subType: {
+        nested: {
+          path: 'nested.field.path',
+        },
+      },
+    };
+    const unnestedField = {
+      name: 'unnested.field',
+      type: 'keyword',
+    };
+    const list = [nestedField, unnestedField];
+
+    const dataView = {
+      fields: {
+        getByName: jest.fn((fieldName) => list.find((field) => field.name === fieldName)),
+        getAll: jest.fn(() => list),
+      },
+    } as unknown as DataView;
+
+    expect(isNestedFieldParent('nested', dataView)).toBe(true);
+    expect(isNestedFieldParent('nested.field', dataView)).toBe(false);
+    expect(isNestedFieldParent('unnested.field', dataView)).toBe(false);
+    expect(isNestedFieldParent('whateverField', dataView)).toBe(false);
+  });
+});

packages/kbn-discover-utils/src/utils/nested_fields.ts

@@ -47,12 +47,15 @@ import type { DataView } from '@kbn/data-views-plugin/public';
  * issue: https://github.com/elastic/kibana/issues/54957
  */
 export function isNestedFieldParent(fieldName: string, dataView: DataView): boolean {
+  const nestedRootRegex = new RegExp(escapeRegExp(fieldName) + '(\\.|$)');
   return (
     !dataView.fields.getByName(fieldName) &&
     !!dataView.fields.getAll().find((patternField) => {
       // We only want to match a full path segment
-      const nestedRootRegex = new RegExp(escapeRegExp(fieldName) + '(\\.|$)');
       const subTypeNested = getDataViewFieldSubtypeNested(patternField);
+      if (!subTypeNested) {
+        return false;
+      }
       return nestedRootRegex.test(subTypeNested?.nested.path ?? '');
     })
   );