[8.5] [Discover] Improve performance of getFieldsToShow (#144672) (#1…

…45424)

# Backport

This will backport the following commits from `main` to `8.5`:
- [[Discover] Improve performance of getFieldsToShow
(#144672)](#144672)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Matthias
Wilhelm","email":"matthias.wilhelm@elastic.co"},"sourceCommit":{"committedDate":"2022-11-16T17:53:18Z","message":"[Discover]
Improve performance of getFieldsToShow (#144672)\n\n##
Summary\r\n\r\nSignificantly improves the performance of
`getFieldsToShow` for very large data views (>=100k). This is done by
reducing the number ofiterations necessary to detect multi fields shat
should not be
displayed.","sha":"1ffb93d687e899ec93b4e1735458e9e2863d6b39","branchLabelMapping":{"^v8.6.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Feature:Discover","release_note:fix","Team:DataDiscovery","v8.6.0","v7.17.8","v8.7.0","v8.5.2"],"number":144672,"url":"#144672
Improve performance of getFieldsToShow (#144672)\n\n##
Summary\r\n\r\nSignificantly improves the performance of
`getFieldsToShow` for very large data views (>=100k). This is done by
reducing the number ofiterations necessary to detect multi fields shat
should not be
displayed.","sha":"1ffb93d687e899ec93b4e1735458e9e2863d6b39"}},"sourceBranch":"main","suggestedTargetBranches":["7.17","8.7","8.5"],"targetPullRequestStates":[{"branch":"main","label":"v8.6.0","labelRegex":"^v8.6.0$","isSourceBranch":true,"state":"MERGED","url":"#144672
Improve performance of getFieldsToShow (#144672)\n\n##
Summary\r\n\r\nSignificantly improves the performance of
`getFieldsToShow` for very large data views (>=100k). This is done by
reducing the number ofiterations necessary to detect multi fields shat
should not be
displayed.","sha":"1ffb93d687e899ec93b4e1735458e9e2863d6b39"}},{"branch":"7.17","label":"v7.17.8","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"},{"branch":"8.7","label":"v8.7.0","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"},{"branch":"8.5","label":"v8.5.2","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Matthias Wilhelm <matthias.wilhelm@elastic.co>

src/plugins/discover/public/utils/get_fields_to_show.ts

@@ -8,24 +8,38 @@
 import { getFieldSubtypeMulti } from '@kbn/data-views-plugin/public';
 import type { DataView } from '@kbn/data-views-plugin/public';
 
+/**
+ * Returns am array of fields to display in the Documents column of the data table
+ * If showMultiFields is set to false, it filters out multifields that have a parent, to prevent entries for multifields
+ * like this: field, field.keyword, field.whatever
+ * @param fields
+ * @param dataView
+ * @param showMultiFields
+ */
 export const getFieldsToShow = (fields: string[], dataView: DataView, showMultiFields: boolean) => {
-  const childParentFieldsMap = {} as Record<string, string>;
-  const mapping = (name: string) => dataView.fields.getByName(name);
+  if (showMultiFields) {
+    return fields;
+  }
+  const fieldSet = new Set();
+  const childParentFieldsMap = new Map();
+  const parentFieldSet = new Set();
   fields.forEach((key) => {
-    const mapped = mapping(key);
+    const mapped = dataView.fields.getByName(key);
     const subTypeMulti = mapped && getFieldSubtypeMulti(mapped.spec);
+    const isMultiField = Boolean(subTypeMulti?.multi);
     if (mapped && subTypeMulti?.multi?.parent) {
-      childParentFieldsMap[mapped.name] = subTypeMulti.multi.parent;
+      childParentFieldsMap.set(key, subTypeMulti.multi.parent);
+    }
+    if (mapped && isMultiField) {
+      parentFieldSet.add(key);
     }
+    fieldSet.add(key);
   });
   return fields.filter((key: string) => {
-    const fieldMapping = mapping(key);
-    const subTypeMulti = fieldMapping && getFieldSubtypeMulti(fieldMapping.spec);
-    const isMultiField = !!subTypeMulti?.multi;
-    if (!isMultiField) {
+    if (!parentFieldSet.has(key)) {
       return true;
     }
-    const parent = childParentFieldsMap[key];
-    return showMultiFields || (parent && !fields.includes(parent));
+    const parent = childParentFieldsMap.get(key);
+    return parent && !fieldSet.has(parent);
   });
 };