[7.x] Optimize performance for document table (#101715)

src/plugins/discover/public/application/helpers/columns.test.ts

@@ -43,4 +43,9 @@ describe('getDisplayedColumns', () => {
       ]
     `);
   });
+  test('returns the same instance of ["_source"] over multiple calls', async () => {
+    const result = getDisplayedColumns([], indexPatternWithTimefieldMock);
+    const result2 = getDisplayedColumns([], indexPatternWithTimefieldMock);
+    expect(result).toBe(result2);
+  });
 });

src/plugins/discover/public/application/helpers/columns.ts

@@ -8,6 +8,11 @@
 
 import { IndexPattern } from '../../../../data/common';
 
+// We store this outside the function as a constant, so we're not creating a new array every time
+// the function is returning this. A changing array might cause the data grid to think it got
+// new columns, and thus performing worse than using the same array over multiple renders.
+const SOURCE_ONLY = ['_source'];
+
 /**
  * Function to provide fallback when
  * 1) no columns are given
@@ -19,5 +24,5 @@ export function getDisplayedColumns(stateColumns: string[] = [], indexPattern: I
     // check if all columns where removed except the configured timeField (this can't be removed)
     !(stateColumns.length === 1 && stateColumns[0] === indexPattern.timeFieldName)
     ? stateColumns
-    : ['_source'];
+    : SOURCE_ONLY;
 }