Improve code quality

Author: kandy

app/code/Magento/Catalog/Model/Indexer/Product/Flat/FlatTableBuilder.php

@@ -261,7 +261,7 @@ protected function _fillTemporaryFlatTable(array $tables, $storeId, $valueFieldS
 
         $select->from(
             ['et' => $entityTemporaryTableName],
-            $allColumns
+            array_merge(...$allColumns)
         )->joinInner(
             ['e' => $this->resource->getTableName('catalog_product_entity')],
             'e.entity_id = et.entity_id',