executor: make SLOW_QUERY support query slow log at any time

executor/builder.go

@@ -1354,12 +1354,16 @@ func (b *executorBuilder) buildMemTable(v *plannercore.PhysicalMemTable) Executo
 				},
 			}
 		case strings.ToLower(infoschema.TableSlowQuery), strings.ToLower(infoschema.ClusterTableSlowLog):
+			retriever := &slowQueryRetriever{
+				table:      v.Table,
+				outputCols: v.Columns,
+			}
+			if v.Extractor != nil {
+				retriever.extractor = v.Extractor.(*plannercore.SlowQueryExtractor)
+			}
 			return &MemTableReaderExec{
 				baseExecutor: newBaseExecutor(b.ctx, v.Schema(), v.ExplainID()),
-				retriever: &SlowQueryRetriever{
-					table:      v.Table,
-					outputCols: v.Columns,
-				},
+				retriever:    retriever,
 			}
 		}
 	}