when changing table reset columns

public/app/plugins/datasource/postgres/query_ctrl.ts

@@ -209,12 +209,22 @@ export class PostgresQueryCtrl extends QueryCtrl {
           this.timeColumnSegment.html = segment.html;
           this.timeColumnSegment.value = segment.value;
 
+          this.target.where = [];
+          this.target.updatePersistedParts();
           this.timeColumnChanged();
         }
       }
     });
 
+    this.datasource.metricFindQuery(this.metaBuilder.buildColumnQuery('value')).then(result => {
+      // check if value columns are still valid
+      if (result.length > 0) {
+        this.target.select = [[{ type: 'column', params: [result[0].text] }]];
+        this.target.group = [];
+      }
+      this.updateProjection();
       this.panelCtrl.refresh();
+    });
   }
 
   getTimeColumnSegments() {