Postgres: Fix aggregate dropdown when TimescaleDB is enabled (#45993)

* add timescaleDB checker function

* use existing fn findAggregateIndex

* build options from empty list

* refactor fn and handle state for existing elem

* remove newline

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

@@ -111,10 +111,27 @@ export class PostgresQueryCtrl extends QueryCtrl {
     this.panelCtrl.refresh();
   }
 
+  timescaleAggCheck() {
+    const aggIndex = this.findAggregateIndex(this.selectParts[0]);
+
+    // add or remove TimescaleDB aggregate functions as needed
+    if (aggIndex !== -1) {
+      const baseOpts = this.selectParts[0][aggIndex].def.params[0].baseOptions;
+      const timescaleOpts = baseOpts.concat(this.selectParts[0][aggIndex].def.params[0].timescaleOptions);
+
+      if (this.datasource.jsonData.timescaledb === true) {
+        this.selectParts[0][aggIndex].def.params[0].options = timescaleOpts;
+      } else {
+        this.selectParts[0][aggIndex].def.params[0].options = baseOpts;
+      }
+    }
+  }
+
   updateProjection() {
     this.selectParts = map(this.target.select, (parts: any) => {
       return map(parts, sqlPart.create).filter((n) => n);
     });
+    this.timescaleAggCheck();
     this.whereParts = map(this.target.where, sqlPart.create).filter((n) => n);
     this.groupParts = map(this.target.group, sqlPart.create).filter((n) => n);
   }
@@ -125,6 +142,7 @@ export class PostgresQueryCtrl extends QueryCtrl {
         return { type: part.def.type, datatype: part.datatype, params: part.params };
       });
     });
+    this.timescaleAggCheck();
     this.target.where = map(this.whereParts, (part: any) => {
       return { type: part.def.type, datatype: part.datatype, name: part.name, params: part.params };
     });

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

@@ -49,7 +49,9 @@ register({
     {
       name: 'name',
       type: 'string',
-      options: ['avg', 'count', 'min', 'max', 'sum', 'stddev', 'variance'],
+      options: [],
+      baseOptions: ['avg', 'count', 'min', 'max', 'sum', 'stddev', 'variance'],
+      timescaleOptions: ['first', 'last'],
     },
   ],
   defaultParams: ['avg'],