fix compile error

modules/sql/src/cluster/local.ts

@@ -1,4 +1,4 @@
-// Copyright (c) 2021, NVIDIA CORPORATION.
+// Copyright (c) 2021-2022, NVIDIA CORPORATION.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -53,7 +53,5 @@ export class LocalSQLWorker implements Worker {
 
   public async dropTable(name: string) { await this.context.dropTable(name); }
 
-  public async sql(query: string, token: number) {
-    return await (await this.context.sql(query, token)).result();
-  }
+  public async sql(query: string, token: number) { return await this.context.sql(query, token); }
 }

modules/sql/src/context.ts

@@ -263,7 +263,7 @@ export class SQLContext {
    * const sqlContext = new SQLContext();
    * sqlContext.createTable('test_table', df);
    *
-   * sqlContext.sql('SELECT a FROM test_table').result(); // [1, 2, 3]
+   * await sqlContext.sql('SELECT a FROM test_table'); // [1, 2, 3]
    * ```
    */
   public sql(query: string, ctxToken: number = Math.random() * Number.MAX_SAFE_INTEGER | 0) {

modules/sql/src/graph.ts

@@ -18,27 +18,50 @@ import {DataFrame, Table} from '@rapidsai/cudf';
 
 let nonce = Math.random() * 1e3 | 0;
 
-export class ExecutionGraph {
+export class ExecutionGraph implements Promise<DataFrame[]> {
   constructor(private _graph?: import('./rapidsai_sql').ExecutionGraph) {}
 
-  start(): void { this._graph?.start(); }
+  get[Symbol.toStringTag]() { return 'ExecutionGraph'; }
 
-  then() { return this.result(); }
+  then<TResult1 = DataFrame[], TResult2 = never>(
+    onfulfilled?: ((value: DataFrame[]) => TResult1 | PromiseLike<TResult1>)|undefined|null,
+    onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>)|undefined|
+    null): Promise<TResult1|TResult2> {
+    return this.result().then(onfulfilled, onrejected);
+  }
 
-  async result() {
-    const {names, tables} =
-      this._graph ? (await this._graph.result()) : {names: [], tables: [new Table({})]};
-    const results: DataFrame[] = [];
-    tables.forEach((table: Table) => {
-      results.push(new DataFrame(
-        names.reduce((cols, name, i) => ({...cols, [name]: table.getColumnByIndex(i)}), {})));
-    });
+  catch<TResult = never>(onrejected?: ((reason: any) => TResult | PromiseLike<TResult>)|undefined|
+                         null): Promise<DataFrame[]|TResult> {
+    return this.result().catch(onrejected);
+  }
+
+  finally(onfinally?: (() => void)|undefined|null): Promise<DataFrame[]> {
+    return this.result().finally(onfinally);
+  }
+
+  private _result: Promise<DataFrame[]>|undefined;
+
+  start() { this._graph?.start(); }
+
+  result() {
+    if (!this._result) {
+      this._result = (async () => {
+        const {names, tables} =
+          this._graph ? (await this._graph.result()) : {names: [], tables: [new Table({})]};
+        const results: DataFrame[] = [];
+        tables.forEach((table: Table) => {
+          results.push(new DataFrame(
+            names.reduce((cols, name, i) => ({...cols, [name]: table.getColumnByIndex(i)}), {})));
+        });
 
-    return results;
+        return results;
+      })();
+    }
+    return this._result;
   }
 
-  async sendTo(id: number) {
-    return await this.result().then((dfs) => {
+  sendTo(id: number) {
+    return this.then((dfs) => {
       const {_graph}                                  = this;
       const inFlightTables: Record<string, DataFrame> = {};
       if (_graph) {

modules/sql/test/sql-context-tests.ts

@@ -1,3 +1,17 @@
+// Copyright (c) 2021-2022, NVIDIA CORPORATION.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 import {DataFrame, Float64, Series, Utf8String} from '@rapidsai/cudf';
 import {SQLContext} from '@rapidsai/sql';