Type conditioned fetching (#2949)

Type conditioned fetching

Fixes #2938
When querying a field that is in a path of 2 or more unions, the query
planner was not able to handle different selections and would
aggressively collapse selections in fetches yielding an incorrect plan.

This change introduces new syntax to express type conditions in (key and
flatten) paths. Type conditioned fetching can be enabled through a flag,
and execution is supported in the router only.

.changeset/olive-papayas-act.md

@@ -0,0 +1,9 @@
+---
+"@apollo/query-planner": patch
+---
+
+Type conditioned fetching
+
+When querying a field that is in a path of 2 or more unions, the query planner was not able to handle different selections and would aggressively collapse selections in fetches yielding an incorrect plan.
+
+This change introduces new syntax to express type conditions in (key and flatten) paths. Type conditioned fetching can be enabled through a flag, and execution is supported in the router only. (#2938)

gateway-js/src/index.ts

@@ -57,6 +57,7 @@ import {
   LocalCompose,
 } from './supergraphManagers';
 import {
+  assert,
   operationFromDocument,
   Schema,
   ServiceDefinition,
@@ -186,7 +187,7 @@ export class ApolloGateway implements GatewayInterface {
       this.pollIntervalInMs = this.config?.pollIntervalInMs;
     }
 
-    this.issueConfigurationWarningsIfApplicable();
+    this.validateConfigAndEmitWarnings();
 
     this.logger.debug('Gateway successfully initialized (but not yet loaded)');
     this.state = { phase: 'initialized' };
@@ -213,7 +214,9 @@ export class ApolloGateway implements GatewayInterface {
     });
   }
 
-  private issueConfigurationWarningsIfApplicable() {
+  private validateConfigAndEmitWarnings() {
+    assert(!this.config.queryPlannerConfig?.typeConditionedFetching, "Type conditions are not supported in the gateway");
+
     // Warn against using the pollInterval and a serviceList simultaneously
     // TODO(trevor:removeServiceList)
     if (this.pollIntervalInMs && isServiceListConfig(this.config)) {
@@ -568,6 +571,7 @@ export class ApolloGateway implements GatewayInterface {
     this.queryPlanStore.clear();
     this.apiSchema = supergraph.apiSchema();
     this.schema = addExtensions(this.apiSchema.toGraphQLJSSchema());
+
     this.queryPlanner = new QueryPlanner(supergraph, this.config.queryPlannerConfig);
 
     // Notify onSchemaChange listeners of the updated schema