Reapply #2639 (with fix for #2680)

.changeset/wild-buses-poke.md

@@ -0,0 +1,12 @@
+---
+"@apollo/query-planner": patch
+"@apollo/federation-internals": patch
+"@apollo/gateway": patch
+---
+
+Reapply #2639:
+
+Try reusing named fragments in subgraph fetches even if those fragment only apply partially to the subgraph. Before this change, only named fragments that were applying entirely to a subgraph were tried, leading to less reuse that expected. Concretely, this change can sometimes allow the generation of smaller subgraph fetches.
+
+Additionally, resolve a bug which surfaced in the fragment optimization logic which could result in invalid/incorrect optimizations / fragment reuse.
+

gateway-js/src/__tests__/buildQueryPlan.test.ts

@@ -744,18 +744,21 @@ describe('buildQueryPlan', () => {
 
     it(`should not get confused by a fragment spread multiple times`, () => {
       const operationString = `#graphql
-        fragment Price on Product {
+        fragment PriceAndCountry on Product {
           price
+          details {
+            country
+          }
         }
 
         query {
           topProducts {
             __typename
             ... on Book {
-              ...Price
+              ...PriceAndCountry
             }
             ... on Furniture {
-              ...Price
+              ...PriceAndCountry
             }
           }
         }
@@ -770,16 +773,20 @@ describe('buildQueryPlan', () => {
                       topProducts {
                         __typename
                         ... on Book {
-                          ...Price
+                          ...PriceAndCountry
                         }
                         ... on Furniture {
-                          ...Price
+                          ...PriceAndCountry
                         }
                       }
                     }
 
-                    fragment Price on Product {
+                    fragment PriceAndCountry on Product {
                       price
+                      details {
+                        __typename
+                        country
+                      }
                     }
                   },
                 }