SERVER-51120 Find queries with MERGE_SORT incorrectly sort the results when the collation is specified

(cherry picked from commit eee7fb8f2c6da144e9d4c3df7887a5ec167f3a6f)

6 files changed, 556 insertions, 227 deletions

diff --git a/jstests/core/explode_for_sort_collation.js b/jstests/core/explode_for_sort_collation.js
deleted file mode 100644
index 2329f6b7987..00000000000
--- a/jstests/core/explode_for_sort_collation.js
+++ /dev/null
@@ -1,217 +0,0 @@
-/**
- * Tests explode for sort query planner behavior with collated queries and indexes. This is a test
- * for SERVER-48993.
- * @tags: [requires_find_command]
- */
-(function() {
-    "use strict";
-
-    const testDB = db.getSiblingDB(jsTestName());
-
-    // Drop the test database.
-    assert.commandWorked(testDB.dropDatabase());
-
-    const coll = testDB.explode_for_sort;
-
-    // Executes a test case that creates an index, inserts documents, issues a find query on a
-    // collection and compares the results with the expected collection.
-    function executeQueryTestCase(testCase) {
-        jsTestLog(tojson(testCase));
-
-        // Create a collection.
-        coll.drop();
-        assert.commandWorked(testDB.createCollection(coll.getName()));
-
-        // Create an index.
-        assert.commandWorked(coll.createIndex(testCase.indexKeyPattern, testCase.indexOptions));
-
-        // Insert some documents into the collection.
-        assert.commandWorked(coll.insert(testCase.inputDocuments));
-
-        // Run a find query with optionally specified collation.
-        let cursor = coll.find(testCase.filter).sort(testCase.sort);
-        if (testCase.findCollation !== undefined) {
-            cursor = cursor.collation(testCase.findCollation);
-        }
-        const actualResults = cursor.toArray();
-
-        // Compare results to expected.
-        assert.eq(actualResults, testCase.expectedResults);
-    }
-
-    const standardInputDocuments =
-        [{_id: 0, a: 0, b: "CC"}, {_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}];
-
-    const testCases = [
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key together with
-          // a
-          // sort on a suffix of the index key returns correct results when the index is a compound
-          // index with a non-simple collation and the query does not have an explicit collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: 0},
-          sort: {b: 1},
-          inputDocuments: standardInputDocuments,
-          expectedResults:
-              [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}, {_id: 2, a: 0, b: "bb"}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key together with
-          // a
-          // sort on a suffix of the index key returns correct results when the index is a compound
-          // index with a non-simple collation  and the query explicitly specifies the simple
-          // collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: 0},
-          sort: {b: 1},
-          findCollation: {locale: "simple"},
-          inputDocuments: standardInputDocuments,
-          expectedResults:
-              [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}, {_id: 2, a: 0, b: "bb"}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key together with
-          // a
-          // sort on a suffix of the index key returns correct results when the index is a compound
-          // index with a simple collation and the query explicitly specifies a non-simple
-          // collation.
-          indexKeyPattern: {a: 1, b: 1},
-          filter: {a: 0},
-          sort: {b: 1},
-          findCollation: {locale: "en_US", strength: 1},
-          inputDocuments: standardInputDocuments,
-          expectedResults:
-              [{_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}, {_id: 0, a: 0, b: "CC"}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key together with
-          // a
-          // sort on a suffix of the index key returns correct results when the index is a compound
-          // index with a simple collation and the query explicitly specifies a non-simple
-          // collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "simple"}},
-          filter: {a: 0},
-          sort: {b: 1},
-          findCollation: {locale: "en_US", strength: 1},
-          inputDocuments: standardInputDocuments,
-          expectedResults:
-              [{_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}, {_id: 0, a: 0, b: "CC"}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key together with
-          // a
-          // sort on a suffix of the index key returns correct results when the index is a compound
-          // index with a non-simple collation that is different from the query's.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 5}},
-          filter: {a: 0},
-          sort: {b: 1},
-          findCollation: {locale: "en_US", strength: 1},
-          inputDocuments: standardInputDocuments,
-          expectedResults:
-              [{_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}, {_id: 0, a: 0, b: "CC"}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key, a collatable
-          // range-query on the suffix, and a sort on the suffix of the index key returns correct
-          // results when the index is a compound index with a non-simple collation and the query
-          // does
-          // not have an explicit collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: 0, b: {$gte: 'A', $lt: 'D'}},
-          sort: {b: 1},
-          inputDocuments: standardInputDocuments,
-          expectedResults: [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key, a collatable
-          // range-query and a sort on a prefix of a suffix of the index key returns correct results
-          // when the index is a compound index with a non-simple collation and the query does not
-          // have an explicit collation.
-          indexKeyPattern: {a: 1, b: 1, c: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: 0, b: {$gte: 'A', $lt: 'D'}},
-          sort: {b: 1},
-          inputDocuments: standardInputDocuments,
-          expectedResults: [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}]
-        },
-        {
-          // Verifies that a non-collatable multi-point query on the prefix of the index key, a
-          // collatable range-query on the suffix, and a sort on the suffix of the index key returns
-          // correct results when the index is a compound index with a non-simple collation and the
-          // query does not have an explicit collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: {$in: [0, 2]}, b: {$gte: 'A', $lt: 'D'}},
-          sort: {b: 1},
-          inputDocuments: [
-              {_id: 0, a: 0, b: "CC"},
-              {_id: 1, a: 0, b: "AA"},
-              {_id: 2, a: 0, b: "bb"},
-              {_id: 3, a: 2, b: "BB"}
-          ],
-          expectedResults:
-              [{_id: 1, a: 0, b: "AA"}, {_id: 3, a: 2, b: "BB"}, {_id: 0, a: 0, b: "CC"}]
-        },
-        {
-          // Verifies that a non-collatable multi-point query on the prefix of the index key, a
-          // non-collatable range-query on the suffix, and a sort on the suffix of the index key
-          // returns correct results when the index is a compound index with a non-simple collation
-          // and the query does not have an explicit collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: {$in: [0, 2]}, b: {$gte: 0, $lt: 10}},
-          sort: {b: 1},
-          inputDocuments: [
-              {_id: 0, a: 0, b: 6},
-              {_id: 1, a: 0, b: 10},
-              {_id: 2, a: 0, b: "bb"},
-              {_id: 3, a: 2, b: 5},
-              {_id: 4, a: 2, b: 4}
-          ],
-          expectedResults: [{_id: 4, a: 2, b: 4}, {_id: 3, a: 2, b: 5}, {_id: 0, a: 0, b: 6}]
-        },
-        {
-          // Verifies that a non-collatable multi-point query on the prefix of the index key, a
-          // non-collatable range-query on the suffix, and a sort on the suffix of the index key
-          // returns correct results when the index is a compound index with a simple collation
-          // and the query explicitly specifies a non-simple collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "simple"}},
-          filter: {a: {$in: [0, 2]}, b: {$gte: 0, $lt: 10}},
-          sort: {b: 1},
-          findCollation: {locale: "en_US", strength: 1},
-          inputDocuments: [
-              {_id: 0, a: 0, b: 6},
-              {_id: 1, a: 0, b: 10},
-              {_id: 2, a: 0, b: "bb"},
-              {_id: 3, a: 2, b: 5},
-              {_id: 4, a: 2, b: 4}
-          ],
-          expectedResults: [{_id: 4, a: 2, b: 4}, {_id: 3, a: 2, b: 5}, {_id: 0, a: 0, b: 6}]
-        },
-        {
-          // Verifies that a non-collatable point-query on the prefix of the index key, a
-          // non-collatable range-query on the suffix, and a sort on the suffix of the index key
-          // returns correct results when the index is a compound index with a non-simple collation
-          // and the query does not have an explicit collation.
-          indexKeyPattern: {a: 1, b: 1},
-          indexOptions: {collation: {locale: "en_US", strength: 1}},
-          filter: {a: 0, b: {$gte: 0, $lt: 10}},
-          sort: {b: 1},
-          inputDocuments: [
-              {_id: 0, a: 0, b: 6},
-              {_id: 1, a: 0, b: 5},
-              {_id: 2, a: 0, b: "bb"},
-              {_id: 3, a: 0, b: 4}
-          ],
-          expectedResults: [{_id: 3, a: 0, b: 4}, {_id: 1, a: 0, b: 5}, {_id: 0, a: 0, b: 6}]
-        }
-    ];
-
-    testCases.forEach(executeQueryTestCase);
-}());
\ No newline at end of file
diff --git a/jstests/core/merge_sort_collation.js b/jstests/core/merge_sort_collation.js
new file mode 100644
index 00000000000..8ebbaf8da70
--- /dev/null
+++ b/jstests/core/merge_sort_collation.js
@@ -0,0 +1,499 @@
+/**
+ * Tests following query related aspects involving collated queries and indexes:
+ *  1) explode for sort query planner behavior related to the selection of MERGE_SORT stage;
+ *  2) MERGE_SORT stage execution.
+ * @tags: [
+ *   requires_find_command,
+ * ]
+ */
+(function() {
+    "use strict";
+
+    const testDB = db.getSiblingDB(jsTestName());
+    const coll = testDB.explode_for_sort;
+
+    // Executes a test case that creates a collection and indexes, inserts documents, issues a find
+    // query on a collection and compares the results with the expected collection.
+    function executeQueryTestCase(testCase) {
+        jsTestLog(tojson(testCase));
+
+        // Drop the test database.
+        assert.commandWorked(testDB.dropDatabase());
+
+        // Create a collection.
+        const collectionOptions = {};
+        if (testCase.collectionCollation !== undefined) {
+            collectionOptions.collation = testCase.collectionCollation;
+        }
+        assert.commandWorked(testDB.createCollection(coll.getName(), collectionOptions));
+
+        // Create index(es).
+        if (testCase.indexes !== undefined) {
+            assert.commandWorked(coll.createIndexes(testCase.indexes, testCase.indexOptions));
+        }
+
+        // Insert some documents into the collection.
+        assert.commandWorked(coll.insert(testCase.inputDocuments));
+
+        // Run a find query with optionally specified collation and projection.
+        let projection = {};
+        if (testCase.projection !== undefined) {
+            projection = testCase.projection;
+        }
+        let cursor = coll.find(testCase.filter, projection).sort(testCase.sort);
+        if (testCase.findCollation !== undefined) {
+            cursor = cursor.collation(testCase.findCollation);
+        }
+        const actualResults = cursor.toArray();
+
+        // Compare results to expected.
+        assert.eq(actualResults, testCase.expectedResults);
+    }
+
+    const standardInputDocuments =
+        [{_id: 0, a: 0, b: "CC"}, {_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}];
+
+    const testCases = [
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key together with
+          // a
+          // sort on a suffix of the index key returns correct results when the index is a compound
+          // index with a non-simple collation and the query does not have an explicit collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: 0},
+          sort: {b: 1},
+          inputDocuments: standardInputDocuments,
+          expectedResults:
+              [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}, {_id: 2, a: 0, b: "bb"}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key together with
+          // a
+          // sort on a suffix of the index key returns correct results when the index is a compound
+          // index with a non-simple collation  and the query explicitly specifies the simple
+          // collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: 0},
+          sort: {b: 1},
+          findCollation: {locale: "simple"},
+          inputDocuments: standardInputDocuments,
+          expectedResults:
+              [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}, {_id: 2, a: 0, b: "bb"}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key together with
+          // a
+          // sort on a suffix of the index key returns correct results when the index is a compound
+          // index with a simple collation and the query explicitly specifies a non-simple
+          // collation.
+          indexes: [{a: 1, b: 1}],
+          filter: {a: 0},
+          sort: {b: 1},
+          findCollation: {locale: "en_US", strength: 1},
+          inputDocuments: standardInputDocuments,
+          expectedResults:
+              [{_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}, {_id: 0, a: 0, b: "CC"}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key together with
+          // a
+          // sort on a suffix of the index key returns correct results when the index is a compound
+          // index with a simple collation and the query explicitly specifies a non-simple
+          // collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "simple"}},
+          filter: {a: 0},
+          sort: {b: 1},
+          findCollation: {locale: "en_US", strength: 1},
+          inputDocuments: standardInputDocuments,
+          expectedResults:
+              [{_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}, {_id: 0, a: 0, b: "CC"}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key together with
+          // a
+          // sort on a suffix of the index key returns correct results when the index is a compound
+          // index with a non-simple collation that is different from the query's.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 5}},
+          filter: {a: 0},
+          sort: {b: 1},
+          findCollation: {locale: "en_US", strength: 1},
+          inputDocuments: standardInputDocuments,
+          expectedResults:
+              [{_id: 1, a: 0, b: "AA"}, {_id: 2, a: 0, b: "bb"}, {_id: 0, a: 0, b: "CC"}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key, a collatable
+          // range-query on the suffix, and a sort on the suffix of the index key returns correct
+          // results when the index is a compound index with a non-simple collation and the query
+          // does
+          // not have an explicit collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: 0, b: {$gte: 'A', $lt: 'D'}},
+          sort: {b: 1},
+          inputDocuments: standardInputDocuments,
+          expectedResults: [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key, a collatable
+          // range-query and a sort on a prefix of a suffix of the index key returns correct results
+          // when the index is a compound index with a non-simple collation and the query does not
+          // have an explicit collation.
+          indexes: [{a: 1, b: 1, c: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: 0, b: {$gte: 'A', $lt: 'D'}},
+          sort: {b: 1},
+          inputDocuments: standardInputDocuments,
+          expectedResults: [{_id: 1, a: 0, b: "AA"}, {_id: 0, a: 0, b: "CC"}]
+        },
+        {
+          // Verifies that a non-collatable multi-point query on the prefix of the index key, a
+          // collatable range-query on the suffix, and a sort on the suffix of the index key returns
+          // correct results when the index is a compound index with a non-simple collation and the
+          // query does not have an explicit collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: {$in: [0, 2]}, b: {$gte: 'A', $lt: 'D'}},
+          sort: {b: 1},
+          inputDocuments: [
+              {_id: 0, a: 0, b: "CC"},
+              {_id: 1, a: 0, b: "AA"},
+              {_id: 2, a: 0, b: "bb"},
+              {_id: 3, a: 2, b: "BB"}
+          ],
+          expectedResults:
+              [{_id: 1, a: 0, b: "AA"}, {_id: 3, a: 2, b: "BB"}, {_id: 0, a: 0, b: "CC"}]
+        },
+        {
+          // Verifies that a non-collatable multi-point query on the prefix of the index key, a
+          // non-collatable range-query on the suffix, and a sort on the suffix of the index key
+          // returns correct results when the index is a compound index with a non-simple collation
+          // and the query does not have an explicit collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: {$in: [0, 2]}, b: {$gte: 0, $lt: 10}},
+          sort: {b: 1},
+          inputDocuments: [
+              {_id: 0, a: 0, b: 6},
+              {_id: 1, a: 0, b: 10},
+              {_id: 2, a: 0, b: "bb"},
+              {_id: 3, a: 2, b: 5},
+              {_id: 4, a: 2, b: 4}
+          ],
+          expectedResults: [{_id: 4, a: 2, b: 4}, {_id: 3, a: 2, b: 5}, {_id: 0, a: 0, b: 6}]
+        },
+        {
+          // Verifies that a non-collatable multi-point query on the prefix of the index key, a
+          // non-collatable range-query on the suffix, and a sort on the suffix of the index key
+          // returns correct results when the index is a compound index with a simple collation
+          // and the query explicitly specifies a non-simple collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "simple"}},
+          filter: {a: {$in: [0, 2]}, b: {$gte: 0, $lt: 10}},
+          sort: {b: 1},
+          findCollation: {locale: "en_US", strength: 1},
+          inputDocuments: [
+              {_id: 0, a: 0, b: 6},
+              {_id: 1, a: 0, b: 10},
+              {_id: 2, a: 0, b: "bb"},
+              {_id: 3, a: 2, b: 5},
+              {_id: 4, a: 2, b: 4}
+          ],
+          expectedResults: [{_id: 4, a: 2, b: 4}, {_id: 3, a: 2, b: 5}, {_id: 0, a: 0, b: 6}]
+        },
+        {
+          // Verifies that a non-collatable point-query on the prefix of the index key, a
+          // non-collatable range-query on the suffix, and a sort on the suffix of the index key
+          // returns correct results when the index is a compound index with a non-simple collation
+          // and the query does not have an explicit collation.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en_US", strength: 1}},
+          filter: {a: 0, b: {$gte: 0, $lt: 10}},
+          sort: {b: 1},
+          inputDocuments: [
+              {_id: 0, a: 0, b: 6},
+              {_id: 1, a: 0, b: 5},
+              {_id: 2, a: 0, b: "bb"},
+              {_id: 3, a: 0, b: 4}
+          ],
+          expectedResults: [{_id: 3, a: 0, b: 4}, {_id: 1, a: 0, b: 5}, {_id: 0, a: 0, b: 6}]
+        },
+        {
+          // Verifies that a collatable multi-point query on the prefix of the index key, and a sort
+          // on the suffix of the index key returns correct results when the index is a compound
+          // index
+          // with a non-simple collation and the query has a collation specified matching collation
+          // of
+          // the index.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {a: {$in: ["1", "2"]}},
+          sort: {b: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: "1", b: "a"},
+              {_id: 1, a: "1", b: "c"},
+              {_id: 2, a: "2", b: "b"},
+              {_id: 3, a: "2", b: "d"}
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", b: "a"},
+              {_id: 2, a: "2", b: "b"},
+              {_id: 1, a: "1", b: "c"},
+              {_id: 3, a: "2", b: "d"}
+          ]
+        },
+        {
+          // Verifies that a collatable multi-point query on the prefix of the index key, and a sort
+          // on a prefix of a suffix of the index key returns correct results when the index is a
+          // compound index with a non-simple collation and the query has a collation specified
+          // matching collation of the index.
+          indexes: [{a: 1, b: 1, c: 1, d: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {a: {$in: ["1", "2"]}, b: "1"},
+          sort: {c: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 1, a: "1", b: "1", c: "c"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 3, a: "2", b: "1", c: "u", d: "a"}
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 1, a: "1", b: "1", c: "c"},
+              {_id: 3, a: "2", b: "1", c: "u", d: "a"}
+          ]
+        },
+        {
+          // Verifies that a collatable $or query on the prefixes of the keys of 3 indexes, and a
+          // sort
+          // on a suffix of the keys of indexes returns correct results when the indexes are
+          // compound
+          // indexes with a non-simple collation and the query has a collation specified matching
+          // collation of the indexes. Also, the second/third operands to $or queries on fields
+          // 'e'/'g' that are not covered by the indexes therefore triggers addition of a FETCH
+          // stage
+          // between MERGE_SORT and IXSCAN. This tests comparison of index versus fetched document
+          // provided sort keys. In addition to that, some documents have objects as sort attribute
+          // values.
+          indexes: [{a: 1, b: 1, c: 1}, {d: 1, c: 1}, {f: 1, c: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {$or: [{a: {$in: ["1", "2"]}, b: "1"}, {d: "3", e: "3"}, {f: "4", g: "3"}]},
+          sort: {c: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 1, a: "1", b: "1", c: "d"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 3, a: "2", b: "1", c: "e"},
+              {_id: 6, a: "2", b: "1", c: {a: "B"}},
+              {_id: 4, d: "3", e: "3", c: "c"},
+              {_id: 5, d: "3", e: "3", c: "f"},
+              {_id: 7, d: "3", e: "3", c: {a: "A"}},
+              {_id: 8, d: "3", e: "3", c: {a: "C"}},
+              {_id: 9, f: "4", g: "3", c: "g"},
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 4, d: "3", e: "3", c: "c"},
+              {_id: 1, a: "1", b: "1", c: "d"},
+              {_id: 3, a: "2", b: "1", c: "e"},
+              {_id: 5, d: "3", e: "3", c: "f"},
+              {_id: 9, f: "4", g: "3", c: "g"},
+              {_id: 7, d: "3", e: "3", c: {a: "A"}},
+              {_id: 6, a: "2", b: "1", c: {a: "B"}},
+              {_id: 8, d: "3", e: "3", c: {a: "C"}},
+          ]
+        },
+        {
+          // Verifies that a multi-point query on the prefix of the index key, and a sort on the
+          // suffix of the index key returns correct results when the collection has a non-simple
+          // collation specified and the index is a compound index.
+          collectionCollation: {locale: "en", strength: 2},
+          indexes: [{a: 1, b: 1, c: 1, d: 1}],
+          filter: {a: {$in: ["1", "2"]}, b: "1"},
+          sort: {c: 1},
+          inputDocuments: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 1, a: "1", b: "1", c: "c"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 3, a: "2", b: "1", c: "d"}
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 1, a: "1", b: "1", c: "c"},
+              {_id: 3, a: "2", b: "1", c: "d"}
+          ]
+        },
+        {
+          // Verifies that a multi-point query on the prefix of the index key, and a sort on the
+          // suffix of the index key returns correct results when the collection has no collation
+          // specified and the index is a compound index.
+          indexes: [{a: 1, b: 1, c: 1, d: 1}],
+          filter: {a: {$in: ["1", "2"]}, b: "1"},
+          sort: {c: 1},
+          inputDocuments: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 1, a: "1", b: "1", c: "c"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 3, a: "2", b: "1", c: "d"}
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", b: "1", c: "a"},
+              {_id: 2, a: "2", b: "1", c: "b"},
+              {_id: 1, a: "1", b: "1", c: "c"},
+              {_id: 3, a: "2", b: "1", c: "d"}
+          ]
+        },
+        {
+          // Verifies that an $or query on the prefixes of index keys, and a sort on the suffix of
+          // the
+          // index keys returns correct results when the collection and the query have the same
+          // non-simple collation specified.
+          indexes: [{a: 1, c: 1}, {b: 1, c: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {$or: [{a: "1"}, {b: "2"}]},
+          sort: {c: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: "1", c: "a"},
+              {_id: 1, a: "1", c: "c"},
+              {_id: 2, b: "2", c: "b"},
+              {_id: 3, b: "2", c: "d"}
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", c: "a"},
+              {_id: 2, b: "2", c: "b"},
+              {_id: 1, a: "1", c: "c"},
+              {_id: 3, b: "2", c: "d"}
+          ]
+        },
+        {
+          // Verifies that an $or query on the prefixes of index keys, and a sort on the suffix of
+          // the
+          // index keys returns correct results when the collection and the query have the same
+          // non-simple collation specified and one $or operand requires a FETCH.
+          indexes: [{a: 1, c: 1}, {b: 1, c: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {$or: [{a: "1"}, {b: "2", d: "3"}]},
+          sort: {c: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: "1", c: "a"},
+              {_id: 1, a: "1", c: "c"},
+              {_id: 2, b: "2", c: "b", d: "3"},
+              {_id: 3, b: "2", c: "d", d: "3"}
+          ],
+          expectedResults: [
+              {_id: 0, a: "1", c: "a"},
+              {_id: 2, b: "2", c: "b", d: "3"},
+              {_id: 1, a: "1", c: "c"},
+              {_id: 3, b: "2", c: "d", d: "3"}
+          ]
+        },
+        {
+          // Verifies that a non-collatable multi-point query on the prefix of the index key, and a
+          // collatable sort on the suffix of the index key returns correct results when the index
+          // is
+          // a compound index with a non-simple collation and the query has a collation specified
+          // matching collation of the index.
+          indexes: [{a: 1, b: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {a: {$in: [1, 2]}},
+          sort: {b: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: 1, b: "a"},
+              {_id: 1, a: 1, b: "c"},
+              {_id: 2, a: 2, b: "b"},
+              {_id: 3, a: 2, b: "d"}
+          ],
+          expectedResults: [
+              {_id: 0, a: 1, b: "a"},
+              {_id: 2, a: 2, b: "b"},
+              {_id: 1, a: 1, b: "c"},
+              {_id: 3, a: 2, b: "d"}
+          ]
+        },
+        {
+          // Verifies that a non-collatable $or query on the prefixes of the index keys, and a sort
+          // on
+          // suffixes of the index keys returns correct results when the index is a compound index
+          // with a non-simple collation that is different from the collation of the query.
+          indexes: [{a: 1, c: 1}, {b: 1, c: 1}],
+          indexOptions: {collation: {locale: "fr"}},
+          filter: {$or: [{a: 1, c: 1}, {b: 2, c: 2}, {b: 2, c: 3}, {b: 2, c: 4}]},
+          sort: {c: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 4, b: 2, c: 4},
+              {_id: 2, b: 2, c: 2},
+              {_id: 0, a: 1, c: 1},
+              {_id: 3, b: 2, c: 3},
+          ],
+          expectedResults: [
+              {_id: 0, a: 1, c: 1},
+              {_id: 2, b: 2, c: 2},
+              {_id: 3, b: 2, c: 3},
+              {_id: 4, b: 2, c: 4},
+          ]
+        },
+        {
+          // Verifies that a fully-index-covered non-collatable multi-point query on a prefix of an
+          // index key, and a sort on a suffix of an index key returns correct results when the
+          // index
+          // is a compound index.
+          indexes: [{a: 1, c: 1}],
+          filter: {a: {$in: [1, 2]}},
+          projection: {_id: 0, a: 1, c: 1},
+          sort: {c: 1},
+          inputDocuments: [
+              {_id: 0, a: 1, c: 5},
+              {_id: 1, a: 1, c: 3},
+              {_id: 2, a: 2, c: 1},
+              {_id: 3, a: 2, c: 4},
+          ],
+          expectedResults: [
+              {a: 2, c: 1},
+              {a: 1, c: 3},
+              {a: 2, c: 4},
+              {a: 1, c: 5},
+          ]
+        },
+        {
+          // Verifies that a non-collatable multi-point query on a prefix of an index key, and a
+          // collatable sort on a suffix of an index key returns correct results when the index is a
+          // compound index with a collation specified that matches a collation of the query. The
+          // query would be eligible to be covered by the index due to a projection, but requires a
+          // FETCH because index bounds include strings that are encoded as collated keys.
+          indexes: [{a: 1, c: 1}],
+          indexOptions: {collation: {locale: "en", strength: 2}},
+          filter: {a: {$in: [1, 2]}},
+          projection: {_id: 0, a: 1, c: 1},
+          sort: {c: 1},
+          findCollation: {locale: "en", strength: 2},
+          inputDocuments: [
+              {_id: 0, a: 1, c: "a"},
+              {_id: 1, a: 1, c: "c"},
+              {_id: 2, a: 2, c: "b"},
+              {_id: 3, a: 2, c: "d"},
+          ],
+          expectedResults: [
+              {a: 1, c: "a"},
+              {a: 2, c: "b"},
+              {a: 1, c: "c"},
+              {a: 2, c: "d"},
+          ]
+        },
+    ];
+
+    testCases.forEach(executeQueryTestCase);
+}());
\ No newline at end of file
diff --git a/jstests/libs/parallelTester.js b/jstests/libs/parallelTester.js
index 87e8fa65a28..870e97193a0 100644
--- a/jstests/libs/parallelTester.js
+++ b/jstests/libs/parallelTester.js
@@ -237,7 +237,7 @@ if (typeof _threadInject != "undefined") {
         // The following tests cannot run when shell readMode is legacy.
         if (db.getMongo().readMode() === "legacy") {
             var requires_find_command = [
-                "explode_for_sort_collation.js",
+                "merge_sort_collation.js",
                 "views/views_aggregation.js",
                 "views/views_change.js",
                 "views/views_drop.js",
diff --git a/src/mongo/db/exec/merge_sort.cpp b/src/mongo/db/exec/merge_sort.cpp
index 7a840617566..a7aceb5aca4 100644
--- a/src/mongo/db/exec/merge_sort.cpp
+++ b/src/mongo/db/exec/merge_sort.cpp
@@ -33,6 +33,7 @@
 #include "mongo/db/exec/scoped_timer.h"
 #include "mongo/db/exec/working_set.h"
 #include "mongo/db/exec/working_set_common.h"
+#include "mongo/db/query/collation/collation_index_key.h"
 #include "mongo/db/query/collation/collator_interface.h"
 #include "mongo/stdx/memory.h"
 #include "mongo/util/mongoutils/str.h"
@@ -206,11 +207,44 @@ bool MergeSortStage::StageWithValueComparison::operator()(const MergingRef& lhs,
         BSONElement lhsElt;
         verify(lhsMember->getFieldDotted(fn, &lhsElt));
 
+        // Determine if the left-hand side sort key part comes from an index key.
+        auto lhsIsFromIndexKey = !lhsMember->hasObj();
+
         BSONElement rhsElt;
         verify(rhsMember->getFieldDotted(fn, &rhsElt));
 
+        // Determine if the right-hand side sort key part comes from an index key.
+        auto rhsIsFromIndexKey = !rhsMember->hasObj();
+
+        // A collator to use for comparing the sort keys. We need a collator when values of both
+        // operands are supplied from a document and the query is collated. Otherwise bit-wise
+        // comparison should be used.
+        const CollatorInterface* collatorToUse = nullptr;
+        BSONObj collationEncodedKeyPart;  // A backing storage for a collation-encoded key part
+                                          // (according to collator '_collator') of one of the
+                                          // operands - either 'lhsElt' or 'rhsElt'.
+
+        if (nullptr == _collator || (lhsIsFromIndexKey && rhsIsFromIndexKey)) {
+            // Either the query has no collation or both sort key parts come directly from index
+            // keys. If the query has no collation, then the query planner should have guaranteed
+            // that we don't need to perform any collation-aware comparisons here. If both sort key
+            // parts come from index keys, we may need to respect a collation but the index keys are
+            // already collation-encoded, therefore we don't need to perform a collation-aware
+            // comparison here.
+        } else if (!lhsIsFromIndexKey && !rhsIsFromIndexKey) {
+            // Both sort key parts were extracted from fetched documents. These parts are not
+            // collation-encoded, so we will need to perform a collation-aware comparison.
+            collatorToUse = _collator;
+        } else {
+            // One of the sort key parts was extracted from fetched documents. Encode that part
+            // using the query's collation.
+            auto& keyPartFetchedFromDocument = rhsIsFromIndexKey ? lhsElt : rhsElt;
+            collationEncodedKeyPart = encodeKeyPartWithCollation(keyPartFetchedFromDocument);
+            keyPartFetchedFromDocument = collationEncodedKeyPart.firstElement();
+        }
+
         // false means don't compare field name.
-        int x = lhsElt.woCompare(rhsElt, false, _collator);
+        int x = lhsElt.woCompare(rhsElt, false, collatorToUse);
         if (-1 == patternElt.number()) {
             x = -x;
         }
@@ -225,6 +259,13 @@ bool MergeSortStage::StageWithValueComparison::operator()(const MergingRef& lhs,
     return false;
 }
 
+BSONObj MergeSortStage::StageWithValueComparison::encodeKeyPartWithCollation(
+    const BSONElement& keyPart) {
+    BSONObjBuilder objectBuilder;
+    CollationIndexKey::collationAwareIndexKeyAppend(keyPart, _collator, &objectBuilder);
+    return objectBuilder.obj();
+}
+
 unique_ptr<PlanStageStats> MergeSortStage::getStats() {
     _commonStats.isEOF = isEOF();
 
diff --git a/src/mongo/db/exec/merge_sort.h b/src/mongo/db/exec/merge_sort.h
index e4d2e9af190..1d1fe200fb0 100644
--- a/src/mongo/db/exec/merge_sort.h
+++ b/src/mongo/db/exec/merge_sort.h
@@ -137,6 +137,9 @@ private:
         bool operator()(const MergingRef& lhs, const MergingRef& rhs);
 
     private:
+        // Encodes sort key part 'keyPart' according to the collation of the query.
+        BSONObj encodeKeyPartWithCollation(const BSONElement& keyPart);
+
         WorkingSet* _ws;
         BSONObj _pattern;
         const CollatorInterface* _collator;
diff --git a/src/mongo/dbtests/query_stage_merge_sort.cpp b/src/mongo/dbtests/query_stage_merge_sort.cpp
index 43160272861..e0ce986fdb2 100644
--- a/src/mongo/dbtests/query_stage_merge_sort.cpp
+++ b/src/mongo/dbtests/query_stage_merge_sort.cpp
@@ -851,7 +851,7 @@ public:
         msparams.pattern = BSON("c" << 1 << "d" << 1);
         CollatorInterfaceMock collator(CollatorInterfaceMock::MockType::kReverseString);
         msparams.collator = &collator;
-        MergeSortStage* ms = new MergeSortStage(&_opCtx, msparams, ws.get(), coll);
+        auto ms = std::make_unique<MergeSortStage>(&_opCtx, msparams, ws.get(), coll);
 
         // a:1
         IndexScanParams params;
@@ -861,17 +861,20 @@ public:
         params.bounds.endKey = objWithMaxKey(1);
         params.bounds.boundInclusion = BoundInclusion::kIncludeBothStartAndEndKeys;
         params.direction = 1;
-        ms->addChild(new IndexScan(&_opCtx, params, ws.get(), NULL));
+        auto idxScan = std::make_unique<IndexScan>(&_opCtx, params, ws.get(), nullptr);
+
+        // Wrap 'idxScan' with a FETCH stage so a document is fetched and MERGE_SORT is forced to
+        // use the provided collator 'collator'. Also, this permits easier retrieval of result
+        // objects in the result verification code.
+        ms->addChild(new FetchStage(&_opCtx, ws.get(), idxScan.release(), nullptr, coll));
 
         // b:1
         params.descriptor = getIndex(secondIndex, coll);
-        ms->addChild(new IndexScan(&_opCtx, params, ws.get(), NULL));
+        idxScan = std::make_unique<IndexScan>(&_opCtx, params, ws.get(), nullptr);
+        ms->addChild(new FetchStage(&_opCtx, ws.get(), idxScan.release(), nullptr, coll));
 
-        unique_ptr<FetchStage> fetchStage =
-            make_unique<FetchStage>(&_opCtx, ws.get(), ms, nullptr, coll);
-        // Must fetch if we want to easily pull out an obj.
-        auto statusWithPlanExecutor = PlanExecutor::make(
-            &_opCtx, std::move(ws), std::move(fetchStage), coll, PlanExecutor::NO_YIELD);
+        auto statusWithPlanExecutor =
+            PlanExecutor::make(&_opCtx, std::move(ws), std::move(ms), coll, PlanExecutor::NO_YIELD);
         ASSERT_OK(statusWithPlanExecutor.getStatus());
         auto exec = std::move(statusWithPlanExecutor.getValue());