SERVER-13618 explode for sort tries reversing index scan direction

(cherry picked from commit ce52f31)

Author: dstorch

src/mongo/db/query/planner_analysis.cpp

@@ -300,7 +300,17 @@ namespace mongo {
             // See if it's the order we're looking for.
             BSONObj possibleSort = resultingSortBob.obj();
             if (!desiredSort.isPrefixOf(possibleSort)) {
-                return false;
+                // We can't get the sort order from the index scan. See if we can
+                // get the sort by reversing the scan.
+                BSONObj reversePossibleSort = QueryPlannerCommon::reverseSortObj(possibleSort);
+                if (!desiredSort.isPrefixOf(reversePossibleSort)) {
+                    // Can't get the sort order from the reversed index scan either. Give up.
+                    return false;
+                }
+                else {
+                    // We can get the sort order we need if we reverse the scan.
+                    QueryPlannerCommon::reverseScans(isn);
+                }
             }
 
             // Do some bookkeeping to see how many ixscans we'll create total.

src/mongo/db/query/query_planner_test.cpp

@@ -1650,6 +1650,37 @@ namespace {
                              "{fetch: {node: {ixscan: {pattern: {a: 1, b: 1, c:1, d:1}}}}}}}");
     }
 
+    // SERVER-13618: test that exploding scans for sort works even
+    // if we must reverse the scan direction.
+    TEST_F(QueryPlannerTest, ExplodeMustReverseScans) {
+        addIndex(BSON("a" << 1 << "b" << 1 << "c" << 1 << "d" << 1));
+        runQuerySortProj(fromjson("{a: {$in: [1, 2]}, b: {$in: [3, 4]}}"),
+                         BSON("c" << -1), BSONObj());
+
+        assertNumSolutions(2U);
+        assertSolutionExists("{sort: {pattern: {c: -1}, limit: 0, node: {cscan: {dir: 1}}}}");
+        assertSolutionExists("{fetch: {node: {mergeSort: {nodes: "
+                                "[{ixscan: {pattern: {a:1, b:1, c:1, d:1}}},"
+                                 "{ixscan: {pattern: {a:1, b:1, c:1, d:1}}},"
+                                 "{ixscan: {pattern: {a:1, b:1, c:1, d:1}}},"
+                                 "{ixscan: {pattern: {a:1, b:1, c:1, d:1}}}]}}}}");
+    }
+
+    // SERVER-13618
+    TEST_F(QueryPlannerTest, ExplodeMustReverseScans2) {
+        addIndex(BSON("a" << 1 << "b" << 1 << "c" << -1));
+        runQuerySortProj(fromjson("{a: {$in: [1, 2]}, b: {$in: [3, 4]}}"),
+                         BSON("c" << 1), BSONObj());
+
+        assertNumSolutions(2U);
+        assertSolutionExists("{sort: {pattern: {c: 1}, limit: 0, node: {cscan: {dir: 1}}}}");
+        assertSolutionExists("{fetch: {node: {mergeSort: {nodes: "
+                                "[{ixscan: {pattern: {a:1, b:1, c:-1}}},"
+                                 "{ixscan: {pattern: {a:1, b:1, c:-1}}},"
+                                 "{ixscan: {pattern: {a:1, b:1, c:-1}}},"
+                                 "{ixscan: {pattern: {a:1, b:1, c:-1}}}]}}}}");
+    }
+
     TEST_F(QueryPlannerTest, InWithSortAndLimitTrailingField) {
         addIndex(BSON("a" << 1 << "b" << -1 << "c" << 1));
         runQuerySortProjSkipLimit(fromjson("{a: {$in: [1, 2]}, b: {$gte: 0}}"),