Choose index based on fields match

src/mango/src/mango_cursor_view.erl

@@ -110,7 +110,11 @@ composite_indexes(Indexes, FieldRanges) ->
     lists:foldl(fun(Idx, Acc) ->
         Cols = mango_idx:columns(Idx),
         Prefix = composite_prefix(Cols, FieldRanges),
-        [{Idx, Prefix} | Acc]
+        % Calcuate the difference between the FieldRanges/Selector
+        % and the Prefix. We want to select the index with a prefix
+        % that is as close to the FieldRanges as possible
+        PrefixDifference = length(FieldRanges) - length(Prefix),
+        [{Idx, Prefix, PrefixDifference} | Acc]
     end, [], Indexes).
 
 
@@ -125,29 +129,40 @@ composite_prefix([Col | Rest], Ranges) ->
     end.
 
 
-% Low and behold our query planner. Or something.
-% So stupid, but we can fix this up later. First
-% pass: Sort the IndexRanges by (num_columns, idx_name)
-% and return the first element. Yes. Its going to
-% be that dumb for now.
+% The query planner
+% First choose the index with the lowest difference between its 
+% Prefix and the FieldRanges. If that is equal, then
+% choose the index with the least number of 
+% fields in the index. If we still cannot break the tie, 
+% then choose alphabetically based on ddocId.
+% Return the first element's Index and IndexRanges.
 %
 % In the future we can look into doing a cached parallel
 % reduce view read on each index with the ranges to find
 % the one that has the fewest number of rows or something.
 choose_best_index(_DbName, IndexRanges) ->
-    Cmp = fun({A1, A2}, {B1, B2}) ->
-        case length(A2) - length(B2) of
+    Cmp = fun({IdxA, _PrefixA, PrefixDifferenceA}, {IdxB, _PrefixB, PrefixDifferenceB}) ->
+        case PrefixDifferenceA - PrefixDifferenceB of
             N when N < 0 -> true;
             N when N == 0 ->
-                % This is a really bad sort and will end
-                % up preferring indices based on the
-                % (dbname, ddocid, view_name) triple
-                A1 =< B1;
+                ColsLenA = length(mango_idx:columns(IdxA)),
+                ColsLenB = length(mango_idx:columns(IdxB)),
+                case ColsLenA - ColsLenB of
+                    M when M < 0 -> 
+                        true;
+                    M when M == 0 ->
+                        % We have no other way to choose, so at this point 
+                        % select the index based on (dbname, ddocid, view_name) triple
+                        IdxA =< IdxB;
+                    _ -> 
+                        false
+                end;
             _ ->
                 false
         end
     end,
-    hd(lists:sort(Cmp, IndexRanges)).
+    {SelectedIndex, SelectedIndexRanges, _} = hd(lists:sort(Cmp, IndexRanges)),
+    {SelectedIndex, SelectedIndexRanges}.
 
 
 handle_message({meta, _}, Cursor) ->

src/mango/test/12-use-correct-index.py

@@ -0,0 +1,100 @@
+# 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 mango
+import copy
+
+DOCS = [
+    {
+        "_id": "_design/my-design-doc",
+    },
+    {
+        "_id": "54af50626de419f5109c962f",
+        "user_id": 0,
+        "age": 10,
+        "name": "Jimi",
+        "location": "UK",
+        "number": 4
+    },
+    {
+        "_id": "54af50622071121b25402dc3",
+        "user_id": 1,
+        "age": 12,
+        "name": "Eddie",
+        "location": "ZAR",
+        "number": 2
+    },
+    {
+        "_id": "54af50622071121b25402dc6",
+        "user_id": 1,
+        "age": 6,
+        "name": "Harry",
+        "location": "US",
+        "number":8
+    },
+    {
+        "_id": "54af50622071121b25402dc9",
+        "name": "Eddie",
+        "occupation": "engineer",
+        "number":7
+    },
+]
+
+class ChooseCorrectIndexForDocs(mango.DbPerClass):
+    def setUp(self):
+        self.db.recreate()
+        self.db.save_docs(copy.deepcopy(DOCS))
+
+    def test_choose_index_with_one_field_in_index(self):
+        self.db.create_index(["name", "age", "user_id"], ddoc="aaa")
+        self.db.create_index(["name"], ddoc="zzz")
+        explain = self.db.find({"name": "Eddie"}, explain=True)
+        assert explain["index"]["ddoc"] == '_design/zzz'
+
+    def test_choose_index_with_two(self):
+        self.db.create_index(["name", "age", "user_id"], ddoc="aaa")
+        self.db.create_index(["name", "age"], ddoc="bbb")
+        self.db.create_index(["name"], ddoc="zzz")
+        explain = self.db.find({"name": "Eddie", "age":{"$gte": 12}}, explain=True)
+        assert explain["index"]["ddoc"] == '_design/bbb'
+
+    def test_choose_index_alphabetically(self):
+        self.db.create_index(["name", "age", "user_id"], ddoc="aaa")
+        self.db.create_index(["name", "age", "location"], ddoc="bbb")
+        self.db.create_index(["name"], ddoc="zzz")
+        explain = self.db.find({"name": "Eddie", "age": {"$gte": 12}}, explain=True)
+        assert explain["index"]["ddoc"] == '_design/aaa'
+
+    def test_choose_index_most_accurate(self):
+        self.db.create_index(["name", "location", "user_id"], ddoc="aaa")
+        self.db.create_index(["name", "age", "user_id"], ddoc="bbb")
+        self.db.create_index(["name"], ddoc="zzz")
+        explain = self.db.find({"name": "Eddie", "age": {"$gte": 12}}, explain=True)
+        assert explain["index"]["ddoc"] == '_design/bbb'
+
+    def test_choose_index_most_accurate_in_memory_selector(self):
+        self.db.create_index(["name", "location", "user_id"], ddoc="aaa")
+        self.db.create_index(["name", "age", "user_id"], ddoc="bbb")
+        self.db.create_index(["name"], ddoc="zzz")
+        explain = self.db.find({"name": "Eddie", "number": {"$lte": 12}}, explain=True)
+        assert explain["index"]["ddoc"] == '_design/zzz'
+
+    def test_chooses_idxA(self):
+        DOCS2 = [
+            {"a":1, "b":1, "c":1},
+            {"a":1000, "d" : 1000, "e": 1000}
+        ]
+        self.db.save_docs(copy.deepcopy(DOCS2))
+        self.db.create_index(["a", "b", "c"])
+        self.db.create_index(["a", "d", "e"])
+        explain = self.db.find({"a": {"$gt": 0}, "b": {"$gt": 0}, "c": {"$gt": 0}}, explain=True)
+        assert explain["index"]["def"]["fields"] == [{'a': 'asc'}, {'b': 'asc'}, {'c': 'asc'}]