Mango: change catch-all field range priority (#1069)

01252f97 introduced a "catch-all" feature to Mango that allowed
queries to fall back on a full database scan (_all_docs) when
no valid index was available.

This worked by creating a special index range representing
the full database scan.

For example, a selector:

{ "_id": "foo" }

would be translated into a field range of:

[{ "startkey": "foo", "endkey": "foo"}]

then prepending the catch-all field range, we would have:

[
{ "startkey": null, "endkey": max_json_value},
{ "startkey": "foo", "endkey": "foo"}
]

This set gets passed into
mango_cursor_view:choose_best_index to determine most selective
index and field range combination to use. Unfortunately, in
the event that we have one possible index (all_docs) and multiple
valid ranges, it just chooses the first range it finds -
the full index scan in this case.

This commit makes the catch-all field range the last
available option, ensuring we use the more selective
range where available.

2 files changed, 13 insertions, 3 deletions

diff --git a/src/mango/src/mango_cursor_special.erl b/src/mango/src/mango_cursor_special.erl
index 78cac7f5d..f4a760d1c 100644
--- a/src/mango/src/mango_cursor_special.erl
+++ b/src/mango/src/mango_cursor_special.erl
@@ -31,10 +31,13 @@
 create(Db, Indexes, Selector, Opts) ->
     InitialRange = mango_idx_view:field_ranges(Selector),
     CatchAll = [{<<"_id">>, {'$gt', null, '$lt', mango_json_max}}],
-    FieldRanges = lists:append(CatchAll, InitialRange),
+    % order matters here - we only want to use the catchall index
+    % if no other range can fulfill the query (because we know)
+    % catchall is the most expensive range
+    FieldRanges = InitialRange ++ CatchAll,
     Composited = mango_cursor_view:composite_indexes(Indexes, FieldRanges),
     {Index, IndexRanges} = mango_cursor_view:choose_best_index(Db, Composited),
-
+    
     Limit = couch_util:get_value(limit, Opts, mango_opts:default_limit()),
     Skip = couch_util:get_value(skip, Opts, 0),
     Fields = couch_util:get_value(fields, Opts, all_fields),
diff --git a/src/mango/test/03-operator-test.py b/src/mango/test/03-operator-test.py
index 239cc7d41..4650c7e84 100644
--- a/src/mango/test/03-operator-test.py
+++ b/src/mango/test/03-operator-test.py
@@ -264,6 +264,13 @@ class OperatorTextTests(mango.UserDocsTextTests, OperatorTests):
 
 
 class OperatorAllDocsTests(mango.UserDocsTestsNoIndexes, OperatorTests):
-    pass
+    def test_range_id_eq(self):
+        doc_id = "8e1c90c0-ac18-4832-8081-40d14325bde0"
+        r = self.db.find({
+                "_id": doc_id
+            }, explain=True, return_raw=True)
+        
+        self.assertEqual(r["mrargs"]["end_key"], doc_id)
+        self.assertEqual(r["mrargs"]["start_key"], doc_id)