mango: mark fields with the `$exists` operator indexable

This is required to make index selection work better with covering
indexes.  The `$exists` operator prescribes the presence of the
given field so that if an index has the field, it shall be
considered because it implies true.  Without this change, it will
not happen, but covering indexes can work if the index is manually
picked.

1 files changed, 94 insertions, 0 deletions

diff --git a/src/mango/src/mango_idx_view.erl b/src/mango/src/mango_idx_view.erl
index 641b8140a..e3db24fbb 100644
--- a/src/mango/src/mango_idx_view.erl
+++ b/src/mango/src/mango_idx_view.erl
@@ -300,6 +300,10 @@ indexable({[{<<"$gt">>, _}]}) ->
     true;
 indexable({[{<<"$gte">>, _}]}) ->
     true;
+% This is required to improve index selection for covering indexes.
+% Making `$exists` indexable should not cause problems in other cases.
+indexable({[{<<"$exists">>, _}]}) ->
+    true;
 % All other operators are currently not indexable.
 % This is also a subtle assertion that we don't
 % call indexable/1 on a field name.
@@ -542,6 +546,96 @@ covers(Idx, Fields) ->
 -ifdef(TEST).
 -include_lib("eunit/include/eunit.hrl").
 
+indexable_fields_of(Selector) ->
+    indexable_fields(test_util:as_selector(Selector)).
+
+indexable_fields_empty_test() ->
+    ?assertEqual([], indexable_fields_of(#{})).
+
+indexable_fields_and_test() ->
+    Selector = #{<<"$and">> => [#{<<"field1">> => undefined, <<"field2">> => undefined}]},
+    ?assertEqual([<<"field1">>, <<"field2">>], indexable_fields_of(Selector)).
+
+indexable_fields_or_test() ->
+    Selector = #{<<"$or">> => [#{<<"field1">> => undefined, <<"field2">> => undefined}]},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_nor_test() ->
+    Selector = #{<<"$nor">> => [#{<<"field1">> => undefined, <<"field2">> => undefined}]},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_all_test() ->
+    Selector = #{<<"field">> => #{<<"$all">> => []}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_elemMatch_test() ->
+    Selector = #{<<"field">> => #{<<"$elemMatch">> => #{}}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_allMatch_test() ->
+    Selector = #{<<"field">> => #{<<"$allMatch">> => #{}}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_keyMapMatch_test() ->
+    Selector = #{<<"field">> => #{<<"$keyMapMatch">> => #{}}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_in_test() ->
+    Selector = #{<<"field">> => #{<<"$in">> => []}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_nin_test() ->
+    Selector = #{<<"field">> => #{<<"$nin">> => []}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_not_test() ->
+    Selector = #{<<"field">> => #{<<"$not">> => #{}}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_lt_test() ->
+    Selector = #{<<"field">> => #{<<"$lt">> => undefined}},
+    ?assertEqual([<<"field">>], indexable_fields_of(Selector)).
+
+indexable_fields_lte_test() ->
+    Selector = #{<<"field">> => #{<<"$lte">> => undefined}},
+    ?assertEqual([<<"field">>], indexable_fields_of(Selector)).
+
+indexable_fields_eq_test() ->
+    Selector = #{<<"field">> => #{<<"$eq">> => undefined}},
+    ?assertEqual([<<"field">>], indexable_fields_of(Selector)).
+
+indexable_fields_ne_test() ->
+    Selector = #{<<"field">> => #{<<"$ne">> => undefined}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_gte_test() ->
+    Selector = #{<<"field">> => #{<<"$gte">> => undefined}},
+    ?assertEqual([<<"field">>], indexable_fields_of(Selector)).
+
+indexable_fields_gt_test() ->
+    Selector = #{<<"field">> => #{<<"$gt">> => undefined}},
+    ?assertEqual([<<"field">>], indexable_fields_of(Selector)).
+
+indexable_fields_mod_test() ->
+    Selector = #{<<"field">> => #{<<"$mod">> => [0, 0]}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_regex_test() ->
+    Selector = #{<<"field">> => #{<<"$regex">> => undefined}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_exists_test() ->
+    Selector = #{<<"field">> => #{<<"$exists">> => true}},
+    ?assertEqual([<<"field">>], indexable_fields_of(Selector)).
+
+indexable_fields_type_test() ->
+    Selector = #{<<"field">> => #{<<"$type">> => undefined}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
+indexable_fields_size_test() ->
+    Selector = #{<<"field">> => #{<<"$size">> => 0}},
+    ?assertEqual([], indexable_fields_of(Selector)).
+
 covers_all_fields_test() ->
     ?assertNot(covers(undefined, all_fields)).