Fix missing mango execution stats (part 2)fix_mango_execution_stats

The previous implementation of Mango execution stats relied on
passing the docs_examined count from each shard to the coordinator
in the view_row record. This failed to collect the count of
documents read which weren't followed by a match (in a given shard).
For example, if an index was scanned but no documents were matched,
the docs_examined would be 0, when it should be equal to the number
of documents in the index.

This commit changes the implementation so that docs examined is passed
only when each shard has completed its index scan.

The work is split into 2 commits to support mixed-version cluster
upgrades - the previous commit adds the message handlers only
so can be safely rolled out without breaking in-flight requests.

2 files changed, 23 insertions, 52 deletions

diff --git a/src/mango/src/mango_cursor_view.erl b/src/mango/src/mango_cursor_view.erl
index f5cd3822f..f1b753bd7 100644
--- a/src/mango/src/mango_cursor_view.erl
+++ b/src/mango/src/mango_cursor_view.erl
@@ -231,32 +231,26 @@ view_cb({row, Row}, #mrargs{extra = Options} = Acc) ->
     },
     case ViewRow#view_row.doc of
         null ->
-            put(mango_docs_examined, get(mango_docs_examined) + 1),
             maybe_send_mango_ping();
         undefined ->
-            ViewRow2 = ViewRow#view_row{
-                value = couch_util:get_value(value, Row)
-            },
-            ok = rexi:stream2(ViewRow2),
-            put(mango_docs_examined, 0),
+            % include_docs=false. Use quorum fetch at coordinator
+            ok = rexi:stream2(ViewRow),
             set_mango_msg_timestamp();
         Doc ->
+            put(mango_docs_examined, get(mango_docs_examined) + 1),
             Selector = couch_util:get_value(selector, Options),
             case mango_selector:match(Selector, Doc) of
                 true ->
-                    ViewRow2 = ViewRow#view_row{
-                        value = get(mango_docs_examined) + 1
-                    },
-                    ok = rexi:stream2(ViewRow2),
-                    put(mango_docs_examined, 0),
+                    ok = rexi:stream2(ViewRow),
                     set_mango_msg_timestamp();
                 false ->
-                    put(mango_docs_examined, get(mango_docs_examined) + 1),
                     maybe_send_mango_ping()
             end
         end,
     {ok, Acc};
 view_cb(complete, Acc) ->
+    % Send shard-level execution stats
+    ok = rexi:stream2({execution_stats, {docs_examined, get(mango_docs_examined)}}),
     % Finish view output
     ok = rexi:stream_last(complete),
     {ok, Acc};
@@ -286,16 +280,16 @@ handle_message({meta, _}, Cursor) ->
     {ok, Cursor};
 handle_message({row, Props}, Cursor) ->
     case doc_member(Cursor, Props) of
-        {ok, Doc, {execution_stats, ExecutionStats1}} ->
+        {ok, Doc, {execution_stats, Stats}} ->
             Cursor1 = Cursor#cursor {
-                execution_stats = ExecutionStats1
+                execution_stats = Stats
             },
             Cursor2 = update_bookmark_keys(Cursor1, Props),
             FinalDoc = mango_fields:extract(Doc, Cursor2#cursor.fields),
             handle_doc(Cursor2, FinalDoc);
-        {no_match, _, {execution_stats, ExecutionStats1}} ->
+        {no_match, _, {execution_stats, Stats}} ->
             Cursor1 = Cursor#cursor {
-                execution_stats = ExecutionStats1
+                execution_stats = Stats
             },
             {ok, Cursor1};
         Error ->
@@ -420,20 +414,14 @@ doc_member(Cursor, RowProps) ->
     Opts = Cursor#cursor.opts,
     ExecutionStats = Cursor#cursor.execution_stats,
     Selector = Cursor#cursor.selector,
-    {Matched, Incr} = case couch_util:get_value(value, RowProps) of
-        N when is_integer(N) -> {true, N};
-        _ -> {false, 1}
-    end,
     case couch_util:get_value(doc, RowProps) of
         {DocProps} ->
-            ExecutionStats1 = mango_execution_stats:incr_docs_examined(ExecutionStats, Incr),
-            case Matched of
-                true ->
-                    {ok, {DocProps}, {execution_stats, ExecutionStats1}};
-                false ->
-                    match_doc(Selector, {DocProps}, ExecutionStats1)
-                end;
+            % only matching documents are returned; the selector
+            % is evaluated at the shard level in view_cb({row, Row},
+            {ok, {DocProps}, {execution_stats, ExecutionStats}};
         undefined ->
+            % an undefined doc was returned, indicating we should
+            % perform a quorum fetch
             ExecutionStats1 = mango_execution_stats:incr_quorum_docs_examined(ExecutionStats),
             Id = couch_util:get_value(id, RowProps),
             case mango_util:defer(fabric, open_doc, [Db, Id, Opts]) of
@@ -443,9 +431,9 @@ doc_member(Cursor, RowProps) ->
                 Else ->
                     Else
             end;
-        null ->
-            ExecutionStats1 = mango_execution_stats:incr_docs_examined(ExecutionStats),
-            {no_match, null, {execution_stats, ExecutionStats1}}
+        _ ->
+            % no doc, no match
+            {no_match, null, {execution_stats, ExecutionStats}}
     end.
 
 
@@ -481,28 +469,8 @@ update_bookmark_keys(Cursor, _Props) ->
 -ifdef(TEST).
 -include_lib("eunit/include/eunit.hrl").
 
-runs_match_on_doc_with_no_value_test() ->
-    Cursor = #cursor {
-        db = <<"db">>,
-        opts = [],
-        execution_stats = #execution_stats{},
-        selector = mango_selector:normalize({[{<<"user_id">>, <<"1234">>}]})
-    },
-    RowProps = [
-        {id,<<"b06aadcf-cd0f-4ca6-9f7e-2c993e48d4c4">>},
-        {key,<<"b06aadcf-cd0f-4ca6-9f7e-2c993e48d4c4">>},
-        {doc,{
-            [
-                {<<"_id">>,<<"b06aadcf-cd0f-4ca6-9f7e-2c993e48d4c4">>},
-                {<<"_rev">>,<<"1-a954fe2308f14307756067b0e18c2968">>},
-                {<<"user_id">>,11}
-            ]
-        }}
-    ],
-    {Match, _, _} = doc_member(Cursor, RowProps),
-    ?assertEqual(Match, no_match).
 
-does_not_run_match_on_doc_with_value_test() ->
+does_not_refetch_doc_with_value_test() ->
     Cursor = #cursor {
         db = <<"db">>,
         opts = [],
@@ -512,7 +480,6 @@ does_not_run_match_on_doc_with_value_test() ->
     RowProps = [
         {id,<<"b06aadcf-cd0f-4ca6-9f7e-2c993e48d4c4">>},
         {key,<<"b06aadcf-cd0f-4ca6-9f7e-2c993e48d4c4">>},
-        {value,1},
         {doc,{
             [
                 {<<"_id">>,<<"b06aadcf-cd0f-4ca6-9f7e-2c993e48d4c4">>},
diff --git a/src/mango/test/15-execution-stats-test.py b/src/mango/test/15-execution-stats-test.py
index 922cadf83..d3687f8c8 100644
--- a/src/mango/test/15-execution-stats-test.py
+++ b/src/mango/test/15-execution-stats-test.py
@@ -53,6 +53,10 @@ class ExecutionStatsTests(mango.UserDocsTests):
         )
         self.assertEqual(resp["execution_stats"]["results_returned"], len(resp["docs"]))
 
+    def test_no_matches_index_scan(self):
+        resp = self.db.find({"age": {"$lt": 35}, "nomatch": "me"}, return_raw=True, executionStats=True)
+        self.assertEqual(resp["execution_stats"]["total_docs_examined"], 3)
+        self.assertEqual(resp["execution_stats"]["results_returned"], 0)
 
 @unittest.skipUnless(mango.has_text_service(), "requires text service")
 class ExecutionStatsTests_Text(mango.UserDocsTextTests):