[couchdb] branch 3.x updated: Make view merge row aggregation in fabric stable

[va...@apache.org]

This is an automated email from the ASF dual-hosted git repository.

vatamane pushed a commit to branch 3.x
in repository https://gitbox.apache.org/repos/asf/couchdb.git


The following commit(s) were added to refs/heads/3.x by this push:
     new 4f33f14  Make view merge row aggregation in fabric stable
4f33f14 is described below

commit 4f33f14deb733f36cf0df03aedceebc746716d8c
Author: Nick Vatamaniuc <va...@gmail.com>
AuthorDate: Thu Sep 23 18:43:14 2021 -0400

    Make view merge row aggregation in fabric stable
    
    We do that by matching the comparator function behavior used during row merging
    [1] in with the comparison function used when sorting the rows on view
    shards [2]. This goes along with the constraint in the lists:merge/3 docs which
    indicates that the input lists should be sorted according to the same
    comparator [3] as the one passed to the lists:merge/3 call.
    
    The stability of returned rows results from the case when both keys match as
    equal. Now `lists:merge/3` will favor the element in the existing rows list
    instead of always replacing [4] the older matching row with the last arriving
    row, since now `less(A, A)` will be `false`, while previously it was `true`.
    
    The fix was found by Adam when discussing issue #3750
    
    https://github.com/apache/couchdb/issues/3750#issuecomment-920947424
    
    Co-author: Adam Kocoloski <ko...@apache.org>
    
    [1] https://github.com/apache/couchdb/blob/3.x/src/fabric/src/fabric_view_map.erl#L244-L248
    [2] https://github.com/apache/couchdb/blob/3.x/src/couch_mrview/src/couch_mrview_util.erl#L1103-L1107
    [3] https://erlang.org/doc/man/lists.html#merge-3
    [4] https://github.com/erlang/otp/blob/master/lib/stdlib/src/lists.erl#L2668-L2675
---
 src/fabric/src/fabric_view.erl     | 1 -
 src/fabric/src/fabric_view_map.erl | 1 -
 2 files changed, 2 deletions(-)

diff --git a/src/fabric/src/fabric_view.erl b/src/fabric/src/fabric_view.erl
index 425f864..bd5e42f 100644
--- a/src/fabric/src/fabric_view.erl
+++ b/src/fabric/src/fabric_view.erl
@@ -298,7 +298,6 @@ transform_row(#view_row{key=Key, id=_Id, value=_Value, doc={error,Reason}}) ->
 transform_row(#view_row{key=Key, id=Id, value=Value, doc=Doc}) ->
     {row, [{id,Id}, {key,Key}, {value,Value}, {doc,Doc}]}.
 
-compare(_, _, A, A) -> true;
 compare(fwd, <<"raw">>, A, B) -> A < B;
 compare(rev, <<"raw">>, A, B) -> B < A;
 compare(fwd, _, A, B) -> couch_ejson_compare:less_json(A, B);
diff --git a/src/fabric/src/fabric_view_map.erl b/src/fabric/src/fabric_view_map.erl
index 8ab8083..adfe7d6 100644
--- a/src/fabric/src/fabric_view_map.erl
+++ b/src/fabric/src/fabric_view_map.erl
@@ -241,7 +241,6 @@ merge_row(Dir, Collation, KeyDict0, Row, Rows0) ->
             {Rows1, KeyDict1}
     end.
 
-compare(_, _, A, A) -> true;
 compare(fwd, <<"raw">>, A, B) -> A < B;
 compare(rev, <<"raw">>, A, B) -> B < A;
 compare(fwd, _, A, B) -> couch_ejson_compare:less_json_ids(A, B);