davisp commented on a change in pull request #1376: Add fields to sort based on
selector
URL: https://github.com/apache/couchdb/pull/1376#discussion_r193523326
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File path: src/mango/src/mango_idx.erl
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@@ -125,6 +126,32 @@ maybe_filter_by_sort_fields(Indexes, SortFields) ->
end.
+% This is a user experience improvement. If a selector has a sort field set,
+% an index is only valid if all the fields in the index are also specified
+% in the sort.
+
+% If a field is in the selector is constant, eg {age: {$eq: 21}} and it's
+% part of the index then we can automatically add it to the sort list because
+% it won't affect sorting.
+
+% This will then increase the likely hood of the index being choosen
+% and make it a little easier for the user.
+normalize_sort_fields(Cols, SortFields, Selector) ->
+ % Keep any fields in the sort that might not be defined in the index
+ Start = lists:subtract(SortFields, Cols),
+ lists:foldl(fun (Col, Sort) ->
+ case lists:member(Col, SortFields) of
+ true ->
+ [Col | Sort];
+ _ ->
+ case mango_selector:is_constant_field(Selector, Col) of
+ true -> [Col | Sort];
+ _ -> Sort
+ end
+ end
+ end, Start, lists:reverse(Cols)).
Review comment:
You can use lists:foldr above instead of reversing `Cols` here.
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