garrensmith commented on a change in pull request #469: Choose index based on
fields match
URL: https://github.com/apache/couchdb/pull/469#discussion_r114286289
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File path: src/mango/src/mango_cursor_view.erl
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@@ -124,25 +128,31 @@ composite_prefix([Col | Rest], Ranges) ->
[]
end.
-
-% Low and behold our query planner. Or something.
-% So stupid, but we can fix this up later. First
-% pass: Sort the IndexRanges by (num_columns, idx_name)
-% and return the first element. Yes. Its going to
-% be that dumb for now.
+% Low and behold our query planner
+% First pass: sort the IndexRanges by KeysPrefixLength,
+% if that is equal, choose the index with the least number of
+% fields in the index. If we still cannot break the tie,
+% then choose alphabetically based on ddocId.
+% Return the first element.
%
% In the future we can look into doing a cached parallel
% reduce view read on each index with the ranges to find
% the one that has the fewest number of rows or something.
choose_best_index(_DbName, IndexRanges) ->
- Cmp = fun({A1, A2}, {B1, B2}) ->
- case length(A2) - length(B2) of
+ Cmp = fun({IdxA, PrefixA, KeysPrefixLengthA}, {IdxB, PrefixB,
KeysPrefixLengthB}) ->
+ case KeysPrefixLengthA - KeysPrefixLengthB of
N when N < 0 -> true;
N when N == 0 ->
- % This is a really bad sort and will end
- % up preferring indices based on the
- % (dbname, ddocid, view_name) triple
- A1 =< B1;
+ ColsLenA = length(mango_idx:columns(IdxA)),
+ ColsLenB = length(mango_idx:columns(IdxB)),
+ case ColsLenA - ColsLenB of
+ M when M < 0 -> true;
Review comment:
What is the general formatting rule, when should it be on a new line?
Does the true on line 144 also need to be on a new line?
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