davisp commented on a change in pull request #469: Choose index based on fields
match
URL: https://github.com/apache/couchdb/pull/469#discussion_r113814945
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File path: src/mango/src/mango_cursor_view.erl
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@@ -107,10 +107,14 @@ execute(#cursor{db = Db, index = Idx} = Cursor0,
UserFun, UserAcc) ->
% check FieldRanges for a, b, c, and d and return
% the longest prefix of columns found.
composite_indexes(Indexes, FieldRanges) ->
+ FieldKeys = [Key || {Key, _} <- FieldRanges],
lists:foldl(fun(Idx, Acc) ->
Cols = mango_idx:columns(Idx),
Prefix = composite_prefix(Cols, FieldRanges),
- [{Idx, Prefix} | Acc]
+ % create a KeysPrefixLength based on how close the prefix of
+ % the index is to the number of keys in the selector
+ KeysPrefixLength = length(FieldKeys) - length(Prefix),
Review comment:
"NumExtraColumns" seems like it'd be better. Also the comment should be
updated to specify that we're looking to use the index with the fewest extra
columns. And we should add a comment below in choose_best_index.
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