[GitHub] tonysun83 commented on a change in pull request #469: Choose index based on fields match

[gi...@git.apache.org]

tonysun83 commented on a change in pull request #469: Choose index based on fields match
URL: https://github.com/apache/couchdb/pull/469#discussion_r110298507
 
 

 ##########
 File path: src/mango/src/mango_cursor_view.erl
 ##########
 @@ -135,14 +139,13 @@ composite_prefix([Col | Rest], Ranges) ->
 % 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, ColPrefixLengthA}, {IdxB, PrefixB, ColPrefixLengthB}) ->
+        case ColPrefixLengthA - ColPrefixLengthB 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;
+                % We have no other way to choose, so at this point 
+                % select index based on (dbname, ddocid, view_name) triple
+                IdxA =< IdxB;
 
 Review comment:
   So with the old code (which is also what you're leaving with here), I ran a simple test, and here's what happens:
   
   IdxA: [a,b,c]
   IdxB: [a,d,e]
   
   I insert two docs, {"a":1, "b":1, "c":1} and {"a":1000, "d" : 1000, "e": 1000}
   
   then I run the query with _explain:
   
   ```
   {
     "selector": {
       "a": {
         "$gt": 0
       },
       "b": {
         "$gt": 0
       },
       "c": {
         "$gt": 0
       }
     }
   }
   ```
   and I get back IdxB
   ```
   "index": {
           "ddoc": "_design/ccf468609d8ca7ef1744797c6b685d82cb552335",
           "name": "IdxB",
           "type": "json",
           "def": {
               "fields": [
                   {
                       "a": "asc"
                   },
                   {
                       "d": "asc"
                   },
                   {
                       "e": "asc"
                   }
               ]
           }
       }
   ```
   I think we've been wrong all this time, and it should be IdxA right?
   Can you try out this scenario in the tests? We might have to change this comparison. 
 
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