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Type:
Improvement
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Resolution: Duplicate
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: Index Maintenance
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None
It appears that find all is not using my index. The supposition is that find all may be ignoring indexes other than BasicCursor? It would be advantageous if the query used the covered index without having to resort to a hint.
Sample "Test" Collection Schema
{
_id: ObjectId(<whatever>),
a: <whatever>,
b: <whatever>,
c: <whatever>,
d: <whatever>,
e: {
f: <whatever>,
g: <whatever>
}
}
Index on "Test"
db.test.ensureIndex( { "a": NumberInt(1), "c": NumberInt(1), "_id": NumberInt(1), "d": NumberInt(1) },
{ name: "a_1_c_1__id_1_d_1", background: true } );
Query without hint and query with hint...
> db.test.find({},{d:1}).explain();
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 752,
"nscannedObjects" : 752,
"nscanned" : 752,
"nscannedObjectsAllPlans" : 752,
"nscannedAllPlans" : 752,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 4,
"nChunkSkips" : 0,
"millis" : 5,
"indexBounds" : {
},
"server" : <whatever>
}
> db.test.find({},{d:1}).hint("a_1_c_1__id_1_d_1").explain();
{
"cursor" : "BtreeCursor a_1_c_1__id_1_d_1",
"isMultiKey" : false,
"n" : 752,
"nscannedObjects" : 752,
"nscanned" : 752,
"nscannedObjectsAllPlans" : 752,
"nscannedAllPlans" : 752,
"scanAndOrder" : false,
"indexOnly" : true,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [ [ { "$minElement" : 1 }, { "$maxElement" : 1 } ] ],
"c" : [ [ { "$minElement" : 1 }, { "$maxElement" : 1 } ] ],
"_id" : [ [ { "$minElement" : 1 }, { "$maxElement" : 1 } ] ],
"d" : [ [ { "$minElement" : 1 }, { "$maxElement" : 1 } ] ]
},
"server" : <whatever>
}
http://stackoverflow.com/questions/18790163/mongodb-index-use-on-find-all-without-hint
- duplicates
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- Closed
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