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Type:
Bug
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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: Querying
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ALL
Firstly, Assume my schema is the below:
{
"_id" : ObjectId("51548ccdc77b96c10579b54f"),
"key" : {
"val" : 557
},
"list" : [
{
"A" : 696,
"B" : 688,
"C" : 743
},
{
"A" : 743,
"B" : 696,
"C" : 688
},
{
"A" : 688,
"B" : 743,
"C" : 696
}
]
}
Second, assume my indexes are the below:
db.test.ensureIndex(
{ "key.val" : 1 } );
db.test.ensureIndex(
);
db.test.ensureIndex(
);
Now consider the query below:
db.test.find( { "$or" : [ { "key.val" : 123456789} ,
{ "list" : { "$elemMatch" :
{ "$or" : [ { "A" : 696} , { "B" : 696 } ] , "C" : 743}} } ] } )
What this is saying is find me all docs where key.val = 12345679 OR ( ( list.A = 696 OR list.B = 696) AND C = 743).
If I execute this query on my DB I get the following explain output.
> db.test.find( { "$or" : [ { "key.val" : 123456789} , { "list" : { "$elemMatch" : { "$or" : [ { "A" : 696} , { "B" : 696 } ] , "C" : 743}} } ] } ).explain()
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 4,
"nscannedObjects" : 100000,
"nscanned" : 100000,
"nscannedObjectsAllPlans" : 100000,
"nscannedAllPlans" : 100000,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 1,
"nChunkSkips" : 0,
"millis" : 117,
"indexBounds" : {
},
"server" : "Oz-Olivo-MacBook-Pro.local:27017"
}
The results show that this DOES NOT use an index. But does correctly return the 4 matching documents in my DB. At the very least, it should be using the val index for the first part of the OR if not all 3 indexes for the different OR clauses. Why?
The four matching documents are below:
> db.test.find( { "$or" : [ { "key.val" : 123456789} , { "list" : { "$elemMatch" : { "$or" : [ { "A" : 696} , { "B" : 696 } ] , "C" : 743}} } ] } ).pretty()
{
"_id" : ObjectId("51548ccdc77b96c10579b54f"),
"key" : {
"val" : 557
},
"list" : [
{
"A" : 696,
"B" : 688,
"C" : 743
},
{
"A" : 743,
"B" : 696,
"C" : 688
},
{
"A" : 688,
"B" : 743,
"C" : 696
}
]
}
{
"_id" : ObjectId("515599fc23a957b837c8a24d"),
"key" : {
"val" : 708
},
"list" : [
{
"A" : 743,
"B" : 647,
"C" : 696
},
{
"A" : 696,
"B" : 743,
"C" : 647
},
{
"A" : 647,
"B" : 696,
"C" : 743
}
]
}
{
"_id" : ObjectId("515599fc23a957b837c8ac95"),
"key" : {
"val" : 505
},
"list" : [
{
"A" : 264,
"B" : 696,
"C" : 743
},
{
"A" : 743,
"B" : 264,
"C" : 696
},
{
"A" : 696,
"B" : 743,
"C" : 264
}
]
}
{
"_id" : ObjectId("515599fc23a957b837c8b30a"),
"key" : {
"val" : 849
},
"list" : [
{
"A" : 323,
"B" : 696,
"C" : 743
},
{
"A" : 743,
"B" : 323,
"C" : 696
},
{
"A" : 696,
"B" : 743,
"C" : 323
}
]
}
Alternatively, I thought this query could be rewritten as
db.test.find({ "$or" : [ { "key.val" : 123456789} ,
{ "list" : { "$elemMatch" : { "A" : 696 , "C" : 743 }}} ,
{ "list" : { "$elemMatch" : { "B" : 696 , "C" :743 }}} ]})
What this is saying is find me all docs where key.val = 12345679 OR ( list.A = 696 AND C = 743 ) OR (list.B = 696 AND C = 743). Which should be semantically equivalent to the above.
When I run the explain on this query I get:
> db.test.find({ "$or" : [ { "key.val" : 123456789} , { "list" : { "$elemMatch" : { "A" : 696 , "C" : 743 }}} ,{ "list" : { "$elemMatch" : { "B" : 696 , "C" :743 }}} ]}).explain()
{
"clauses" : [
{
"cursor" : "BtreeCursor key.val_1",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 0,
"nscanned" : 0,
"nscannedObjectsAllPlans" : 0,
"nscannedAllPlans" : 0,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"key.val" : [
[
123456789,
123456789
]
]
}
},
{
"cursor" : "BtreeCursor list.A_1",
"isMultiKey" : true,
"n" : 4,
"nscannedObjects" : 297,
"nscanned" : 297,
"nscannedObjectsAllPlans" : 594,
"nscannedAllPlans" : 594,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 1,
"indexBounds" : {
"list.A" : [
[
696,
696
]
]
}
},
{
"cursor" : "BtreeCursor list.B_1",
"isMultiKey" : true,
"n" : 0,
"nscannedObjects" : 297,
"nscanned" : 297,
"nscannedObjectsAllPlans" : 594,
"nscannedAllPlans" : 594,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 1,
"indexBounds" : {
"list.B" : [
[
696,
696
]
]
}
}
],
"n" : 4,
"nscannedObjects" : 594,
"nscanned" : 594,
"nscannedObjectsAllPlans" : 1188,
"nscannedAllPlans" : 1188,
"millis" : 3,
"server" : "Oz-Olivo-MacBook-Pro.local:27017"
}
This DOES use the indexes and finds 4 matches.
Why does query 1 not use any index while query 2 does?
I've uploaded my initial data load script in case it can be of any help. You may have to change the values for A, B, and C in the queries to find anything reasonable though, as the values are randomly generated
- is related to
-
SERVER-3327 use indexes for nested $or clauses
-
- Closed
-