MongoDB Index boundry constraints - mongodb

Question

During my hands on with MongoDB I came to understand about a problem with MongoDB indexes. Problem is that MongoDB indexes sometimes doesn't enforce the two-end boundaries to query. Here's one of the output I encountered while querying the database:
Query:
db.user.find({transaction:{$elemMatch:{product:"mobile", firstTransaction:{$gte:ISODate("2015-01-01"), $lt:ISODate("2015-01-02")}}}}).hint("transaction.product_1_transaction.firstTransaction_1").explain()
Output:
"cursor" : "BtreeCursor transaction.firstTransaction_1_transaction.product_1",
"isMultiKey" : true,
"n" : 622,
"nscannedObjects" : 350931,
"nscanned" : 6188185,
"nscannedObjectsAllPlans" : 350931,
"nscannedAllPlans" : 6188185,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 235851,
"nChunkSkips" : 0,
"millis" : 407579,
"indexBounds" : {
"transaction.firstTransaction" : [
[
true,
ISODate("2015-01-02T00:00:00Z")
]
],
"transaction.product" : [
[
"mobile",
"mobile"
]
]
},
As you can see in above example for firstTransaction field one end of the bound is true instead of date I mentioned. I found the workaround for this is min(), max() functions. I tried those but they not seem to be working with embedded document (transaction is an array of sub document which contains fields like firstTransaction, product etc). I get following error:
Query:
db.user.find({transaction:{$elemMatch:{product:'mobile'}}}).min({transaction:{$elemMatch:{firstTransaction:ISODate("2015-01-01")}}}).max({transaction:{$elemMatch:{firstTransaction:ISODate("2015-01-02")}}})
Output:
planner returned error: unable to find relevant index for max/min query
firstTransaction field is indexed though as well as product & their compound index too. I don't know what is going wrong here.
Sample document:
{
_id: UUID (indexed by default),
name: string,
dob: ISODate,
addr: string,
createdAt: ISODate (indexed),
.
.
.,
transaction:[
{
firstTransaction: ISODate(indexed),
lastTransaction: ISODate(indexed),
amount: float,
product: string (indexed),
.
.
.
},...
],
other sub documents...
}

Answer 1

This is the correct behavior. You cannot always intersect the index bounds for $lte and $gte - sometimes it would give incorrect results. For example, consider the document
{ "x" : [{ "a" : [4, 6] }] }
This document matches the query
db.test.find({ "x" : { "$elemMatch" : { "a" : { "$gte" : 5, "$lte" : 5 } } } });
If we define an index on { "x.a" : 1 }, the two index bounds would be [5, infinity], and [-infinity, 5]. Intersecting them would give [5, 5] and using this index bound would not match the document - incorrectly!
Can you provide a sample document and tell us more about what you're trying to do with the query? With context, there may be another way to write the query that uses tighter index bounds.