CODEWITHSUNDEEP
mongodb
Twinsen
Twinsen

Reputation: 893

When Create ensureIndex in mongoDb

I'm new in mongoDb, And I don't understand when (and why) use ensureIndex

for example i have this collection

db.films.insert( { "Type" : "DVD", "Title" : "Matrix, The", "Released" :1999} )
db.films.insert( { "Type" : "DVD", "Title" : "Blade Runner", "Released" :1982 } )
db.films.insert( { "Type" : "DVD", "Title" : "Toy Story 3", "Released" :2010} )

if I try with

db.films.find(). min ( { Released: 1995 } ) . max ( { Released : 2005 } )

i have this error

error: {
    "$err" : "no index found for specified keyPattern: {} min: { Released: 1995.0 } max: { Released: 2005.0 }",
    "code" : 10367
}

I have to do 

db.films.ensureIndex( { "Released": 1 } )

and all Works.

why this does not work?

db.films.find(). min ( { "Released": 1995 } ) . max ( { "Released" : 2005 } )

and when i have to use 

. hint ( { Released : 1 } )

I try to do

db.films.ensureIndex( { "Released": 1 } )
db.films.ensureIndex( { "Released": -1 } )

and

db.films.find() . min ( { Released: 1995 } ) . max ( { Released : 2005 } ). hint ( { Released : -1 } )

this is my error

error: {
    "$err" : "requested keyPattern does not match specified keys",
    "code" : 10365
}

Upvotes: 0

Views: 236

Answers (2)

Sammaye
Sammaye

Reputation: 43884

min() and max() are index operators. Instead of constrainng a set of results from an index by a range ($lt, $lte, $gt and $gte) they actually decide how much of the index is scanned by selecting a lower and upper limit of data points. You can see this most noticeably if you also do an explain().

Without an index MongoDB cannot restrict the index.

It should be noted that if you wish to go the route of using min and max then for descending indexes you will need to reverse the values within the two functions to match the reversed index.

In all honesty it is better to leave the range of index scanning to MongoDB most of the tme.

If your query:

db.films.find() . min ( { Released: 1995 } ) . max ( { Released : 2005 } ). hint ( { Released : -1 } )

Doesn't work it means that MongoDB cannot find the range you specify in your index.

Upvotes: 1

Sumeet Kumar Yadav
Sumeet Kumar Yadav

Reputation: 13015

Your query is not working because min() requires an index on a field and forces the query to use the index.

Upvotes: 1

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