Create a Haystack Index¶

A haystack index must reference two fields: the location field and a second field. The second field is used for exact matches. Haystack indexes return documents based on location and an exact match on a single additional criterion. These indexes are not necessarily suited to returning the closest documents to a particular location.

To build a haystack index, use the following syntax:

db.coll.createIndex( { <location field> : "geoHaystack" ,
                       <additional field> : 1 } ,
                     { bucketSize : <bucket value> } )

To build a haystack index, you must specify the bucketSize option when creating the index. A bucketSize of 5 creates an index that groups location values that are within 5 units of the specified longitude and latitude. The bucketSize also determines the granularity of the index. You can tune the parameter to the distribution of your data so that in general you search only very small regions. The areas defined by buckets can overlap. A document can exist in multiple buckets.

Example

If you have a collection with documents that contain fields similar to the following:

{ _id : 100, pos: { lng : 126.9, lat : 35.2 } , type : "restaurant"}
{ _id : 200, pos: { lng : 127.5, lat : 36.1 } , type : "restaurant"}
{ _id : 300, pos: { lng : 128.0, lat : 36.7 } , type : "national park"}

The following operations create a haystack index with buckets that store keys within 1 unit of longitude or latitude.

db.places.createIndex( { pos : "geoHaystack", type : 1 } ,
                       { bucketSize : 1 } )

This index stores the document with an _id field that has the value 200 in two different buckets:

In a bucket that includes the document where the _id field has a value of 100
In a bucket that includes the document where the _id field has a value of 300

To query using a haystack index you use the geoSearch command. See Query a Haystack Index.

By default, queries that use a haystack index return 50 documents.

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