Wildcard Index Restrictions

Incompatible Index Types or Properties

Wildcard indexes do not support the following index types or properties:

• Compound

• TTL

• text

• 2d (Geospatial)

• 2dsphere (Geospatial)

• Hashed

• Unique

Unsupported Query and Aggregation Patterns

Field does not exist

Wildcard indexes are sparse and do not index empty fields. Wildcard indexes therefore cannot support querying for documents where a field does not exist.

For example, consider a collection inventory with a wildcard index on product_attributes. The wildcard index cannot support the following queries:

db.inventory.find( {"product_attributes" : { $exists : false } } )
db.inventory.aggregate([
  { $match : { "product_attributes" : { $exists : false } } }
])

Field is equal to a document or an array

Wildcard indexes generate entries for the contents of a document or array, and not the document/array itself. Wildcard indexes therefore cannot support exact document/array equality matches. Wildcard indexes can support querying where the field equals an empty document {}.

For example, consider a collection inventory with a wildcard index on product_attributes. The wildcard index cannot support the following queries:

db.inventory.find({ "product_attributes" : { "price" : 29.99 } } )
db.inventory.find({ "product_attributes.tags" : [ "waterproof", "fireproof" ] } )
db.inventory.aggregate([{
  $match : { "product_attributes" : { "price" : 29.99 } }
}])
db.inventory.aggregate([{
  $match : { "product_attributes.tags" : ["waterproof", "fireproof" ] } }
}])

Field is not equal to a document or array

Wildcard indexes generate entries for the contents of a document or array, and not the document/array itself. Wildcard indexes therefore cannot support exact document/array inequality matches.

For example, consider a collection inventory with a wildcard index on product_attributes. The wildcard index cannot support the following queries:

db.inventory.find( { $ne : [ "product_attributes", { "price" : 29.99 } ] } )
db.inventory.find( { $ne : [ "product_attributes.tags",  [ "waterproof", "fireproof" ] ] } )
db.inventory.aggregate([{
  $match : { $ne : [ "product_attributes", { "price" : 29.99 } ] }
}])
db.inventory.aggregate([{
  $match : { $ne : [ "product_attributes.tags", [ "waterproof", "fireproof" ] ] }
}])

Array Field is equal or not equal to null

If a given field is an array in any document in the collection, wildcard indexes cannot support queries for documents where that field is equal or not equal to null.

For example, consider a collection inventory with a wildcard index on product_attributes. The wildcard index cannot support the following queries if product_attributes.tags is an array in any document in the collection:

db.inventory.find( { "product_attributes.tags": { $ne: null } } )
db.inventory.find( { "product_attributes.tags": null } )
db.inventory.aggregate([{
  $match : { "product_attributes.tags": { $ne: null } }
}])
db.inventory.aggregate([{
  $match : { "product_attributes.tags": null }
}])

Field is equal to null

Wildcard indexes cannot support queries for documents where a field is equal to null. The query { $eq: null } matches all documents where the field is null or missing. Wildcard are sparse, meaning they do not index the documents where the indexed field is missing, and therefore cannot support a match on null.

For example, consider a collection inventory with a wildcard index on product_attributes. The wildcard index cannot support the following queries:

db.inventory.find( { "product_attributes.price": { $eq: null } } )
db.inventory.aggregate([{
   $match : { "product_attributes.price": { $eq: null } }
}])

Sharding

You cannot shard a collection using a wildcard index. Create a non-wildcard index on the field or fields you want to shard on. For more information on shard key selection, see Shard Keys.