Compound Wildcard Indexes

Search Using the Attribute Pattern

The attribute pattern is a useful technique for searching documents that share common characteristics.

Unfortunately, it is expensive to create a lot of individual indexes to cover all of the possible queries. A wildcard index is a good alternative to creating a large number of individual indexes because one wildcard index can efficiently cover many potential queries.

Consider a schema like:

{
     tenantId: <Number>,
     tenantRegion: <Number>,
     customFields: {
                     addr: <String>,
                     name: <String>,
                     blockId: <Number>,
                     ...
     }
    dateOpened: <Date>
 }

You might want to query aspects of the customFields field for tenants that have a particular tenantId. You could create a series of individual indexes:

{ tenantId: 1, "customFields.addr": 1 }
{ tenantId: 1, "customFields.name": 1 }
{ tenantId: 1, "customFields.blockId": 1 }
...

This approach is difficult to maintain and you are likely to reach the maximum number of indexes per collection (64).

Use a compound wildcard index instead. The compound wildcard index is easier to write, easier to maintain, and is unlikely to reach the 64 index collection limit.

This example creates a compound wildcard index on the salesData collection:

db.runCommand(
   {
       createIndexes: "salesData",
       indexes: [
          {
             key: {
                tenantId: 1,
                "customFields.$**": 1
             },
             name: "tenant_customFields"
          }
       ]
   }
)

The wildcard, "customFields.$**", specifies all of the sub-fields in the customFields field. The other index term, tenantId, is not a wildcard specification; it is a standard field specification.

General Considerations for Wildcard Indexes

• Wildcard indexes omit the _id field by default. To include the _id field in a wildcard index, you must explicitly include it in the wildcardProjection document.

  db.salesData.createIndex(
     { "$**" : 1 },
     { "wildcardProjection" :
        { "_id": 1, "customers.lastName": 1, "customers.FirstName": 1, }
     }
  )

• You can create more than one wildcard index on a collection.

• A wildcard index may cover the same fields as other indexes in the collection.

• Wildcard indexes are sparse. They only include entries for documents that contain the indexed field.

  The document is not indexed if all of the fields in the compound wildcard index are missing.

Compound Wildcard Index Considerations

• Compound wildcard indexes are sparse indexes.

• Documents are included in the index if they are missing the wildcard field but have one of the compound fields.

• Index fields, including wildcard fields, can be sorted in ascending (1) or descending (-1) order.

Filter Fields with a wildcardProjection

You can use a wildcardProjection to specify individual sub-fields.

db.runCommand(
   {
       createIndexes: "salesData",
       indexes: [
          {
             key: {
                tenantId: 1,
                "$**": 1
             },
             name: "tenant_customFields_projection",
             wildcardProjection: {
                "customFields.addr": 1,
                "customFields.name": 1
             }
          }
       ]
   }
)

The wildcard index term, "$**", specifies every field in the collection. The wildcardProjection limits the index to the specified fields, "customFields.addr" and "customFields.name".

You can only use a wildcardProjection when the wildcard term is $**.

Use a Helper Method to Create a Wildcard Index

MongoDB provides shell helper methods for most database commands. These shell methods offer a simplified syntax and are functionally equivalent to the database commands.

The shell helper for the first example is:

db.salesData.createIndex(
   { tenantId: 1, "customFields.$**": 1 },
   {
      name: "tenant_customFields_shellHelper"
   }
)

The shell helper for the second example is:

db.salesData.createIndex(
  { tenantId: 1, "$**": 1 },
  { "wildcardProjection": {
        "customFields.addr": 1,
        "customFields.name": 1
     },
     name: "tenant_customFields_projection_helper"
  }
)

If you want to compare the shell commands and the database commands, you must drop the indexes between command invocations. You cannot create the same index twice, even with different names.

To drop an index, insert the index name and run db.collection.dropIndex().

db.salesData.dropIndex( "tenant_customFields" )

The preceding command removes the "tenant_customFields" index from the salesData database.