How are Salesforce records queued for Indexing?

Hi,

The platform builds a standard database index on this index table. The index table places upper limits on the number of records that an indexed search can effectively return.

By default, the index tables do not include records that are null (records with empty values). You can work with Salesforce Customer Support to create custom indexes that include null rows. Even if you already have custom indexes on your custom fields, you must explicitly enable and rebuild them to get the empty-value rows indexed.

The platform automatically maintains indexes on the following fields for most objects.
RecordTypeId.
Division.
CreatedDate.
Systemmodstamp (LastModifiedDate)
Email (for contacts and leads)
Foreign key relationships (lookups and master-detail)
The unique Salesforce record ID, which is the primary key for each object.

Hope this helps.