Reference from original idea by JP Seabury 4 years ago which was incorrectly merged:
"Use Case: We have a custom object named "Order", which as a Lookup relationship to the standard Account object. This allows us to group all the orders associated with an Account.
When users enter an account name via Chatter / global search, the search results currently return accounts, but also related objects (like Contacts and Opportunities associated with that account). I want those search results to return related Order records, as well. In this manner, custom objects would behave similar to standard objects, creating a more uniform user experience.
Currently, Lookup fields are not part of the standard indexing for custom objects, so they are not a searchable field. Below is the article which lists all field types for each object that is searchable: https://help.salesforce.com/HTViewHelpDoc?id=search_fields.htm&language=en_US#CustomObjects
As a workaround, Salesforce Support has suggested we create a hidden text field containing the Account name, and auto-populate it with a workflow rule when the record is saved. It's a hack, and we'll implement it .. but I hate adding redundant data fields to my database. This enhancement request calls for allowing Lookup fields to be indexed / searchable in custom objects."
PLEASE DO NOT MERGE INCORRECTLY
This relates to Global Search not Report and Dashboards :(Report Filters Should Be Able To Compare Fields To Each Other)