Question:

A SOQL Query uses a filter that is unique and indexed. It may return incorrect results in the Sandbox. 

Example:

SELECT Id,External_ID_vod__c,Name from Address_vod__c  where Account_vod__r.External_ID_vod__c = '123456'

No results are returned even though there are actual results matching there.

Answer:

This is a Salesforce-known issue. It is due to an index on the custom field which gets corrupted during a Sandbox refresh.

Remove and Recreate Unique properties of the custom field External.

Related Documentation:

Salesforce Help Documentation: SOQL Queries returning incorrect results in sandbox  

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