When a message having at least one attachment is obtained for indexing, it is indexed as N+1 separate documents, where N is the number of attached documents. If the message is part of a message thread, then information regarding the last message in the thread is retrieved, and search index attachment meta data for the last message is extracted. A unique identifier is computed for the newly obtained attachments, and used to search for matches in the attachments for the last message in the thread. If there is a match, then the newly obtained attachment is not indexed, but the unique identifier of the previously indexed matching attachment is added to a body index document for the new message. A unique identifier associated with the new message is also added to a list of parent identifiers associated with the attachment.