In search result page, when you click on the “View Profile” page, it does call the database to retrieve the data. So Authentication mode is important here. If you specify “PassThrough”, make sure the “Default content access account” (seem in Search Settings page) has access to that DB. Or, change to “RevertToSelf” and the identify of the application pool will be used – please note this is the SSP application pool, not the central admin application pool. However, for unknown reason, The “Default content access account” still need access to the database, otherwise the crawling will fail even the “View Profile” URL can retrieve the data from database. Also, don’t forget to give “Default content access account” (the account used to crawl) appropriate rights to the BDC application and entity.
Please note that there might be a delay of a few minutes after the permission is changed on BDC application and entity. If the crawling still has “Access denied b BDC” error, wait for a few minutes.
You can delete and re-import the BDC application definition at the anytime – no need to re-crawl as long as the BDC name is not changed.
Incremental Crawl – it’s said that if you add a LastModifiedDate column to the record, SharePoint indexing service will use that as a time stamp and incremental crawling is enabled. However in my various tests, it took virtually same time in full and incremental crawling. Either this doesn’t work, or in my test case, most of the time was spent on retrieving the data instead of indexing them.
Some good articles to read: