@navneet

That query results in a single record being returned.  I know that batch apex is an option, but I don't see why I should have to fall back on that in this situation.  While the number of records in the object is large, the number I'm actually working with is quite small.

@zachelrath

You are correct about the fields in my WHERE clause except for one important detail: the source__c text field IS indexed now as I have marked it as an external ID.   I also just tried using the LastModifiedDate, but the error still occurs:

trigger errorTest on Property__c (after update, before update) {
    List<Asset> assetsToClear = new List<Asset>();
	assetsToClear =[Select id, name, SerialNumber from Asset where IsVisionLink__c = false and Exception_from_Disable__c = true
    	and source__c !=null and source__c = 'CN_CCFL_OU'
    	and LastModifiedDate < :(System.now().addDays(-1))];
    
}

Hmm --- I wonder if there is some sort of lag time on indexing?

Not sure if this will help, but since you're in a trigger, try refactoring your code like this:

trigger errorTest on Property__c (after update, before update) {
    for (List<Asset> assetsToClear : 
           [Select id, name, SerialNumber from Asset
            where IsVisionLink__c = false 
            and Exception_from_Disable__c = true
    	    and source__c !=null and source__c = 'CN_CCFL_OU'
    	    and LastModifiedDate < :(System.now().addDays(-1))] {
        for (Asset a : assetsToClear) {
            // Do something to each Asset
        }
    }
    
}

 I'm wondering if the reason the error is being thrown in the Trigger but not elsewhere is due to the SOQL query not being batched, as the for (List<SObject> lst : [SOQL]) syntax will do. Just an idea...

Correction to that last post: using LastModifiedDate does work if I switch the sign to greater than.  Still, I don't understand why it's not recognizing source__c as an indexed field.

Yeah, I actually tried that  exact refactoring with the thought that maybe the internal batching would do something, but the result was the same - still error.  :-(

Okay, that makes more sense. If LastModifiedDate didn't count as indexed, I'd definitely be worried. But as far as your ExternalId field, I'd bet that there's lag time on the indexing. Since you have some 700,000 Asset records, I'd bet that Salesforce is still indexing the Source__c field. I'd be interested to see if you tried this again later today, without the LastModifiedDate criterion, to see if it worked. That would probably confirm the indexing-lag-time theory.