Misleading error messages.

Salesforce has governor limits, I understand. But it is frustrating when the methods for checking limits throws an exception that it is exceeding limits.

For example:

if(myList.size()>=Limits.getLimitQueryLocatorRows())

throws this error:

System.QueryException: Aggregate query has too many rows for direct assignment, use FOR loop

And this:

if(u.Tasks!=null)

throws this error:

System.QueryException: Aggregate query has too many rows for direct assignment, use FOR loop

And this:

SELECT count() FROM myObject

throws this error:

System.Exception: Too many query rows: 10001

Then assigning the rows within the loop like this:

for( MyChild__c c : parent.MyChild__r )

throws this error:

INVALID_QUERY_LOCATOR: invalid query locator

This looks pretty hopeless - can't exceed limits, and there's no way to check if we're exceeding limits.

There are actually several possibilities for this sequence of errors:

1. You really do have too much data for a single query
2. There's a null somewhere, most likely the parent record is null. This could be a result of #4 below. A queryLocator is not available indefinitely.
3. You've exceeded your allowance of having max 5 queryLocators open. The 5 cursor limit is per userId, not per session.
4. You've exceeded 10 minute inactivity timeout of a queryLocator
5. Checking for "done" invalidates the queryLocator

Best Practices:

1. Call queryMore at least every 15 minutes to keep the cursor alive, otherwise the queryLocator gets invalidated
2. Check your queryLocator value before and after each call
3. If you know your Ids prior, use the retrieve call instead.