[SalesForce] Fetching records with SOQL in batch apex

Hold on, I have just read your comment on the other answer.

You already have your filter criteria built up into a string?

In that case, you need to build the rest of your query into the string, and use Database.query like so:

// This deduced however you like
String filterValue = '(accountname = \'account1\') and (contactname =\'contact1\')';

String queryHeader = 'SELECT Id FROM Object WHERE ';
List<sObject> results = Database.query(queryHeader + filterValue);

Or any other manipulation of strings that you like.

You can find more out about dynamic SOQL here.

Turned out that something had gone wrong with some of the records when creating the sandbox. It was a full sandbox with a copy of all the data from production.

In my case, I was able to reproduce the issue by creating a new column on Opportunity and a batch process that did nothing except update that column. I could see that some rows weren't being updated. Support were able to reproduce the issue in my sandbox as well, so they were able to diagnose and solve it.

Here was the cut down batch process I used to demonstrate the problem:

global class NBTypeBatchUpdate implements Database.Batchable<sObject> {
    global Database.QueryLocator start(Database.BatchableContext BC)
    {

        String query = 'SELECT Id, WasUpdatedByBatchUpdate__c ';
        query += 'FROM Opportunity ';

        return Database.getQueryLocator(query);
    }

    global void execute(Database.BatchableContext BC, List<sObject> scope)
    {
        system.debug('Processing ' + scope.size() + ' opportunities');

        for (Opportunity opportunity : (List<Opportunity>)scope){
            opportunity.WasUpdatedByBatchUpdate__c = true;
        }

        update scope;
    }  

    global void finish(Database.BatchableContext BC)
    {
    }
}

I didn't get any error messages from the update dml statement. The trouble records just didn't get updated.

Here's exactly what support told me (in case anyone else has this problem):

it seems this was an unfortunate consequence of the non blocking copy process used when refreshing a sandbox which left some data in an inconsistent state.

We ran scrutiny to fix the issue. It seems the issue is resolve for now.