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I have a relativley complex batch apex process that needs to pull in a large number of master objects based on a field on the master OR the detail object. However in my apex logic I need to ensure that each master object is processed fully within a single execute method.

This means that I can't really use the detail object as my query locator as if I do there's a chance that I'll get different detail objects for the same master in different execute chunks.

So this brings me to using a semi-join in the query locator my batch job starts with. Something like: SELECT id FROM Master__c WHERE Id IN (SELECT master__c FROM Detail__c WHERE master__r.theFieldICareAbout__c = true OR theFieldICareAbout__c = true)

However the data sets this needs to work on are poorly defined at best, in some cases I could be working on as few as 20,000 records, but in others I could be working on something close to 1.5 million detail objects.

I don't have any orgs available for testing at the higher end of this spectrum, so I have to ask, will I hit any governors trying to handle a colossal semi-join like this? Will it just fizzle out and die?

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If the Detail__c part of this query exceeds your 100,000 rows, you will receive an exception, before you even try to receive the Master__c records. I'm not sure exactly what you're going for, but here are a couple of approaches:

  • Query on the Master object with a subquery on the related Details within the execute() method. You might run with a limit on the batch size if that is a safe way to limit how many Detail rows you'll receive. I have run this kind of process with as few as 1 records per batch.
  • Query on the Detail object but sort by Master Id, so you can finish and update objects as you go along. Implementing Database.stateful will allow you to keep a running tally of any incomplete objects between execute runs. This will eliminate the requirement that one Master object fit within the governor restrictions of one execute() mathod.
  • Thanks, that 100k limit was exactly what I needed. I think I'll end up going the stateful route since there's a possibility of having large numbers of non-relevant records and sorting through them in apex would be a pretty serious performance hit. – ca_peterson Sep 26 '12 at 2:37
  • Where is this 100,000 limit on sub queries documented? – Vid L Nov 7 '12 at 7:04
  • The limit is not on sub-queries, it's on the total number of rows returned. For a query that gets Master and related Detail objects, the total number of Master rows + total number of Detail rows is what's compared to the governor limit. – Jeremy Nottingham Nov 12 '12 at 14:59
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Thefieldicareabout__c in the where clause should be an external Id so that it is indexed and available for optimised querying.

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