[SalesForce] How to retrieve more than 50000 Records in a single query

As sfdcfox points out there isn't support for this natively in SOQL, however formula fields can in many cases be leveraged to do this type of work.

Creating a new formula field on the account object of type checkbox with a formula of ISNUMBER(Name) should get you a new boolean column you can filter on for this effect. As formula field values are generated on the fly during the query, much like SQL functions, this has a very similar result - it just has to be prepared ahead of time in a formula field instead of being embedded in the query at runtime.

Don't calculate them all at once. The whole point of a batch is you break the execution into multiple chunks. So calculate comparesn in your execute method for each individual chunk.

public void execute(Database.BatchableContext context, List<OpportunityLineItem> lineItems)
{
    // you only need to map the child records specific to your lineItems in scope
    Map<Id, Id> myMap = new Map<Id, Id>();
    for (MyChildObject__c child : [
        SELECT LineItem__c FROM MyChildObject__c
        WHERE LineItem__c IN :new Map<Id, SObject>(lineItems).keySet()
    ]){
        myMap.put(child.LineItem__c, child.Id);
    }

    // other logic
}