Aggregate query has too many rows for direct assignment, use FOR loop' Salesforce Error

Solution 1: Use a FOR Loop to Iterate Child Records

Instead of directly assigning the entire child collection to a variable, use a FOR loop to iterate through child records one at a time. This avoids the direct assignment row limit.

for (childtype ch : Parent.children) {

//do some logic...

}

In this pattern, Parent.children refers to the child relationship collection on the parent record. Each iteration of the loop processes one child record, keeping memory usage within governor limits.

Solution 2: Use a Map of Parent to Child Objects

Alternatively, build a map of parent records to their child collections and use .size() on individual map keys rather than on the full child relationship directly.
Instead of:

if (parent.child.size() > 0)

Use a map-based approach to check the size of child collections per parent record. This avoids the direct assignment error by not attempting to load the entire child collection into a single variable.