$push used too much memory and cannot spill to disk

Working with Data
1

To execute queries doing analysis using aggregate and find similar records, $push is used. But, it gives error on certain MongoDB version i.e. 4.2.3.

For example, to execute query “Find Employees with the highest salary”, I have used $push in aggregate functions and allowDiskUse (code mentioned below). But, it shows an error in MongoDB version 4.2.3. " $push used too much memory and cannot spill to disk" .

What is an alternate options other than application-level join?

resultE=db.emp.aggregate([
{ "$group":{ 
    "_id":None ,
    "maxSalary": { "$max": "$salary" },
    "empgrp": { "$push": {
        "_id": "$_id",
        "name": "$emp_name",
        "sal": "$salary"
    }}
}},
{ "$project": {
    "maxSalary": 1,
    "emps": {
        "$setDifference": [
           { "$map": {
               "input": "$empgrp",
               "as": "emp",
               "in": {
                   "$cond": [ 
                       { "$eq": [ "$maxSalary", "$$emp.sal" ] },
                       "$$emp",
                       False
                   ]
               }
           }},
           [False]
        ]
    }
}} ]

, allowDiskUse=True
)

2

Why not just just:

db.emp.aggregate([
  {
    $sort: {
      salary: -1,
    },
  },
  {
    $limit: 1,
  },
]);

OR

db.emp.find({}).sort({ salary: -1 }).limit(1);

?