When two $setWindowFields stages appear in a row, it may be beneficial to combine them into one stage.

For example, this query:

{$setWindowFields: {
    partitionBy: "$zip",
    output: {total: {$sum: "$x"}},
}},
{$setWindowFields: {
    partitionBy: "$zip",
    output: {avg: {$avg: "$x"}},
}},

has the same result as this one:

{$setWindowFields: {
    partitionBy: "$zip",
    output: {
        total: {$sum: "$x"},
        avg: {$avg: "$x"},
    },
}},

After desugaring and dropping a redundant $sort (SERVER-55464), the original query is:

{$sort: {zip: 1}},
{$_internalSetWindowFields: {
    partitionBy: "$zip",
    output: {total: {$sum: "$x"}},
}},
{$sort: {zip: 1}},
{$_internalSetWindowFields: {
    partitionBy: "$zip",
    output: {avg: {$avg: "$x"}},
}},

I think a valid set of criteria for combining the two _internalSetWindowFields stages would be:

• The second stage does not read from any 'output' field of the first.
• The two stages don't have any 'output' field name in common.
• The two stages have identical partitionBy and sortBy arguments.