What do set, addFields, skip and count do?

Definition:

These stages shape or limit pipeline output: $set/$addFields create or replace fields, $skip removes a leading number of results, and $count summarizes how many documents remain. Their position determines whether they operate on source records or already transformed results. A small final result does not mean the earlier work was small.

Key points:

• $set and $addFields add or replace fields.

• $skip discards the first N results; deep offsets can be slow.

• $count returns the number of documents reaching that stage.

• Place a deterministic $sort before page boundaries when results must be stable across requests.

Interview point:

• $skip still requires advancing past earlier results, which is why deep offset pagination can become expensive.