ฟังก์ชันหน้าต่างด้วย $setWindowFields

Window functions compute values over a set of documents related to the current document—a 'window'—without collapsing them into a single group like $group does. They originated in SQL (SQL:2003) and were added to MongoDB in version 5.0 through the $setWindowFields pipeline stage. Common use cases include running totals, moving averages, rank, and cumulative metrics.

The $setWindowFields stage has three main configuration keys: partitionBy defines how to divide documents into independent windows (like GROUP BY in SQL), sortBy orders documents within each partition, and output specifies the new fields to compute along with their window operator and window bounds.

Window bounds define which documents contribute to the computation for each row. Document-based bounds use the documents key with a two-element array: the start position and end position relative to the current document. 'unbounded' means 'from the beginning (or to the end)', 'current' means the current document, and numeric offsets like -1 mean 'one document before'. Common patterns: ['unbounded', 'current'] for a running total, [-1, 1] for a 3-document moving window.

Range-based bounds define the window using value ranges on the sort key rather than document offsets. This is especially useful for time-series data where you want 'the last 7 days' rather than 'the last 7 documents'. Use the range key with a unit for date fields. This correctly handles gaps in data where days might be missing.

A running total (cumulative sum) is computed by setting the window to span from the first document in the partition to the current document. As MongoDB processes each document in sort order, it adds that document's value to all previous values. This produces a monotonically increasing total per partition, useful for cumulative revenue or progressive download counts.

A moving average smooths out short-term fluctuations in time-series data to reveal underlying trends. Configure the window to span a fixed number of periods in both directions (or only backwards for a 'trailing' average). Moving averages are common in financial charts, performance monitoring dashboards, and IoT sensor analysis.

The $rank operator assigns a rank number to each document within its partition based on the sort order. Tied documents receive the same rank, and the next rank skips accordingly (1, 2, 2, 4). $denseRank assigns consecutive ranks without gaps for ties (1, 2, 2, 3). Neither takes a window specification—they always rank across the full partition.

$documentNumber assigns a sequential integer starting from 1 to each document within its partition, in sort order. Unlike $rank, it never repeats numbers—every document gets a unique number. This is useful for pagination, sequence numbering, or when you need to identify which row within a partition a document occupies.

$shift returns the value of an expression from a document at a specified offset relative to the current document within the partition. Use by: -1 to access the previous document's value (e.g., yesterday's price), by: 1 for the next document, and specify a default for when the offset falls outside the partition boundary.

$setWindowFields benefits from indexes on the partition and sort fields. An index that covers both the partition key and the sort key allows MongoDB to efficiently retrieve each partition's documents in sorted order without a full collection scan. Without an index, MongoDB must sort in-memory (up to allowDiskUse limits). For large collections, ensure indexes align with your window functions' partition and sort specifications.

A real-world sales report might need each salesperson's transactions enriched with their running total, their rank within their team, and the team cumulative total—all computed in a single pipeline without multiple joins or application-side computation. This is exactly the kind of analytical query $setWindowFields was built for.

Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.

In this lesson you learned: $setWindowFields computes values over a sliding window of related documents without collapsing them like $group, document-based and range-based window bounds control which documents contribute to each computation, and operators like $rank, $denseRank, $documentNumber, and $shift enable ranking, numbering, and cross-row comparisons. Next up we explore ACID guarantees in MongoDB's distributed document store.