Composite Sort Keys in DynamoDB

A composite primary key is a partition key plus a sort key. The trick that makes it powerful is what you put in the sort key: encode a hierarchy as one delimited string, and a single Query reads an entire subtree in sort order — no joins, no recursion, no second round-trip.

How do composite sort keys work in DynamoDB?

A composite sort key packs a hierarchy into one delimited string — root/photos/2026/ — that DynamoDB stores in UTF-8 byte order. Because the layout already matches the tree, a single Query with begins_with(SK, "root/photos/") reads an entire subtree in path order. No joins, no recursion, no second round-trip — just a prefix scan over a contiguous slice.

• The sort key is a sortable string, not just an ID. Pack a path into it — root/photos/2026/ — and DynamoDB stores the partition's items in UTF-8 byte order automatically.
• A delimiter turns prefix matches into subtree reads. begins_with(SK, "root/photos/") returns every descendant of that folder in one query.
• Sort keys support range conditions, not arbitrary filters. You get begins_with, between, >, < — design the key so the read you need is a prefix or a range, not a Scan.
• The delimiter is load-bearing. Pick one that can't appear in a path segment, or two unrelated branches collide.

Why the sort key is the whole game

Coming from SQL, you'd model a folder tree with a parent_id self-join and walk it recursively — one query per level. In DynamoDB that's an N+1 footgun against a key-value store that has no joins.

DynamoDB stores every item under a partition key sorted by its sort key, in UTF-8 byte order for strings (AWS: Query key conditions). So if your sort key is the path, the physical layout already matches the tree. A read becomes a prefix scan over a contiguous slice — not a graph walk.

That's the shift: the sort key isn't an identifier you match exactly. It's a sortable address. Design it and the query falls out for free.

Model a file-system tree

Say you're storing per-account file trees. One drive per account is the natural partition; the path inside it is the sort key.

Two original conventions doing the work here:

• PK = DRIVE#<account> keeps one account's whole tree in a single item collection, so any subtree read is a single-partition Query.
• SK is the full path with a trailing / on folders. The trailing slash is deliberate — it makes a folder sort before its own children and keeps root/photos/ distinct from a sibling file named root/photos.

Read a subtree in one query

List everything under root/photos/ — folder, subfolders, and files, recursively:

Query
KeyConditionExpression = PK = :drive AND begins_with(SK, :prefix)
:drive   = "DRIVE#a91"
:prefix  = "root/photos/"

That returns root/photos/, root/photos/2026/, beach.jpg, and sunset.jpg — in path order, in one billed read. You pay only for the items in that slice, not the whole drive.

In DynoTable, you run exactly this begins_with query against the path sort key and the folder plus its descendants come back in path order — no placeholder syntax to hand-write.

Need the raw KeyConditionExpression (names, values, and begins_with) for your own code? Build and copy it in the DynamoDB Expression Builder.

List one level, not the whole subtree

begins_with gives you the recursive read. For a non-recursive directory listing — the immediate children of root/photos/ and nothing deeper — store a depth attribute and add a sort-key range plus a filter, or split the path into a parent GSI. The simplest version: keep a parent attribute (root/photos/) and a GSI keyed on it.

The point: a sort key answers prefix and range questions cheaply. "Direct children only" is a different question — model it explicitly rather than hoping a FilterExpression makes it efficient. A filter runs after the read and you pay for every item it discards.

Pick the delimiter carefully

The delimiter is part of your data contract. Two rules:

• It must never appear inside a path segment. If filenames can contain /, / is the wrong delimiter — a file named a/b is indistinguishable from a folder a holding b. Pick a reserved byte (some teams use # or a control char) and forbid it in segments.
• Mind the sort order at boundaries. / (0x2F) sorts before digits and letters, which is usually what you want for tree order. Change the delimiter and you change the ordering — verify it against real data.

Composite sort key vs. a separate sort attribute

Composite keys win when reads are subtree-shaped and ordering matters; the flat-ID model wins when the tree mutates constantly. Most read-heavy hierarchies — file trees, category trees, org charts — lean composite.

Pitfalls and next steps

• Don't over-stuff the key. Everything you encode is immutable and indexed by prefix only. Attributes you query by equality belong in their own fields or a GSI, not jammed into the sort key.
• A sort key can't do arbitrary WHERE. Only begins_with, between, and comparisons. If you find yourself reaching for a FilterExpression, you've probably modelled the key wrong — see Query vs. Scan.
• Going deeper on key design lives in single-table design; for when a subtree read needs an index instead of the base table, see GSI vs. LSI.

Build the begins_with key condition with the Expression Builder, then download DynoTable to run these prefix queries against your own tables and watch a subtree come back in path order.