Indexes

Indexes support access patterns in a collection. Create them deliberately for the fields you filter or sort on, then measure your actual workload.

Create an index from Go

Inside the Go application from the Go tour, after obtaining todos, add:

err = todos.CreateIndex(ctx, "by_done", []meldbase.IndexField{
	{Field: "done", Order: 1},
}, meldbase.IndexOptions{})
if err != nil {
	log.Fatal(err)
}

Order is 1 for ascending and -1 for descending. Compound definitions list fields in order:

err = todos.CreateIndex(ctx, "by_done_created", []meldbase.IndexField{
	{Field: "done", Order: 1},
	{Field: "createdAt", Order: -1},
}, meldbase.IndexOptions{})
if err != nil {
	log.Fatal(err)
}

Set Unique: true in IndexOptions to enforce complete-tuple uniqueness. Index creation returns errors for invalid definitions and conflicting data; handle those errors before accepting the index as available.

Know what an index does

An index is not a promise that every query avoids a scan. Filters such as $size, $type, $all, and $elemMatch can need residual evaluation. Compound field order and query sort order matter. Each additional index also adds work to writes and occupies storage.

Start with a query that needs help, add the corresponding index, and verify its behavior with representative data. Keep correctness independent of which execution path the planner chooses.

Next step

Review query semantics and pagination, then add live queries if your application needs updates.