Datastore Queries

A Datastore query retrieves entities from Cloud Datastore that meet a specified set of conditions.

A typical query includes the following:

  • An entity kind to which the query applies
  • Optional filters based on the entities' property values, keys, and ancestors
  • Optional sort orders to sequence the results
When executed, a query retrieves all entities of the given kind that satisfy all of the given filters, sorted in the specified order. Queries execute as read-only.

This page describes the structure and kinds of queries used within App Engine to retrieve data from Cloud Datastore.

Filters

A query's filters set constraints on the properties, keys, and ancestors of the entities to be retrieved.

Property filters

A property filter specifies

  • A property name
  • A comparison operator
  • A property value
For example:

q := datastore.NewQuery("Person").Filter("Height <=", maxHeight)

The property value must be supplied by the application; it cannot refer to or be calculated in terms of other properties. An entity satisfies the filter if it has a property of the given name whose value compares to the value specified in the filter in the manner described by the comparison operator.

The comparison operator can be any of the following:

Operator Meaning
= Equal to
< Less than
<= Less than or equal to
> Greater than
>= Greater than or equal to

Key filters

To filter on the value of an entity's key, use the special property __key__:

q := datastore.NewQuery("Person").Filter("__key__ >", lastSeenKey)

When comparing for inequality, keys are ordered by the following criteria, in order:

  1. Ancestor path
  2. Entity kind
  3. Identifier (key name or numeric ID)

Elements of the ancestor path are compared similarly: by kind (string), then by key name or numeric ID. Kinds and key names are strings and are ordered by byte value; numeric IDs are integers and are ordered numerically. If entities with the same parent and kind use a mix of key name strings and numeric IDs, those with numeric IDs precede those with key names.

Queries on keys use indexes just like queries on properties and require custom indexes in the same cases, with a couple of exceptions: inequality filters or an ascending sort order on the key do not require a custom index, but a descending sort order on the key does. As with all queries, the development web server creates appropriate entries in the index configuration file when a query that needs a custom index is tested.

Ancestor filters

You can filter your Datastore queries to a specified ancestor so that the results returned will include only entities descended from that ancestor:

q := datastore.NewQuery("Person").Ancestor(ancestorKey)

Special query types

Some specific types of query deserve special mention:

Kindless queries

A query with no kind and no ancestor filter retrieves all of the entities of an application from Datastore. This includes entities created and managed by other App Engine features, such as statistics entities and Blobstore metadata entities (if any). Such kindless queries cannot include filters or sort orders on property values. They can, however, filter on entity keys by specifying __key__ as the property name:

q := datastore.NewQuery("").Filter("__key__ >", lastSeenKey)

Ancestor queries

A query with an ancestor filter limits its results to the specified entity and its descendants:

// Create two Photo entities in the datastore with a Person as their ancestor.
tomKey := datastore.NewKey(ctx, "Person", "Tom", 0, nil)

wPhoto := Photo{URL: "http://example.com/some/path/to/wedding_photo.jpg"}
wKey := datastore.NewKey(ctx, "Photo", "", 0, tomKey)
_, err := datastore.Put(ctx, wKey, wPhoto)
// check err

bPhoto := Photo{URL: "http://example.com/some/path/to/baby_photo.jpg"}
bKey := datastore.NewKey(ctx, "Photo", "", 0, tomKey)
_, err = datastore.Put(ctx, bKey, bPhoto)
// check err

// Now fetch all Photos that have tomKey as an ancestor.
// This will populate the photos slice with wPhoto and bPhoto.
q := datastore.NewQuery("Photo").Ancestor(tomKey)
var photos []Photo
_, err = q.GetAll(ctx, &photos)
// check err
// do something with photos

Kindless ancestor queries

A kindless query that includes an ancestor filter will retrieve the specified ancestor and all of its descendants, regardless of kind. This type of query does not require custom indexes. Like all kindless queries, it cannot include filters or sort orders on property values, but can filter on the entity's key:

q := datastore.NewQuery("").Ancestor(ancestorKey).Filter("__key__ >", lastSeenKey)

The following example illustrates how to retrieve all entities descended from a given ancestor:

tomKey := datastore.NewKey(ctx, "Person", "Tom", 0, nil)

weddingPhoto := &Photo{URL: "http://example.com/some/path/to/wedding_photo.jpg"}
_, err := datastore.Put(ctx, datastore.NewIncompleteKey(ctx, "Photo", tomKey), weddingPhoto)

weddingVideo := &Video{URL: "http://example.com/some/path/to/wedding_video.avi"}
_, err = datastore.Put(ctx, datastore.NewIncompleteKey(ctx, "Video", tomKey), weddingVideo)

// The following query returns both weddingPhoto and weddingVideo,
// even though they are of different entity kinds.
q := datastore.NewQuery("").Ancestor(tomKey)
t := q.Run(ctx)
for {
	var x interface{}
	_, err := t.Next(&x)
	if err == datastore.Done {
		break
	}
	if err != nil {
		log.Errorf(ctx, "fetching next Photo/Video: %v", err)
		break
	}
	// Do something (e.g. switch on types)
	doSomething(x)
}

Keys-only queries

A keys-only query returns just the keys of the result entities instead of the entities themselves, at lower latency and cost than retrieving entire entities:

q := datastore.NewQuery("Person").KeysOnly()

It is often more economical to do a keys-only query first, and then fetch a subset of entities from the results, rather than executing a general query which may fetch more entities than you actually need.

Note that a keys-only query can return more than 1000 results, but GetAll can only retrieve 1000 keys at a time, and will fail with an error if called on a larger result. Therefore, we recommend adding a limit of 1000 keys to the query.

Projection queries

Sometimes all you really need from the results of a query are the values of a few specific properties. In such cases, you can use a projection query to retrieve just the properties you're actually interested in, at lower latency and cost than retrieving the entire entity; see the Projection Queries page for details.

Sort orders

A query sort order specifies

  • A property name
  • A sort direction (ascending or descending)

In Go, descending sort order is denoted by a hyphen (-) preceding the property name; omitting the hyphen specifies ascending order by default. For example:

// Order alphabetically by last name:
q := datastore.NewQuery("Person").Order("LastName")

// Order by height, tallest to shortest:
q = datastore.NewQuery("Person").Order("-Height")

If a query includes multiple sort orders, they are applied in the sequence specified. The following example sorts first by ascending last name and then by descending height:

q := datastore.NewQuery("Person").Order("LastName").Order("-Height")

If no sort orders are specified, the results are returned in the order they are retrieved from Cloud Datastore.

Note: Because of the way Cloud Datastore executes queries, if a query specifies inequality filters on a property and sort orders on other properties, the property used in the inequality filters must be ordered before the other properties.

Indexes

Every Datastore query computes its results using one or more indexes, which contain entity keys in a sequence specified by the index's properties and, optionally, the entity's ancestors. The indexes are updated incrementally to reflect any changes the application makes to its entities, so that the correct results of all queries are available with no further computation needed.

App Engine predefines a simple index on each property of an entity. An App Engine application can define further custom indexes in an index configuration file named index.yaml. The development server automatically adds suggestions to this file as it encounters queries that cannot be executed with the existing indexes. You can tune indexes manually by editing the file before uploading the application.

Query interface example

The Go Datastore API provides a Query type for preparing and executing queries.

type Person struct {
	FirstName string
	LastName  string
	City      string
	BirthYear int
	Height    int
}

func handle(w http.ResponseWriter, r *http.Request) {
	ctx := appengine.NewContext(r)

	// The Query type and its methods are used to construct a query.
	q := datastore.NewQuery("Person").
		Filter("LastName =", "Smith").
		Filter("Height <=", maxHeight).
		Order("-Height")

	// To retrieve the results,
	// you must execute the Query using its GetAll or Run methods.
	var people []Person
	if _, err := q.GetAll(ctx, &people); err != nil {
		// Handle error.
	}
	// ...
}

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