Optimizing Indexes

This page describes concepts to consider when selecting Firestore in Datastore mode indexes for your app.

Firestore in Datastore mode delivers high query performance by using indexes for all queries. Performance for most queries depends on the size of the result set and not on the total size of the database.

Firestore in Datastore mode defines built-in indexes for each property in an entity. These single property indexes support many simple queries. Firestore in Datastore mode supports an index merging feature that lets your database merge built-in indexes to support additional queries. For more complex queries, you must define composite indexes ahead of time.

This page focuses on the index merging feature, because it affects two important index optimization opportunities:

  • Speeding up queries
  • Reducing the number of composite indexes

The following example demonstrates the index merging feature.

Filtering Photo entities

Consider a Datastore mode database with entities of kind Photo:

Property Value type Description
owner_id String User id
tag Array of strings Tokenized keywords
size Integer Enumeration:
  • 1 icon
  • 2 medium
  • 3 large
coloration Integer Enumeration:
  • 1 black & white
  • 2 color

Imagine that you require an app feature that lets users query Photo entities based on a logical AND of the following:

  • Up to three filters based on the properties:

    • owner_id
    • size
    • coloration
  • A tag search string. The app tokenizes the search string into tags and adds a filter for each tag.

    For example, the app turns the search string outside, family into the query filters tag=outside and tag=family.

Using built-in indexes and Firestore in Datastore mode's index merging feature, you can meet the index requirements of this Photo filter feature without adding additional composite indexes.

The built-in indexes for Photo entities support single-filter queries like:


query_owner_id = client.query(
    filters=[('owner_id', '=', 'user1234')])

query_size = client.query(
    filters=[('size', '=', 2)])

query_coloration = client.query(
    filters=[('coloration', '=', 2)])

The Photo filter feature also requires queries that combine multiple equality filters with a logical AND:


query_all_properties = client.query(
    filters=[('owner_id', '=', 'user1234'),
             ('size', '=', 2),
             ('coloration', '=', 2),
             ('tag', '=', 'family')])

Firestore in Datastore mode can support these queries by merging built-in indexes.

Index merging

Firestore in Datastore mode can use index merging when your query and your indexes meet all of the following constraints:

  • The query uses only equality (=) filters
  • No composite index exists that perfectly matches the filters and ordering of the query
  • Each equality filter matches at least one existing index with the same ordering as the query

In this situation, Firestore in Datastore mode can use existing indexes to support the query instead of requiring you to configure an additional composite index.

When two or more indexes are sorted by the same criteria, Firestore in Datastore mode can merge the results of multiple index scans to find the results that are common to all such indexes. Firestore in Datastore mode can merge built-in indexes, because they all sort values by entity key.

By merging built-in indexes, Firestore in Datastore mode supports queries with equality filters on multiple properties:


query_all_properties = client.query(
    filters=[('owner_id', '=', 'user1234'),
             ('size', '=', 2),
             ('coloration', '=', 2),
             ('tag', '=', 'family')])

Firestore in Datastore mode can also merge index results from multiple sections of the same index. By merging different sections of the built-in index for the tag property, Firestore in Datastore mode supports queries that combine multiple tag filters in a logical AND:


query_tag = client.query(
    filters=[('tag', '=', 'family'),
             ('tag', '=', 'outside'),
             ('tag', '=', 'camping')])

query_owner_size_color_tags = client.query(
    filters=[('owner_id', '=', 'user1234'),
             ('size', '=', 2),
             ('coloration', '=', 2),
             ('tag', '=', 'family'),
             ('tag', '=', 'outside'),
             ('tag', '=', 'camping')])

The queries supported by merged built-in indexes complete the set of queries required by the Photo filtering feature. Note that supporting the Photo filtering feature did not require any additional composite indexes.

When selecting the optimal indexes for your app, it's important to understand the index merging feature. Index merging gives Firestore in Datastore mode greater query flexibility but with a possible trade-off to performance. The next section describes the performance of index merging, and how to improve performance by adding composite indexes.

Optimizing your index selection

This section describes the performance characteristics of index merging and two optimization opportunities related to index merging:

  • Add composite indexes to speed up queries that rely on merged indexes
  • Reduce the number composite indexes by leveraging merged indexes

Index merge performance

In an index merge, Firestore in Datastore mode efficiently merges indexes using a zig-zag merge join algorithm. Using this algorithm, Datastore mode joins potential matches from multiple index scans to produce a result set that matches a query. Index merging combines filter components at read-time instead of write-time. Unlike most Firestore in Datastore mode queries where performance depends only on the size of the result set, performance for index-merge queries depends on the filters in the query and how many potential matches the database considers.

The best-case performance of an index merge happens when every potential match in an index satisfies the query filters. In this case, performance is O(R * I) where R is the size of the result set and I is the number of indexes scanned.

The worst-case performance happens when the database must consider many potential matches but few of them satisfy the query filters. In this case, performance is O(S) where S is the size of the smallest set of potential entities from a single index scan.

The actual performance depends on the shape of the data. The average number of entities considered for each result returned is O(S/(R * I)). Queries perform worse when many entities match each index scan but few entities match the query as a whole, meaning R is small and S is large.

Four things mitigate this risk:

  • The query planner does not look up an entity until it knows the entity matches the entire query.

  • The zig-zag algorithm does not need to find all results to return the next result. If you request the first 10 results, you only pay the latency for finding those 10 results.

  • The zig-zag algorithm skips large sections of false positive results. The worst-case performance happens only if false positive results are perfectly interwoven (in sort order) between scans.

  • Latency depends on the number of entities found in each index scan, not the number of entities that match each filter. As shown in the next section, you can add composite indexes to improve index merge performance.

Speeding up an index-merge query

When Firestore in Datastore mode merges indexes, each index scan often maps to a single filter in the query. You can improve query performance by adding composite indexes that match multiple filters in the query.

Consider this query:


query_owner_size_tag = client.query(
    filters=[('owner_id', '=', 'username'),
             ('size', '=', 2),
             ('tag', '=', 'family')])

Each filter maps to one index scan in the following built-in indexes:

Index(Photo, owner_id)
Index(Photo, size)
Index(Photo, tag)

If you add the composite index Index(Photo, owner_id, size), the query maps to two index scans instead of three:

#  Satisfies both 'owner_id=username' and 'size=1'
Index(Photo, owner_id, size)
Index(Photo, tag)

Consider a scenario with many large images, many black-and-white images, but few large, panoramic images. A query filtering for both panoramic and black-and-white images will be slow if it merges built-in indexes:


query_size_coloration = client.query(
    filters=[('size', '=', 2),
             ('coloration', '=', 1)])

To improve query performance, you can lowers the value of S (smallest set of entities in a single index scan) in O(S/(R * I)) by adding the following composite index:

Index(Photo, size, coloration)

Compared to using two built-in indexes, this composite index produces fewer potential results for the same two query filters. This approach substantially improves performance at the cost of one more index.

Reducing the number of composite indexes with index merging

Although composite indexes that exactly match the filters in a query perform best, it's not always best or possible to add a composite index for every combination of filters. You must balance your composite indexes against the following:

  • Composite index limits:

    Limit Amount
    Maximum number of composite indexes for a project 200
    Maximum sum of the sizes of an entity's composite index entries 2 MiB
    Maximum sum of the following for an entity:
    • the number of indexed property values
    • the number of composite index entries
  • Storage costs of each additional index.

Indexing issues often arise with multi-value fields like the tag property of the Photo entities.

For example, imagine that the Photo filtering feature now needs to support descending ordering clauses based on four additional properties:

Property Value type Description
date_added Integer Date/time
rating Float Aggregate user rating
comment_count Integer Number of comments
download_count Integer Number of downloads

If you disregard the tag field, it's possible to select composite indexes that match every combination of Photo filters:

Index(Photo, owner_id, -date_added)
Index(Photo, owner_id, -comments)
Index(Photo, size, -date_added)
Index(Photo, size, -comments)
Index(Photo, owner_id, size, -date_added)
Index(Photo, owner_id, size, -comments)
Index(Photo, owner_id, size, coloration, -date_added)
Index(Photo, owner_id, size, coloration, -comments)

The total number of composite indexes is:

2^(number of filters) * (number of different orders) = 2 ^ 3 * 4 = 32 composite indexes

If you try to support up to 3 tag filters, the total number of composite indexes exceeds the limit of 200:

2 ^ (3 + 3 tag filters) * 4 = 256 indexes.

Indexes that include multi-value properties like tag also lead to exploding index issues that increase storage costs.

To support filters on the tag field for this feature, you can reduce the total number of indexes by relying on merged indexes. The following set of composite indexes is the minimum required to support the Photo filtering feature with ordering:

Index(Photo, owner_id, -date_added)
Index(Photo, owner_id, -rating)
Index(Photo, owner_id, -comments)
Index(Photo, owner_id, -downloads)
Index(Photo, size, -date_added)
Index(Photo, size, -rating)
Index(Photo, size, -comments)
Index(Photo, size, -downloads)
Index(Photo, tag, -date_added)
Index(Photo, tag, -rating)
Index(Photo, tag, -comments)
Index(Photo, tag, -downloads)

The number of composite indexes defined is:

(number of filters + 1) * (number of orders) = 7 * 4 = 28

Index merging also provides the following benefits:

  • Allows a Photo entity to support up to 1000 tags with no limit on the number of tag filters per query.
  • Reduces the total number of indexes which reduces storage costs.

Selecting indexes for your app

You can select optimal indexes for your Datastore mode database by using two approaches:

  • Use index merging to support additional queries

    • Requires less composite indexes
    • Reduces storage cost per entity
    • Avoids exploding indexes
    • Performance depends on the shape of the data
  • Define a composite index that matches multiple filters in a query

    • Improves query performance
    • Consistent query performance that's not dependent on shape of data
    • Must stay under the limit of 200 composite indexes
    • Increased storage cost per entity

When figuring out the optimal indexes for your app, the answer can change as the shape of your data changes. Sampling query performance gives you a good idea of your app's common queries and its slow queries. With this information, you can add indexes to improve the performance of queries that are both common and slow.