Introduction to materialized views

Materialized views are precomputed views that periodically store the results of a SQL query. In some use cases, materialized views reduce the total processing time and related charges by reducing the amount of data to be scanned for each query. You can query materialized views as you would other data resources.

Benefits of materialized views

The following use cases highlight the value of materialized views:

Pre-process data. Improve query performance by preparing aggregates, filters, joins, and clusters.
Dashboard acceleration. Empower BI tools like Looker that frequently query the same aggregate metrics—for example, daily active users.
Real-time analytics on large streams. Can provide faster responses on tables that receive high-velocity streaming data.
Cost management. Reduce the cost of repetitive, expensive queries over large datasets.

Key characteristics

Key characteristics of materialized views include the following:

Zero maintenance. BigQuery precomputes materialized views in the background when base tables change. BigQuery automatically adds incremental data changes from base tables to materialized views, with no user action required.
Fresh data. Materialized views return fresh data. If changes to base tables might invalidate the materialized view, then BigQuery reads the data directly from the base tables. If the changes to the base tables don't invalidate the materialized view, then BigQuery reads the rest of the data from the materialized view and reads only the changes from the base tables.
Smart tuning. If any part of a query against a base table can be resolved by querying the materialized view, then BigQuery reroutes the query to use the materialized view for improved performance and efficiency. For information about how and when smart tuning can improve queries, see Use materialized views.

Types of materialized views

There are two basic kinds of materialized views:

Incremental materialized views support a limited set of features. To learn more about supported SQL syntax for materialized views, see Create materialized views. Only incremental materialized views can take advantage of smart tuning.
Non-incremental materialized views support most of the syntaxes that incremental materialized views don't support.

When you create materialized views, by default BigQuery only lets you create views based on incremental queries. To create a non-incremental view, you can specify allow_non_incremental_definition = true in the materialized view's definition.

The best type of materialized view to use depends on your situation. The following table compares the features of incremental and non-incremental materialized views:

Category	Incremental	Non-incremental
Query supported	Limited	Most queries
Maintenance cost	Can reduce the cost of frequently used queries. To learn how materialized views are updated, see incremental updates.	Every refresh runs the full query.
Smart tuning support	Supported for most views queries.	No
Always fresh results	Supported. Incremental views return fresh query results even when the base tables have changed since the last refresh.	No

Authorized materialized views

You can create an authorized materialized view to share a subset of data from a source dataset to a view in a secondary dataset. You can then share this view with specific users and groups (principals). Principals can query the data you provide in a view, but they can't access the source dataset directly.

Authorized views and authorized materialized views are authorized in the same way. For details, see Authorized views.

Interaction with other BigQuery features

The following BigQuery features work transparently with materialized views:

Query plan explanation. The query plan shows which materialized views are scanned (if any), and how many bytes are read from the materialized views and base tables combined.
Query caching. The results of a query that BigQuery rewrites using a materialized view can be cached subject to the usual limitations (using deterministic functions, no streaming into the base tables, etc.).
Cost restriction. If you specify maximum bytes billed, and a query reads data beyond that limit, the query fails without incurring a charge whether the query uses materialized views, the base tables, or both.
Cost estimation using dry run. A dry run repeats query rewrite logic using the available materialized views and provides a cost estimate. You can use this feature as a way to test whether a specific query uses any materialized views.
Cross-region data replication. Materialized views can be created over BigQuery tables that have cross-region replication enabled, but only on the primary region. If you use the secondary region, you can encounter the following error message: The dataset replica of the cross region dataset {PROJECT}:{DATASET} in region {REGION} is read-only because it's not the primary replica.

In addition to these features, you can create materialized views over tables with specific features, as described in the following sections.

Tables with active change data capture

You can create materialized views over tables with active change data capture (CDC). These materialized views function like materialized views over BigQuery tables, including the benefits of automatic refresh. Materialized views can't perform runtime merge queries, so you must configure materialized views with a sufficient max_staleness to avoid runtime merge jobs. For more information, see Limitations of materialized views over tables with active change data capture.

BigLake metadata cache-enabled tables

Materialized views over BigLake metadata cache-enabled tables can reference structured data stored in Cloud Storage and Amazon Simple Storage Service (Amazon S3). These materialized views function like materialized views over BigQuery-managed storage tables, including the benefits of automatic refresh and smart tuning. Other benefits include pre-aggregating, pre-filtering, and pre-joining data stored outside of BigQuery. Materialized views over BigLake tables are stored in and have all of the characteristics of BigQuery managed storage.

When you create a materialized view over an Amazon S3 BigLake table, the data in the materialized view isn't available for joins with BigQuery data. To make Amazon S3 data in a materialized view available for joins, create a replica of the materialized view. You can only create materialized view replicas over authorized materialized views.

Limitations

Materialized views are subject to the following limitations:

Limits on base table references and other restrictions might apply. For more information about materialized view limits, see Quotas and limits.
You can't directly update or manipulate materialized view data using operations such as COPY, EXPORT, LOAD, WRITE, or data manipulation language (DML) statements.
You can't update the query for a materialized view after you create it.
A materialized view must reside in the same organization as its base tables, or in the same project if the project does not belong to an organization.
Materialized views use a restricted SQL syntax and a limited set of aggregation functions. For more information, see Materialized views query support.
You can't nest materialized views over other materialized views.
Materialized views can't query external or wildcard tables, logical views¹, or snapshots.
Materialized views don't support system variables—for example, the @@session_id system variable.
The value of the max_staleness option must be between 30 minutes and 3 days, inclusive.
Materialized views support only the GoogleSQL dialect.
You can set descriptions for materialized views, but not for individual columns in the materialized view.
If you delete a base table without first deleting the materialized view, queries and refreshes of the materialized view fail. If you recreate the base table, you must also recreate the materialized view.
Only non-incremental materialized views can have Spanner external dataset base tables. If a non-incremental materialized view's last refresh occurred outside the max_staleness interval, then the query reads the base Spanner external dataset tables. To learn more about Spanner external dataset tables, see Create materialized views over Spanner external datasets.
BigQuery doesn't cache query results if the query runs against non-incremental materialized views that reference Spanner external dataset tables.
Materialized views can't inherit or explicitly define parameterized data types, such as STRING(n), because parameterized data types are only supported for base table columns and script variables.

¹Logical view reference support is in preview. For more information, see Reference logical views.

Limitations of materialized views over tables with active CDC

Materialized views with active change data capture base tables have the following limitations:

If a materialized view has a base table with active change data capture, then you can't reference that table in a query that also references the materialized view.
When you create a materialized view over a table with active change data capture, the materialized view can't perform the runtime merge jobs of the underlying CDC table. Set the max_staleness value of the materialized view to at least twice the max_staleness value of the base table. Queries against a materialized view fail if the current version of the underlying CDC table is staler than the materialized view max_staleness.
You can't use smart tuning for materialized views over tables with active change data capture.

Limitations of materialized views over BigLake tables

Materialized views over BigLake tables have the following limitations:

You can't partition the materialized view. The base tables can use Hive partitioning but you can't partition materialized view storage in BigLake tables. This means that any deletion in a base table causes a full refresh of the materialized view. For more details, see Incremental updates.
The --max_staleness option value of the materialized view must be greater than that of the BigLake base table.
You can't perform a join between BigQuery managed tables and BigLake tables in a single materialized view definition.
BigQuery BI Engine doesn't support acceleration of materialized views over BigLake tables.

Materialized views pricing

Materialized views incur costs in the following ways:

Querying materialized views.
Maintaining materialized views, such as when materialized views are refreshed. The cost for automatic refresh is billed to the project where the view resides. The cost for manual refresh is billed to the project in which the manual refresh job is run. For more information about controlling maintenance cost, see Refresh job maintenance.
Storing materialized view tables.

The following table outlines the pricing components for materialized views:

Component	On-demand pricing	Capacity-based pricing
Querying	Bytes processed by materialized views and any necessary portions of the base tables.¹	Slots are consumed during query time.
Maintenance	Bytes processed during refresh time.	Slots are consumed during refresh time.
Storage	Bytes stored in materialized views.	Bytes stored in materialized views.

¹Where possible, BigQuery reads only the changes since the last time the view was refreshed. For more information, see Incremental updates.

Storage cost details

The way that BigQuery stores certain aggregate values affects how storage size is calculated. For AVG, ARRAY_AGG, and APPROX_COUNT_DISTINCT aggregate values in a materialized view, the final value isn't stored directly. Instead, BigQuery internally stores a materialized view as an intermediate sketch, which is used to produce the final value.

As an example, consider a materialized view that's created with the following command:

CREATE MATERIALIZED VIEW project-id.my_dataset.my_mv_table AS
SELECT date, AVG(net_paid) AS avg_paid
FROM project-id.my_dataset.my_base_table
GROUP BY date

While the avg_paid column appears as NUMERIC or FLOAT64, internally it is stored as BYTES, with its content being an intermediate sketch in a proprietary format. For data size calculation, the column is treated as BYTES.