What is materialized view refresh?

Materialized view refresh is the process of updating a materialized view's data to reflect changes made in its underlying base tables. Unlike a regular view, which is a stored query, a materialized view stores the query results physically, making its data potentially stale if the source tables are modified. The refresh operation ensures the materialized view contains the most current data according to its defining query.

What is a Materialized View?

A materialized view (MV) is a database object that contains the results of a query. It's essentially a pre-computed cache of data derived from one or more base tables. MVs are often used to improve query performance by pre-calculating expensive joins or aggregations, thereby reducing the need for these operations at query time.

The Need for Refresh

Since a materialized view stores a physical copy of data, any modifications (INSERT, UPDATE, DELETE) to the underlying base tables will cause the data in the materialized view to become out-of-sync or 'stale'. To ensure that queries against the materialized view return current and accurate results, its data must be periodically updated, which is what the refresh process accomplishes.

Types of Refresh

There are primarily two main types of refresh operations, with different triggers for execution:

• Complete Refresh: This type rebuilds the materialized view from scratch by re-executing its defining query against the base tables. It's generally slower and resource-intensive, especially for large MVs, but guarantees data consistency.
• Fast (Incremental) Refresh: This method only applies the changes that have occurred in the base tables since the last refresh. It requires specific conditions (like log tables or rowid information) on the base tables and is much faster than a complete refresh, making it suitable for frequent updates.
• On Demand Refresh: The refresh operation is manually triggered by a user or an automated job (e.g., via a scheduler). This gives control over when the refresh occurs, typically during off-peak hours.
• On Commit Refresh: The refresh occurs automatically whenever a transaction commits changes to the underlying base tables. This keeps the materialized view almost instantly up-to-date but can add overhead to transaction commit times, making it suitable for smaller, frequently updated MVs or scenarios where near real-time consistency is critical.

When to Refresh

The frequency and type of refresh depend on several factors: the acceptable data staleness, the volume and frequency of changes in base tables, the performance impact of the refresh operation, and available system resources. For critical real-time reporting, fast refresh on commit might be chosen. For less critical, large datasets, a complete refresh during nightly batch windows might be sufficient.

SQL Commands for Refresh

REFRESH MATERIALIZED VIEW materialized_view_name;

-- Example for specific refresh type (Oracle syntax)
EXEC DBMS_MVIEW.REFRESH('materialized_view_name', 'C'); -- 'C' for complete refresh
EXEC DBMS_MVIEW.REFRESH('materialized_view_name', 'F'); -- 'F' for fast refresh

The exact syntax for refreshing a materialized view can vary slightly between different SQL database systems (e.g., Oracle, PostgreSQL, SQL Server). Generally, a simple REFRESH MATERIALIZED VIEW statement is used. In systems like Oracle, specific procedures or options are available to specify the refresh method (complete or fast) and timing (on commit or on demand).