What is the difference between Views and Materialized Views in Oracle?

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Oracle Views vs. Materialized Views: A Comprehensive Comparison

A conceptual diagram illustrating the difference between a standard view (dynamic query) and a materialized view (stored data snapshot) in an Oracle database environment. Shows data flowing from tables to a view, and separately to a materialized view with a refresh mechanism. Use blue for tables, green for views, orange for materialized views, and arrows for data flow.

Explore the fundamental differences between Oracle Views and Materialized Views, understanding their use cases, performance implications, and how to choose the right one for your database needs.

In Oracle databases, both Views and Materialized Views provide mechanisms to simplify complex queries and present data in a customized format. While they share a common goal of abstracting data, their underlying implementation, performance characteristics, and ideal use cases differ significantly. Understanding these distinctions is crucial for efficient database design and query optimization.

What is an Oracle View?

A View in Oracle is a logical table based on the result set of a SQL query. It does not store data itself but rather acts as a stored query. When you query a view, Oracle executes the underlying SQL statement defined in the view and returns the results. Views are primarily used for data abstraction, security, and simplifying complex queries.

CREATE VIEW employee_details_v AS
SELECT e.employee_id, e.first_name, e.last_name, d.department_name, j.job_title
FROM employees e
JOIN departments d ON e.department_id = d.department_id
JOIN jobs j ON e.job_id = j.job_id
WHERE e.status = 'ACTIVE';

Creating a simple view named employee_details_v

What is an Oracle Materialized View?

A Materialized View (MV), also known as a snapshot, is a physical copy of a table or the result of a query, stored on disk. Unlike a regular view, an MV actually stores data. This stored data can be refreshed periodically to reflect changes in the underlying base tables. Materialized Views are primarily used for query performance optimization, especially in data warehousing, distributed databases, and mobile computing environments where data needs to be readily available without re-executing complex queries.

CREATE MATERIALIZED VIEW sales_summary_mv
BUILD IMMEDIATE
REFRESH COMPLETE ON DEMAND
AS
SELECT p.product_category, SUM(s.amount) AS total_sales
FROM sales s
JOIN products p ON s.product_id = p.product_id
GROUP BY p.product_category;

Creating a materialized view named sales_summary_mv with immediate build and on-demand refresh

Key Differences and Use Cases

The core distinction lies in data storage and refresh mechanisms. Views are dynamic and always reflect the current state of the base tables, while Materialized Views store data and require explicit refresh operations. This fundamental difference dictates their optimal use cases.

A comparison table highlighting the differences between Oracle Views and Materialized Views across several criteria: Data Storage, Data Freshness, Performance, Complexity, and Use Cases. Use a two-column table format with clear headings for 'View' and 'Materialized View'.

Comparison of Oracle Views and Materialized Views

Data Storage

  • View: Does not store data. It's a virtual table.
  • Materialized View: Stores data physically on disk.

Data Freshness

  • View: Always up-to-date, reflecting real-time changes in base tables.
  • Materialized View: Data can be stale, depending on the last refresh. Requires explicit refresh to synchronize with base tables.

Performance

  • View: Query performance depends entirely on the complexity of the underlying query and the performance of the base tables. No performance gain for repeated complex queries.
  • Materialized View: Significantly improves read performance for complex queries, as the pre-computed results are stored. Trades off write performance (due to refresh) for read performance.

Complexity

  • View: Simpler to create and manage. No refresh strategy needed.
  • Materialized View: More complex to manage due to refresh strategies (e.g., FAST, COMPLETE, FORCE, ON COMMIT, ON DEMAND) and potential for stale data.

Use Cases

  • View: Data abstraction, security, simplifying complex queries, presenting data in different formats, renaming columns.
  • Materialized View: Data warehousing (summary tables), distributed databases (local copies of remote data), mobile computing (offline data access), query rewrite optimization, improving performance of aggregate queries or joins.

Choosing Between Views and Materialized Views

The decision to use a View or a Materialized View depends on your specific requirements:

  • Choose a View when:

    • You need real-time data and cannot tolerate any data staleness.
    • The underlying query is not excessively complex or frequently executed.
    • Your primary goal is data abstraction, security, or simplifying data access.
    • Storage space is a concern.

  • Choose a Materialized View when:

    • You need to improve the performance of complex, frequently executed queries (especially those involving aggregations, joins, or remote tables).
    • You can tolerate some data staleness (e.g., daily, hourly, or even minute-level staleness is acceptable).
    • You are working in a data warehousing environment where pre-calculated summaries are beneficial.
    • You need to store a local copy of data from a remote database.
    • Storage space is less of a concern than query performance.