Materialized view pattern

Materialized view pattern TL,DR; Versus "normal" view

A normal view provides ease-of-use and flexibility features, but it DOES NOT speed up data access
A Materialized view is the cache of views. It pre-computes, stores, and optimizes data access when created, and automatically refreshes to ensure real-time data availability

Usecases

In data warehouses that have a large number of complex queries on large tables, consume a lot of time & resource, materialized views can eliminate the overhead of expensive joins and aggregations by responding to queries by pre-computed results.
Especially useful for queries that can be anticipated and repeatedly use the same subquery results.

Two main refresh strategies 1/ Complete refresh

Running within one transaction
At the beginning, the old data of the materialized view is deleted
Then, the new data is inserted by running the underlying SQL query.
At the end of the refresh, the transaction is committed, and the new data is visible for all users. Pros: During this process, users can still use the materialized view and see the old data Cons: This process can take a long time as the number of rows that the materialized view contains

2/ Fast refresh (“incremental refresh” would be more appropriate) In most cases, this method is much faster than a Complete Refresh

A fast refresh requires having a materialized view log on each of the source tables that are referenced in the materialized view
There are several preconditions to enable Fast Refresh, and if only one of them is missing, the Fast Refresh method does not work (can debug by using dbms_mview.explain_mview)

References

#data @brain master