How to Optimize View Performance When WHERE Clause Impacts Count Queries?

MySQL View Performance Optimization

Question

A query for counting users in a specific state is showing a higher cost with a view compared to a direct table scan. How does the WHERE clause work in views, and how can this performance issue be resolved?

Answer

The behavior of the WHERE clause in views is determined by the view algorithm used.

Typically, in a view, the WHERE clause is applied after the view retrieves all rows from the underlying table. This algorithm is known as temptable. However, in this case, the temptable algorithm was not used. Instead, the merge algorithm was employed.

The merge algorithm is more efficient in most cases but is not supported when the view contains aggregate functions (like COUNT()) or a GROUP BY clause.

To fix this issue, the view definition can be modified to replace COUNT(*) with COUNT(DISTINCT state). This will force MySQL to use the merge algorithm, which can significantly improve performance.

<code class="sql">CREATE OR REPLACE VIEW vw_users AS
SELECT
  state,
  COUNT(DISTINCT state) AS cnt
FROM
  users;

EXPLAIN
SELECT
  cnt
FROM
  vw_users
WHERE
  state = 'ca';</code>

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