How Does a SQL Engine Determine the Execution Order of Queries and Subqueries?

Query and Subquery Execution Order in SQL

A common question arises: how does a SQL engine determine the order in which queries and subqueries are executed?

Answer 4: Dynamic Query Interpretation

The correct answer is that there is no fixed sequence of interpretation. Instead, the query parser dynamically makes a decision based on the following factors:

Type of Subquery

• Correlated Subqueries: These are dependent on the outer query, requiring its execution first.
• Uncorrelated Subqueries: These are independent and can be executed before or after the outer query.

Cost Optimization

The SQL engine evaluates the potential execution costs and determines the most efficient sequence. For example, with an uncorrelated subquery and a small result set, it may be advantageous to execute the subquery first and store the results in memory.

Database Optimization

Databases employ proprietary algorithms and optimizations that may alter the execution order to enhance performance. These optimizations may include:

• Caching: Subquery results may be cached for subsequent reuse.
• Join Optimization: The order of joins can be adjusted to reduce processing time.
• Predicate Pushdown: Filter criteria from outer queries may be pushed down into subqueries.

To delve further into this topic, consider the following resource:

• [Subquery Execution Order in SQL](https://www.sqlshack.com/understanding-subquery-execution-order-in-sql-server/)

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