Physical Query Plans and Operators
After a SQL query passes through logical planning, the database engine must decide how to actually execute each operation. A physical query plan is a detailed blueprint that specifies not only what operations to perform, but also how to perform them. Instead of just saying "join these tables," a physical plan selects specific algorithms for each operation—such as a hash join, nested loop join, index scan, or sequential scan. Each operator in the plan corresponds to a real execution strategy, tailored to the available data structures and indexes.
For example, when filtering employees by salary, the engine might use an index scan if an index exists on the salary column. This can be much faster than scanning the entire table, since the index allows the engine to jump directly to relevant rows.
1234-- Suppose an index exists on employees.salary. SELECT name, salary FROM employees WHERE salary > 70000;
The SQL engine often has several possible physical operators to choose from for the same logical operation. For instance, when joining two tables, the engine might consider a nested loop join, a merge join, or a hash join. The decision is based on factors like data size, indexing, and estimated execution cost. The optimizer evaluates these options and selects the operator that is expected to be most efficient for the current data and query structure.
1234-- Join employees and departments; physical plan may use nested loop or hash join. SELECT e.name, d.name AS department FROM employees e JOIN departments d ON e.department_id = d.id;
Understanding physical operators helps you predict why certain queries run faster than others and how indexes and data distribution affect performance. The database engine's optimizer analyzes the logical plan, considers available algorithms and data structures, and chooses the physical operators that are likely to yield the best performance.
1. What distinguishes a physical query plan from a logical query plan?
2. Which of the following is a physical operator in SQL execution?
3. Fill in the blank: The choice between a hash join and a nested loop join is made at the ________ planning stage.
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After a SQL query passes through logical planning, the database engine must decide how to actually execute each operation. A physical query plan is a detailed blueprint that specifies not only what operations to perform, but also how to perform them. Instead of just saying "join these tables," a physical plan selects specific algorithms for each operation—such as a hash join, nested loop join, index scan, or sequential scan. Each operator in the plan corresponds to a real execution strategy, tailored to the available data structures and indexes.
For example, when filtering employees by salary, the engine might use an index scan if an index exists on the salary column. This can be much faster than scanning the entire table, since the index allows the engine to jump directly to relevant rows.
1234-- Suppose an index exists on employees.salary. SELECT name, salary FROM employees WHERE salary > 70000;
The SQL engine often has several possible physical operators to choose from for the same logical operation. For instance, when joining two tables, the engine might consider a nested loop join, a merge join, or a hash join. The decision is based on factors like data size, indexing, and estimated execution cost. The optimizer evaluates these options and selects the operator that is expected to be most efficient for the current data and query structure.
1234-- Join employees and departments; physical plan may use nested loop or hash join. SELECT e.name, d.name AS department FROM employees e JOIN departments d ON e.department_id = d.id;
Understanding physical operators helps you predict why certain queries run faster than others and how indexes and data distribution affect performance. The database engine's optimizer analyzes the logical plan, considers available algorithms and data structures, and chooses the physical operators that are likely to yield the best performance.
1. What distinguishes a physical query plan from a logical query plan?
2. Which of the following is a physical operator in SQL execution?
3. Fill in the blank: The choice between a hash join and a nested loop join is made at the ________ planning stage.
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