The Role of Indexes in Query Plans
Indexes are essential tools in SQL databases that improve query performance by allowing the database engine to locate rows more efficiently. An index is a data structure, often a B-tree, that stores the values of one or more columns and provides quick lookup capabilities. There are several types of indexes:
- Primary indexes: automatically created on primary key columns;
- Unique indexes: enforce uniqueness on one or more columns;
- Composite indexes: cover multiple columns together;
- Full-text indexes: support searching within text fields;
- Regular (non-unique) indexes: created on columns to speed up searches, filtering, or sorting.
When you run a query, the query planner decides whether to use an index based on the available indexes and the conditions in your query. Using an index can drastically reduce the number of rows scanned, especially for large tables. If no suitable index exists, the database may perform a full table scan, which is much slower for large datasets.
12345678-- Query without an index on the genre column EXPLAIN SELECT * FROM books WHERE genre = 'Dystopian'; -- Create an index on the genre column CREATE INDEX idx_books_genre ON books(genre); -- Query with an index on the genre column EXPLAIN SELECT * FROM books WHERE genre = 'Dystopian';
When you examine the output of the EXPLAIN command, you will see several fields that indicate whether and how an index is being used. The key field shows the actual index that the query planner has chosen to use for your query. If this field is NULL, it means no index is being used and a full table scan is likely. The possible_keys field lists all the indexes that could potentially be used for the query, given the WHERE clause and the table's schema. If you see an index name in both possible_keys and key, it means the index is available and is being used.
12-- EXPLAIN output for a query using an index in the WHERE clause EXPLAIN SELECT * FROM books WHERE genre = 'Adventure';
1. What does the key field in EXPLAIN output represent?
2. How can you tell if an index is being used in a query plan?
3. Fill in the blank: Adding an index on a frequently filtered column can ______ query performance.
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Indexes are essential tools in SQL databases that improve query performance by allowing the database engine to locate rows more efficiently. An index is a data structure, often a B-tree, that stores the values of one or more columns and provides quick lookup capabilities. There are several types of indexes:
- Primary indexes: automatically created on primary key columns;
- Unique indexes: enforce uniqueness on one or more columns;
- Composite indexes: cover multiple columns together;
- Full-text indexes: support searching within text fields;
- Regular (non-unique) indexes: created on columns to speed up searches, filtering, or sorting.
When you run a query, the query planner decides whether to use an index based on the available indexes and the conditions in your query. Using an index can drastically reduce the number of rows scanned, especially for large tables. If no suitable index exists, the database may perform a full table scan, which is much slower for large datasets.
12345678-- Query without an index on the genre column EXPLAIN SELECT * FROM books WHERE genre = 'Dystopian'; -- Create an index on the genre column CREATE INDEX idx_books_genre ON books(genre); -- Query with an index on the genre column EXPLAIN SELECT * FROM books WHERE genre = 'Dystopian';
When you examine the output of the EXPLAIN command, you will see several fields that indicate whether and how an index is being used. The key field shows the actual index that the query planner has chosen to use for your query. If this field is NULL, it means no index is being used and a full table scan is likely. The possible_keys field lists all the indexes that could potentially be used for the query, given the WHERE clause and the table's schema. If you see an index name in both possible_keys and key, it means the index is available and is being used.
12-- EXPLAIN output for a query using an index in the WHERE clause EXPLAIN SELECT * FROM books WHERE genre = 'Adventure';
1. What does the key field in EXPLAIN output represent?
2. How can you tell if an index is being used in a query plan?
3. Fill in the blank: Adding an index on a frequently filtered column can ______ query performance.
Thanks for your feedback!
