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Table Indexing

1. What is Indexing?

An index is a data structure that allows the database to find and retrieve specific rows much faster than scanning the entire table. Think of it like the index in a book, which helps you quickly locate a topic without reading every page.

2. How Indexing Improves Query Performance

  • Indexes reduce the amount of data the database needs to scan:

  • Without an index: The database performs a full table scan, checking each row. With an index: The database can quickly locate the rows matching the query criteria.

3. Types of Indexes

  • Primary Index: Automatically created on the primary key.
  • Unique Index: Ensures that all values in a column are unique.
  • Composite Index: An index on multiple columns.
  • Full-Text Index: Optimized for text searching.
  • Clustered Index: Sorts the table data based on the index, improving range queries.
  • Non-Clustered Index: Stores pointers to the actual data, useful for specific lookups.

4. When to Use Indexes

  • Frequently queried columns: Columns used in WHERE, JOIN, ORDER BY, or GROUP BY clauses.
  • Foreign key columns: To speed up joins.
  • Large tables: To avoid full table scans.

5. Creating Indexes

In SQL, you can create an index using:

CREATE INDEX index_name ON table_name (column_name);

For a composite index:

CREATE INDEX index_name ON table_name (column1, column2);

6. Best Practices

  • Avoid too many indexes: Indexes use extra storage and slow down INSERT, UPDATE, and DELETE operations.
  • Choose columns wisely: Index columns that are frequently searched or sorted.
  • Use composite indexes appropriately: Order matters in composite indexes. For example, an index on (column1, column2) can be used for queries involving column1 or both, but not for column2 alone.
  • Monitor and fine-tune: Use tools like EXPLAIN or EXPLAIN PLAN to analyze query execution and understand how indexes are being used.
  • Index maintenance: Periodically rebuild or reorganize indexes for optimal performance.

7. Analyzing Query Performance

Run the query with EXPLAIN to check if indexes are being used effectively:

EXPLAIN SELECT * FROM table_name WHERE column_name = 'value';

This will show whether the database uses an index or performs a full table scan.

8. Downsides of Indexing

  • Storage overhead: Each index takes up additional disk space.
  • Slower write operations: Every INSERT, UPDATE, or DELETE requires updating the indexes.
  • Complexity in maintenance: Indexes require periodic maintenance to prevent fragmentation.

By strategically applying indexes and monitoring their effectiveness, you can significantly speed up query performance in your database.

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