How Can Database Indexing Improve Query Performance?

Understanding Database Indexing

Introduction

Indexing is a fundamental concept in database management systems that significantly enhances query performance. By understanding how indexing works at a database-agnostic level, developers can optimize their database designs and improve query efficiency.

Why is Indexing Important?

Storing data on disk entails accessing entire blocks of data, known as disk blocks. Searching for a specific value in an unsorted table requires a linear search, which has a complexity of (N 1)/2 block accesses. If the search field is not sorted and contains non-unique values, the entire table must be scanned, requiring N block accesses.

Understanding Indexing

Indexing involves creating additional data structures that sort records based on specific fields. Each index entry consists of the field value and a pointer to the corresponding record. By sorting these index structures, binary searches can be performed, reducing the complexity to log2 N block accesses.

How Indexing Works

Consider a sample database table with five million rows and a record length of 204 bytes. The table size is approximately 1GB, using a 1,024-byte block size.

• Sorting vs Unsorting: A linear search on the sorted primary key field would require 500,000 block accesses, while a binary search would require only 20. Searching on an unsorted field with non-unique values would require N block accesses.
• Indexing: An index containing only the indexed field and a record pointer has a record size of 54 bytes. The index size is approximately 278,000 blocks. Searching on the indexed field using a binary search would require only 20 block accesses plus one additional access to retrieve the actual record.

When to Use Indexing

Indexing is beneficial when:

• Fields are frequently used in searches
• Fields have a high cardinality (i.e., they contain unique values)
• Query results are large (i.e., they retrieve many records)

However, it's important to weigh the disk space overhead and potential performance issues associated with maintaining too many indexes.

Conclusion

By understanding how indexing works, developers can make informed decisions on which fields to index and improve the efficiency of their database queries. Indexing plays a crucial role in optimizing database performance, particularly in applications where data sets are large and queries are frequent.

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