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What are the Key Differences Between PRIMARY, UNIQUE, INDEX, and FULLTEXT Indexes in MySQL?

Barbara Streisand
Release: 2024-12-13 03:44:10
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What are the Key Differences Between PRIMARY, UNIQUE, INDEX, and FULLTEXT Indexes in MySQL?

Understanding MySQL Index Types: PRIMARY, UNIQUE, INDEX, and FULLTEXT

MySQL provides several index types that enhance database performance. This article explains the key differences between the four commonly used indexes: PRIMARY, UNIQUE, INDEX, and FULLTEXT.

PRIMARY Index

Similar to a UNIQUE index, a PRIMARY index ensures the uniqueness of all rows in the index. However, it has the following unique characteristics:

  • There can only be one PRIMARY index per table.
  • It is typically named 'PRIMARY' and provides a primary means of identifying rows.
  • It should be on the smallest number of columns that uniquely define a row.
  • Some database systems store table data within the PRIMARY index's B-tree structure.

UNIQUE Index

A UNIQUE index enforces the uniqueness of rows within the specified columns. It:

  • Prevents identical non-NULL values in multiple rows.
  • Allows for NULL values, potentially allowing multiple rows with NULLs to be identical.
  • Can be used for data restraints by preventing duplicate records.

INDEX (Non-Unique)

A basic INDEX or KEY type refers to a non-unique index. It:

  • Allows non-distinct values in the index.
  • Does not enforce any data constraints.
  • Is used solely for faster data access and retrieval.

FULLTEXT Index

Unlike the previous types, a FULLTEXT index is specialized for full-text search functionality:

  • Indexes the full text content of columns, such as strings.
  • Facilitates efficient searching using the MATCH() / AGAINST() clause.
  • Does not follow the same column ordering rules as other index types.

Similarities

Despite their differences, all of these index types share the following characteristics:

  • Can contain multiple columns.
  • Require column order to match query criteria, starting from the leftmost column.

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