MySQL VIRTUAL vs. STORED Column Indexing Indexes

Indexes in MySQL are used to speed up the retrieval of rows by creating a data structure that allows quick searches. VIRTUAL and STORED columns provide ways to define indexed generated columns, which are calculated from other columns in the table.

Usage

Indexes on VIRTUAL and STORED columns are used to optimize query performance by allowing faster access to computed data. VIRTUAL columns are calculated on the fly, while STORED columns are computed and stored physically in the table.

Syntax

CREATE TABLE table_name (
    column_name data_type,
    generated_column_name data_type [VIRTUAL | STORED] AS (expression) [INDEX],
    ...
);

In this syntax, `generated_column_name` can be indexed, and its value is derived from the `expression`. The keyword `VIRTUAL` or `STORED` specifies whether the column is computed on demand or stored.

Examples

1. Basic VIRTUAL Column Indexing

CREATE TABLE products (
    price DECIMAL(10, 2),
    quantity INT,
    total_cost DECIMAL(10, 2) AS (price * quantity) VIRTUAL,
    INDEX (total_cost)
);

In this example, `total_cost` is a VIRTUAL column that is indexed, allowing for efficient searches based on computed values.

2. STORED Column Indexing

CREATE TABLE sales (
    unit_price DECIMAL(10, 2),
    units_sold INT,
    revenue DECIMAL(10, 2) AS (unit_price * units_sold) STORED,
    INDEX (revenue)
);

This example creates an indexed STORED column `revenue`, which is physically stored in the table and can be queried quickly.

3. Use with Multiple Indexes

CREATE TABLE orders (
    item_price DECIMAL(10, 2),
    discount DECIMAL(5, 2),
    final_price DECIMAL(10, 2) AS (item_price - discount) STORED,
    quantity INT,
    total_value DECIMAL(10, 2) AS (final_price * quantity) VIRTUAL,
    INDEX idx_final_price (final_price),
    INDEX idx_total_value (total_value)
);

Here, both `final_price` as a STORED column and `total_value` as a VIRTUAL column are indexed for optimized queries involving these computed fields.

Additional Considerations

Version Compatibility

VIRTUAL and STORED columns were introduced in MySQL 5.7. Ensure your version supports these features if you plan to use them.

Performance Implications

Indexing VIRTUAL columns may impact read performance since values are calculated on the fly during query execution. STORED columns, while faster to read due to pre-computation, may increase write times and storage requirements.

Limitations and Restrictions

Not all expressions or data types are suitable for generated columns. Complex expressions or unsupported data types can lead to errors or performance issues. Generated columns cannot be used directly as primary keys or unique constraints.

Replication and Backup

Generated columns, particularly STORED ones, can affect replication and backup strategies. Ensure that the logic for generating column values is consistent across different database environments.

Tips and Best Practices

• Choose STORED for heavy calculations. Use STORED columns when the expression is computationally expensive and doesn't change often. Consider the frequency of updates to the underlying columns.
• Use VIRTUAL for lightweight calculations. Opt for VIRTUAL columns when recalculating values is not costly and the storage footprint needs to be minimized.
• Index selectively. Index only those generated columns that are frequently used in search or sort operations to minimize overhead.
• Consider storage implications. Understand that STORED columns increase table size, whereas VIRTUAL columns do not.
• Generated columns in constraints. Be cautious when using generated columns in primary keys or unique constraints as this can complicate schema design.