`SELECT *` vs. Explicit Column Selection: When is One More Efficient Than the Other?

SQL query efficiency: SELECT *with explicit column selection

Database query efficiency is an important consideration in SQL commands. There are two main ways to retrieve data from a table: SELECT * and explicit column selection. Some people think SELECT * is less efficient than explicitly specifying the required columns. But is the efficiency difference significant when selecting all columns in the table?

Efficiency considerations

SQL Server uses indexes to optimize data access. When a specific column is requested, SQL Server can leverage existing indexes to retrieve data faster. This is especially important for large tables where scanning the entire table contents is inefficient.

Stability and maintainability

Explicit column selection provides a more robust and maintainable approach. As the table schema evolves, adding or removing columns does not affect the structure of the data retrieved by queries that use explicit column selection. This reduces the risk of code breaking due to schema changes.

Optimization under specific circumstances

While SELECT * may seem more convenient, it is generally not recommended for optimal performance. However, there are exceptions. For example, if you determine that you always need all columns in a table, and the schema remains stable, SELECT * may provide slightly better performance by reducing the number of times SQL Server needs to parse the query syntax.

Conclusion

Given the advantages of index coverage, maintainability, and performance optimization, it is generally recommended to explicitly specify the required columns in the SELECT statement, even if all columns are retrieved. While SELECT * may be slightly efficient in special cases, in most cases its disadvantages outweigh its advantages.

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