SELECT * vs. SELECT Column: What are the Performance Implications?

*Performance difference of `SELECT 与SELECT Column` in database query**

In a database query, the choice of using SELECT * (selects all columns) or SELECT Column (selects only the required columns) has an impact on performance. This article will analyze the performance differences between these two options.

Network overhead and memory usage

Using SELECT * will retrieve all columns in the database, even if only some of them are required. This increases network overhead because more data needs to be transmitted. Additionally, the database engine must process the entire tuple, resulting in increased memory usage to remove unnecessary columns.

Data Retrieval

Whether the database engine retrieves atomic tuples or only the requested columns depends on the database implementation and query optimizer. In some cases, the entire tuple is retrieved, resulting in the same I/O overhead regardless of the query type. However, in other cases, the database engine may optimize the query to retrieve only the specifically requested columns, thereby reducing I/O.

Summary of performance differences

Based on the above factors, SELECT Column queries generally have the following advantages over SELECT * queries:

• Reduce network overhead by transmitting fewer columns;
• Reduce memory usage by selectively retrieving data;
• Better performance may be achieved if the database can use a covering index for the requested column.

Conclusion

For best performance, it is recommended to avoid using SELECT * and instead specify only necessary columns in the query. This approach optimizes data retrieval, reduces network and memory overhead, and enables the database to utilize indexes more efficiently.

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