SELECT * vs. SELECT column: When Does Retrieving All Columns Impact Performance and Memory?

*SELECT Pitfalls: Performance Impact and Memory Consumption**

In a database query, choosing to use SELECT * (retrieve all columns) or SELECT column (specify a specific column) has an impact on performance and resource utilization.

Performance impact:

• Heavy I/O load: SELECT * retrieves all columns, including unnecessary columns. This increases network traffic and can cause I/O bottlenecks if the amount of unwanted data is large.
• Disk read: Databases typically retrieve the entire tuple from disk rather than just fetching the requested columns. Therefore, SELECT * incurs the same I/O overhead as selecting all columns.

Memory consumption:

• Tuple filtering: If the database engine fetches the entire tuple, then SELECT column requires additional memory to fetch the required columns.
• Index Optimization: SELECT * may impact performance if the non-clustered index can cover the query (i.e. include all requested columns). By specifying only the necessary columns, you enable the database to utilize the index efficiently.

*Reasons to avoid using SELECT : **

Beyond performance considerations, there are other reasons to avoid using SELECT * in production code:

• Database lookup is overloaded: SELECT * forces the database to check the table definition for all columns, adding unnecessary overhead.
• Data order dependence: It is dangerous to rely on the order of returned columns if the table structure changes.
• Plan Freeze: SELECT * prevents the query optimizer from selecting the best execution plan.

Conclusion:

While SELECT looks like a shortcut, it often incurs a performance hit and increases memory consumption. By specifying only the necessary columns, you can optimize the speed, efficiency, and flexibility of your database queries. Therefore, always prefer SELECT column over SELECT in production code to avoid these potential pitfalls.

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