Why is 'SELECT *' Harmful to Database Performance and Maintainability?

*Avoid using “SELECT ”: comprehensive analysis of its negative effects**

In database queries, "SELECT " is a common practice, but its widespread use has raised concerns about its performance and code maintainability. This article delves into why "SELECT " is considered harmful and provides alternatives to ensure efficient data retrieval.

1. Inefficient data transmission:

"SELECT *" retrieves all columns in the table, even if the application uses only some of them. This excessive data movement places a burden on server and client machines, increasing latency and resource consumption. Additionally, adding new columns to a table may inadvertently introduce performance bottlenecks.

2. Index performance degradation:

Indices improve query performance by quickly identifying relevant rows of data. "SELECT *" prevents the creation of optimized indexes for the specific columns required. Adding more columns may invalidate these indexes, resulting in slower query execution times.

3. Binding issues and data corruption:

A "SELECT *" query may cause binding issues if multiple tables in the join share the same column names. This ambiguity can cause data consumers to crash, or produce meaningless results if the view is affected by changes in the underlying table structure.

* Acceptable scenarios for “SELECT ”: **

While using "SELECT *" is generally not recommended, there are some situations where it is appropriate:

• Ad hoc query: For quick debugging, it provides a comprehensive data snapshot of the table without having to drill down into column names.
• Counting rows or aggregates: In an aggregate function (such as COUNT()), "SELECT " explicitly expresses the intention to count rows, avoiding ambiguity caused by NULL values.
• Subquery to check if a row exists: In a subquery to check if a row exists, "SELECT *" simply means that a row exists without retrieving unnecessary columns.

The above is the detailed content of Why is 'SELECT *' Harmful to Database Performance and Maintainability?. For more information, please follow other related articles on the PHP Chinese website!