SELECT * vs. Explicit Column Selection: When Does It Really Matter?

*Tradeoffs between `SELECT ` and explicit column selection in SQL queries**

The use of SELECT * (select all columns) versus explicitly naming specific columns in a query has been controversial in the SQL world. As we all know, using SELECT * is generally not recommended due to performance issues. But when should you use explicit column selection, and does it matter in all cases?

Efficiency Dilemma

One of the main reasons to avoid using SELECT * is its potential impact on performance. When using SELECT *, the query engine must retrieve all columns in the table, even if only some of them are actually needed. For tables with many columns, this overhead can be significant, especially when the query involves filtering or sorting.

In contrast, explicit column selection allows the query engine to retrieve only the necessary data, thereby reducing the amount of data that needs to be transferred and processed. This can lead to significant performance improvements.

Index utilization

Another advantage of explicit column selection is that it improves index utilization. Indexes are physical structures that allow faster access to table data. However, they are only valid if the query conditions involve columns included in the index.

When using SELECT *, it is unlikely that all columns in the query will match the index columns. This reduces the likelihood that the query engine will be able to utilize the index, resulting in a more resource-intensive full-text scan.

Future-proof and consistent

Explicit column selection also provides advantages in terms of maintenance and code reliability. By naming specific columns in your query, you ensure that your code remains intact even if new columns are added to the table in the future. This reduces the risk of breaking queries and helps maintain consistent data structures.

Best Practices

Based on the above considerations, it is generally recommended to use explicit column selection in SQL queries whenever possible. This approach not only improves performance, but also improves maintainability and robustness.

Exceptions

While explicit column selection is the preferred method in most cases, there are some exceptions to consider:

• Temporary table or staging area: For fast and temporary data operations, you can use SELECT * if performance is not the main concern.
• Pattern Discovery or Exploratory Analysis: When you are exploring a new database or schema, SELECT * can provide an easy way to examine all columns in a table.

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