How Does Choosing Between INT and VARCHAR Affect SQL SELECT Query Speed?

Does Data Type Selection Affect SQL SELECT Query Speed?

When designing a relational database table, one often faces the choice between storing categorical data as integers (int) or character strings (varchar). While both options have their advantages and disadvantages, it's important to consider their potential impact on SQL query performance.

Integer vs Varchar for Categorical Data

For categorical data, such as car makes like "BMW" or "Audi," storing them as integers is a common practice. Ints are compact, using only 2-8 bytes of storage space. In contrast, varchars store the actual string value, which can vary in length and consume more space.

Query Speed Considerations

The choice between int and varchar can affect SQL SELECT query speed:

• Int Comparisons: Int comparisons, like WHERE make = 5, are generally faster than varchar comparisons, such as WHERE make = 'audi'. This is because ints take up less space and require less processing time to compare.
• Indexed Access: If the make column is indexed, both int and varchar comparisons can be performed efficiently. However, indexing an int column is typically faster than indexing a varchar column, as varchars introduce additional complexities related to string matching.

Space Implications

In PostgreQL, int fields use 2-8 bytes, while character types use 4 bytes plus the actual string length. For example, storing "BMW" as varchar requires at least 4 bytes (for the 3-character string) plus the overhead of storing the string length, while storing it as int using an appropriate code (e.g., 5) requires only 4 bytes.

Conclusion

When selecting a data type for categorical data, both performance and space considerations should be taken into account. For efficient query speed, ints are preferable for unindexed columns, while indexed int columns offer the optimal combination of speed and space efficiency. If space is a significant concern, varchars can be used, but they may impact query performance.

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