What is the Maximum Number of Columns Retrievable in a PostgreSQL SELECT Query?

Maximum number of columns in PostgreSQL SELECT query

Understanding the limitations of a database system is critical to planning project requirements. When working with PostgreSQL, a common question is: How many columns can be retrieved with a single SELECT query?

Maximum number of columns

According to the PostgreSQL documentation, the maximum number of columns that a SELECT query can return is between 250 and 1600, with the exact value depending on the column's data type.

Factors affecting the maximum number of columns

The maximum number of columns is limited by the PostgreSQL page size, which is set to 8KB by default. Each row in the table occupies one page, and the total size of all columns in a row cannot exceed 8KB.

Wider data types, such as large strings or arrays, reduce the maximum number of columns that can fit into a single row. This is because PostgreSQL uses a technology called TOAST to store large values ​​separately from row data. However, columns processed by TOAST still occupy some space within the row, affecting the space available for other columns.

Practical considerations

While it is technically possible to approach the maximum number of columns, this is generally not recommended. High column counts can negatively impact performance and maintenance.

Instead, consider using technologies such as arrays, composite types, or JSON/XML to store complex data in a single column. This approach helps keep the number of columns manageable while maintaining data integrity.

In some cases, it may be necessary to use spreadsheet analysis tools to identify potential problems in wide spreadsheets before converting them to a relational database. This helps avoid performance bottlenecks and ensure optimal database design.

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