Скачать или смотреть Understanding the NULL::character varying Default in PostgreSQL Alter Table Statements

Explore why PostgreSQL sets default values to 'NULL::character varying' in alter table statements and how to resolve it effectively.
---
This video is based on the question https://stackoverflow.com/q/67701577/ asked by the user 'Thor A. Pedersen' ( https://stackoverflow.com/u/900774/ ) and on the answer https://stackoverflow.com/a/67702994/ provided by the user 'Laurenz Albe' ( https://stackoverflow.com/u/6464308/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Postgres alter table defaults to 'NULL::character varying' instead of NULL

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the NULL::character varying Default in PostgreSQL Alter Table Statements

When working with database management, particularly in PostgreSQL, you may encounter some unexpected outputs when altering tables. One such instance is encountering a default value set to 'NULL::character varying' instead of a straightforward NULL. This issue can often leave users puzzled, especially when they see that other varchar fields have NULL expressed correctly in their definitions. In this post, we will delve into this peculiar behavior, clarify why it occurs, and suggest a method to fix it should you find it visually displeasing.

The Problem Explained

You might be running a simple alter statement like the following to add a new column to your database table:

[[See Video to Reveal this Text or Code Snippet]]

However, after executing this statement, you might notice that the CREATE TABLE statement reveals the new column as:

[[See Video to Reveal this Text or Code Snippet]]

What’s Going On Here?

The key to understanding this behavior lies in how PostgreSQL handles default values for columns. When you explicitly set a default value to NULL, PostgreSQL interprets this choice as following the syntax of NULL::character varying. The "::" signifies a type cast, which is a way for PostgreSQL to ensure that the value fits the column’s data type correctly.

It can be confusing because you might have expected it to remain NULL as seen in other columns. So why does PostgreSQL choose this representation? Here’s a breakdown:

Storage Mechanism: Default values in PostgreSQL are not stored as simple strings. Instead, they are stored in binary form, following a parse tree structure. This means that whenever you view the table definition, PostgreSQL "deparses" that binary data leading to a display format that may appear less than intuitive.

Consistency in Representation: The database is designed to maintain consistency for how type casts and defaults are represented, which can lead to outputs that seem over-complicated.

A Practical Solution

If the appearance of 'NULL::character varying' in your table definition seems unsatisfactory or unnecessary, there's a simple way to remedy this. You can drop the default value set for the column. Here's how you can do it:

[[See Video to Reveal this Text or Code Snippet]]

Key Points to Remember

Behavior Consistency: Removing the default value does not change the behavior of the column. PostgreSQL will still treat it as having a default value of NULL (the implicit default value) when no value is supplied during inserts.

No Data Loss: This operation is safe and does not impact any existing data or create any data loss.

In conclusion, encountering the NULL::character varying representation in your PostgreSQL ALTER TABLE statement can be perplexing at first glance. Understanding the underlying mechanics can alleviate the confusion, and adjusting the default value is a straightforward fix for those who prefer a cleaner output. Next time you encounter this issue, you’ll know exactly what to do!