Enhancing Data Integrity: How to Get Enum Values in PostgreSQL

In PostgreSQL, enums (enumerated types) offer a way to define a set of predefined string literals that a column can hold. This enforces data integrity by restricting the column's values to the explicitly listed options. It also improves code readability, as developers can use meaningful enum labels instead of raw string values.

Retrieving Enum Values with information_schema

PostgreSQL provides a built-in schema called information_schema that stores information about the database itself, including details about user-defined data types like enums. To get all allowed values for a specific enum type, you can leverage the enum_range function:

SELECT enum_label AS enum_value
FROM information_schema.enum_type et
WHERE et.enum_name = 'your_enum_type_name';

Explanation:

1. SELECT enum_label AS enum_value: This part of the query selects the enum_label column from the result set and renames it to enum_value for clarity. The enum_label column stores the actual string values defined within the enum.
2. FROM information_schema.enum_type et: This specifies that we're retrieving data from the enum_type table within the information_schema schema. The alias et is used for convenience.
3. WHERE et.enum_name = 'your_enum_type_name': This filters the results to only include rows where the enum_name (the name you assigned to the enum type) matches the specified your_enum_type_name. Replace 'your_enum_type_name' with the actual name of your enum type.

Example Scenario

Suppose you have an enum type named order_status defined with these possible values:

CREATE TYPE order_status AS ENUM ('pending', 'shipped', 'delivered', 'canceled');

Executing the Query

If you run the following query:

SELECT enum_label AS enum_value
FROM information_schema.enum_type et
WHERE et.enum_name = 'order_status';

You'll get the following output:

enum_value
----------
pending
shipped
delivered
canceled

This confirms that the order_status enum can have the values pending, shipped, delivered, and canceled.

Assuming you have an order_status enum type defined in your PostgreSQL database, here's how to retrieve its allowed values:

SELECT enum_label AS enum_value
FROM information_schema.enum_type et
WHERE et.enum_name = 'order_status';

1. SELECT enum_label AS enum_value: This selects the enum_label column and renames it to enum_value for clarity.
2. WHERE et.enum_name = 'order_status': This filters the results to only include rows where the enum_name matches 'order_status', which should be the actual name of your enum type.

Running the Query:

Execute this query in your PostgreSQL client tool. If the order_status enum exists with defined values, you'll see the following output:

enum_value
----------
pending
shipped
delivered
canceled  -- Assuming these are the defined values for order_status

PostgreSQL provides a built-in view named enum_typename_enum_vals (where typename is the actual name of your enum type). This view directly exposes the enum labels as separate columns. Here's how to use it:

SELECT *
FROM information_schema.enum_typename_enum_vals;

• SELECT *: This selects all columns from the view.
• FROM information_schema.enum_typename_enum_vals: This specifies the view to retrieve data from. Replace typename with the actual name of your enum type.

Example:

Assuming your enum type is named order_status, the query would be:

SELECT *
FROM information_schema.order_status_enum_vals;

This approach can be convenient if you want to access the enum values directly as separate columns without renaming. However, it requires memorizing the specific view name for each enum type.

Dynamic Query Construction (Less Recommended):

This method involves constructing a dynamic query string based on the enum name. It's generally less recommended due to potential for SQL injection vulnerabilities and reduced readability. However, it's included for completeness.

Steps:

1. Retrieve enum name: Use system functions like pg_catalog.current_schemas() and pg_catalog.enum_types() to get the list of enums and their names within your current schema.
2. Build query string: Construct a string with the table and column names of the information_schema.enum_type table, filtering by the retrieved enum name.
3. Execute dynamic query: Use functions like EXECUTE to dynamically execute the constructed string as a query.

Important Note:

• Security: Be cautious when using dynamic queries. Ensure proper input validation and escaping mechanisms to prevent SQL injection vulnerabilities.
• Readability: This method can make code less readable compared to the information_schema approaches.