PostgresDB 21000: Cardinality Violation

What is PostgresDB 21000: Cardinality Violation

When you encounter the error 21000: Cardinality Violation in PostgreSQL, it typically means you've attempted an operation that expects a single result row, but the operation returned multiple rows. Here's what you can do immediately:

Identify the Query Causing the Error: Look at the application logs to find the specific query that caused the error. This will help you understand where the problem lies.Analyze the Query: Once you've identified the problematic query, analyze it to understand why it might be returning multiple rows. Look for SELECT statements within your query that should return a single row but might be returning multiple. Common areas to check are subqueries, especially those used with operators that expect a single value (=, >, <, etc.), or in places where a single row is expected like the SET clause of an UPDATE or the VALUES clause of an INSERT.Run the Query Manually: Isolate the part of the query causing issues and run it manually in your PostgreSQL client. For example, if you suspect a subquery is returning multiple rows, run just that subquery.SELECT * FROM (YOUR_SUSPECTED_SUBQUERY) AS sub;Check for Expected Cardinality: Use LIMIT to test the query's behavior, ensuring it should indeed return only one row. If it's logically supposed to return multiple rows, your query logic might need to be revised to ensure only one row is expected.(YOUR_QUERY) LIMIT 1;Debug the Query: If a subquery is the culprit, consider adding conditions to narrow down the result set to a single row. This might involve using DISTINCT, LIMIT, or more specific WHERE clauses.SELECT DISTINCT column FROM table WHERE condition;Or,SELECT column FROM table WHERE condition LIMIT 1;Fix and Test: After adjusting your query to ensure it returns a single row, test it in your development environment before deploying the changes to production. Review Schema and Data: If the issue persists, review your database schema and the data within the tables involved. Ensure that there are not unexpected duplicates or data integrity issues that might cause your query to behave unexpectedly.

By following these steps, you can diagnose and potentially solve the 21000: Cardinality Violation error in PostgreSQL.