How to diagnose and fix the 21000 cardinality_violation error code in Postgres.

The 21000 error code in PostgreSQL, known as cardinality_violation, occurs when a query returns a different number of rows than expected in a situation where only a single row is permitted. This can happen in various scenarios, such as when a subquery used as an expression returns more than one row, or when a UNION operator is improperly used.

To diagnose and fix a cardinality_violation error, follow these steps:

  1. Identify the Query Causing the Error: Look at the query that is throwing the error. The error message will typically indicate which part of the query is responsible.
  2. Check Subqueries: If the error is due to a subquery that should return a single row but returns multiple, you need to modify the subquery to ensure it only returns one row. This can be done by using aggregation functions, adding a LIMIT 1 clause, or refining the WHERE clause to be more specific. Example:
   SELECT (SELECT store_key FROM store WHERE match_name = 'some_value') FROM sales;

If ‘some_value’ matches multiple rows in the store table, the above query will throw a cardinality_violation. To fix this, you could use a LIMIT 1 to ensure only one row is returned:

   SELECT (SELECT store_key FROM store WHERE match_name = 'some_value' LIMIT 1) FROM sales;
  1. Examine UNION Queries: Ensure that all SELECT statements within a UNION have the same number of columns in the result sets. The columns also need to have compatible data types.
  2. Check INSERT and UPDATE Statements: For INSERTSELECT or UPDATEFROM queries, ensure that the subquery returns a single row if that’s what the context requires.
  3. Review ON CONFLICT Clauses: When using ON CONFLICT DO UPDATE, make sure that there are no duplicate keys specified that could lead to a cardinality_violation.
  4. Use Error Handling: Implement error handling in your application to catch and properly deal with cardinality_violation errors.

For more specific guidance, you can refer to the PostgreSQL Error Codes documentation, which lists cardinality_violation under Class 21.

Remember that the exact solution will depend on the context of the query or the transaction that is causing the error. Careful examination and understanding of the SQL operation that is being performed will guide you to the appropriate fix.

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