How to diagnose and fix the 42P19 invalid_recursion error code in Postgres. 

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The 42P19 error code in PostgreSQL, which stands for invalid_recursion, typically occurs when there is an issue with a recursive Common Table Expression (CTE) in a query. A recursive CTE is a CTE that references itself to repeatedly execute subqueries to return a hierarchical or iterative set of results.

To diagnose and fix a 42P19 error, you should:

  1. Understand Recursive CTE Structure: Ensure that your recursive CTE has two parts: the initial, non-recursive term (the anchor), and the recursive term, which must be joined with the anchor using a UNION or UNION ALL.
  2. Check Recursive References: The recursive term should reference the CTE only once. Multiple references to the recursive CTE within the recursive term can cause the 42P19 error.
  3. Review Recursive Conditions: Make sure that the recursive term has a condition that will eventually lead to the termination of the recursion. Without such a condition, the query could result in infinite recursion.

Here are some examples and sample code to illustrate common issues and how to fix them:

Example 1: Multiple References to Recursive CTE

WITH RECURSIVE ancestor AS (
    SELECT id, parent_id FROM my_table WHERE parent_id IS NULL
    UNION ALL
    SELECT a.id, a.parent_id FROM my_table a, ancestor
    WHERE a.parent_id = ancestor.id AND ancestor.id > 0 -- This is incorrect
)
SELECT * FROM ancestor;

This example will cause a 42P19 error because the recursive CTE ancestor is referenced more than once in the recursive term. To fix this, ensure that the recursive CTE is referenced only once:

WITH RECURSIVE ancestor AS (
    SELECT id, parent_id FROM my_table WHERE parent_id IS NULL
    UNION ALL
    SELECT a.id, a.parent_id FROM my_table a
    JOIN ancestor ON a.parent_id = ancestor.id
)
SELECT * FROM ancestor;

Example 2: Incorrect Recursive Term Join

WITH RECURSIVE ancestor AS (
    SELECT id, parent_id FROM my_table WHERE parent_id IS NULL
    UNION ALL
    SELECT a.id, a.parent_id FROM ancestor, my_table a -- This is incorrect
    WHERE a.parent_id = ancestor.id
)
SELECT * FROM ancestor;

In this example, the recursive term incorrectly joins ancestor with my_table a, which will cause a 42P19 error. The recursive term should use a proper JOIN clause:

WITH RECURSIVE ancestor AS (
    SELECT id, parent_id FROM my_table WHERE parent_id IS NULL
    UNION ALL
    SELECT a.id, a.parent_id FROM my_table a
    JOIN ancestor ON a.parent_id = ancestor.id
)
SELECT * FROM ancestor;

Example 3: Infinite Recursion Without Termination

WITH RECURSIVE numbers AS (
    SELECT 1 AS n
    UNION ALL
    SELECT n + 1 FROM numbers -- This will cause infinite recursion without termination
)
SELECT * FROM numbers;

This query will result in an infinite loop because there is no termination condition for the recursion. To fix this, add a termination condition:

WITH RECURSIVE numbers AS (
    SELECT 1 AS n
    UNION ALL
    SELECT n + 1 FROM numbers WHERE n < 10 -- Termination condition added
)
SELECT * FROM numbers;

When working with recursive CTEs, it’s crucial to structure them correctly and ensure that they have proper termination conditions. For more information on writing recursive queries in PostgreSQL, you can refer to the official PostgreSQL Documentation on Recursive Queries. Additionally, community resources such as Stack Overflow provide practical examples and discussions that can help understand and resolve recursive CTE issues, as seen in a related Stack Overflow question about a 42P19 error encountered with recursive queries.

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