hierarchical sum in PostgreSQL

Question

This is a simplified version of a problem I am encountering in PostgreSQL.

I have the following table A:

[ ID INTEGER | VALUE NUMERIC(10,2) | PARENT INTEGER ]

Where 'PARENT' is a self-referencing FK to column ID.

The table definition is:

CREATE TABLE A(ID INTEGER IDENTITY, VALUE NUMERIC(10,2), PARENT INTEGER)                                    
ALTER  TABLE A ADD CONSTRAINT FK FOREIGN KEY (PARENT) REFERENCES A(ID) 

This simple table allows one to define tree data structures of arbitrary depth. Now I need to write a SQL (I prefer not to use server-side PL-SQL) that reports for each node, the total value of the sub-tree "hanging" under it. For instance, with the following table:

|  ID  | VALUE | PARENT |
-------------------------
|   1  | NULL  | NULL   |
|   2  | 3.50  |    1   |
|   3  | NULL  | NULL   |
|   4  | NULL  |    3   |
|   5  | 1.50  |    4   |
|   6  | 2.20  |    4   |

I should get the following result set:

| ID  |  Total-Value-of-Subtree |
|  1  |                  3.50   |
|  2  |                  3.50   |
|  3  |                  3.70   |
|  4  |                  3.70   |
|  5  |                  1.50   |
|  6  |                  2.20   |

For simplicitly, you can assume that only leaf nodes have values, non-leaf nodes always have a value of NULL in the VALUE column. Is there a way to do this in SQL, even utilizing PostgreSQL-specific extensions?

Answer 1

Maybe something like this:

WITH RECURSIVE CTE
AS
(
    SELECT
        t.ID,
        t.VALUE,
        t.PARENT
    FROM
        t
    WHERE NOT EXISTS
        (
            SELECT NULL FROM t AS t2 WHERE t2.PARENT=t.ID
        )
    UNION ALL
    SELECT
        t.ID,
        COALESCE(t.VALUE,CTE.VALUE),
        t.PARENT
    FROM
        t
        JOIN CTE
            ON CTE.PARENT=t.ID
)
SELECT
    CTE.ID,
    SUM(CTE.VALUE)
FROM
    CTE
GROUP BY
    CTE.ID
ORDER BY 
    ID;

This will start with the children that has no children. Then go up the tree to the parents. The result will be like this:

1   3.50
2   3.50
3   3.70
4   3.70
5   1.50
6   2.20

Answer 2

In PostgreSQL you can use recursive CTEs (Common Table Expression) to walk trees in your queries.

Here are two relevant links into the docs:

• Syntax
• Examples

EDIT

Since there is no subselect required it might run a little better on a larger dataset than Arion's query.

WITH RECURSIVE children AS (
    -- select leaf nodes
    SELECT id, value, parent
        FROM t
        WHERE value IS NOT NULL
    UNION ALL
    -- propagate values of leaf nodes up, adding rows 
    SELECT t.id, children.value, t.parent
        FROM children JOIN t ON children.parent = t.id
)
SELECT id, sum(value) 
    FROM children 
    GROUP BY id   -- sum up appropriate rows
    ORDER BY id;