Question

I have strange issue with nested left-joins in postgresql… It’s hard to explain, but easy to show =) here we are:

SELECT * FROM
(
    SELECT 1 as key1
) sub1
LEFT JOIN 
(
    SELECT sub3.key3, sub4.value2 FROM
    (
        SELECT 1 as key3
    ) sub3
    LEFT JOIN 
    (
        SELECT sub5.key5, COALESCE(sub6.value1, 1) as value2
        FROM
        (
            SELECT 1 as key5
        ) sub5
        LEFT JOIN
        (
            SELECT 1 as key6, value1
            FROM
            (
                SELECT NULL::integer as value1
            ) sub7
            WHERE false
        ) sub6 ON false

    )
    sub4 ON sub4.key5=sub3.key3
)
sub2 ON sub1.key1 = sub2.key3

The result of this query:

key1;key3;value2
1;1;NULL

And this is the problem – value2 can’t be NULL because of COALESCE in sub4 (at least I think that it can’t be =))
Anyway if we’ll change

SELECT sub3.key3, sub4.value2 FROM

with

SELECT sub3.key3, value2 FROM

we will got correct result:

key1;key3;value2
1;1;1

Is there something wrong with my mind&hands? or is it a bug?

Thanks in advance!

Answer 1

I got answer from postgresql team.
Verdict:

This is on HEAD from today.  Clearly there's a problem.

So, it was (and it is) a bug. Thanks for all, who participated in this issue! =)