Re: Problem with nested left-joins and coalesce

Hi!

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, 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!

 

Kind regards

Alex