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