On Jul 23, 2009, at 9:34 PM, Brendan Jurd wrote:
Well, a ROW is an ordered set of values, each one of which may be either NULL or NOT NULL.
Right.
It doesn't really make sense to talk about the ROW itself being NULL or NOT NULL, only its member values (but for extra confusion, contrast with the treatment of arrays, which can themselves be NULL).
Well then maybe a record (row) should *never* be null.
It does make sense, however, to talk about the ROW's member values being entirely NULL or entirely NOT NULL, and that's what the IS NULL and IS NOT NULL tests tell you about.
Ah! So that's where the three-valued logic comes in to play with records:
try=# SELECT ROW(1, NULL) IS NULL, ROW (1, 1) IS NULL, ROW(NULL, NULL) IS NULL;
?column? | ?column? | ?column? ----------+----------+---------- f | f | t
I guess the spec authors figured they might as well make IS [NOT] NULL do something useful when applied to a row rather than throwing an error. I tend to agree.
Frankly, I find the state where a record with a NULL and a not-null value being neither NULL nor not NULL bizarre.
I hope that provides some clarity.
It's useful to learn that `ROW(NULL, NULL)` is NULL, but I find the whole thing totally bizarre. Is it me?
Best, David -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
