NOT NULL markings for BKI columns

Hi,

8b38a538c0aa5a432dacd90f10805dc667a3d1a0 changed things so that all
columns are checked for NOT NULL ness. Which is fine in general, but it
currently does make it impossible to have a varlena column (except
OID/INT2 vector...) as a key column in syscache. Even if the column is
factually NOT NUL.

The rule for determining attribute's NOT NULL setting in bootstrap is:
/*
* Mark as "not null" if type is fixed-width and prior columns are too.
* This corresponds to case where column can be accessed directly via C
* struct declaration.
*
* oidvector and int2vector are also treated as not-nullable, even though
* they are no longer fixed-width.
*/
#define MARKNOTNULL(att) \
((att)->attlen > 0 || \
(att)->atttypid == OIDVECTOROID || \
(att)->atttypid == INT2VECTOROID)

if (MARKNOTNULL(attrtypes[attnum]))
{
int i;

for (i = 0; i < attnum; i++)
{
if (!MARKNOTNULL(attrtypes[i]))
break;
}
if (i == attnum)
attrtypes[attnum]->attnotnull = true;
}
(the rule is also encoded in genbki.pl)

Now, you can argue that it's a folly to use the syscache code to access
something via a text column (which is what I want to do). I don't agree,
but even if you're of that position, it seems worthwhile to mark further
catalog columns as NOT NULL in the catalog if that's what the code
expects?

E.g. pg_(sh)?seclabel.provider should certainly be NOT
NULL. pg_extension.extversion as well. There's a couple more I think.

So, how about providing bootstrap infrastructure for marking columns as
NOT NULL?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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