Re: clarify equalTupleDescs()

On Tue, Feb 6, 2024 at 8:59 PM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> In a recent patch thread it was discussed[0] which fields should be
> compared by equalTupleDescs() and whether it is ok to remove a field
> from tuple descriptors and how that should affect their equality
> (attstattarget in that case).
>
> After analyzing all the callers, I have noticed that there are two
> classes of callers of equalTupleDescs():
>
> The first want to compare what I call row-type equality, which means
> they want to check specifically for equal number of attributes, and the
> same attribute names, types, and typmods for each attribute. Most
> callers actually want that behavior. The remaining callers just want to
> compare the tuple descriptors as they are, they don't care why the
> fields are in there, they just want to compare all of them.
>
> In the attached patch, I add a new function equalRowTypes() that is
> effectively a subset of equalTupleDescs() and switch most callers to that.
>
> The purpose of this patch is to make the semantics less uncertain.
> Questions like the one in [0] about attstattarget now have a clear
> answer for both functions. I think this would be useful to have, as we
> are thinking about more changes in pg_attribute and tuple descriptors.
>
>
> [0]:
> https://www.postgresql.org/message-id/202401101316.k4s3fomwjx52@alvherre.pgsql

function name record_type_typmod_hash imply that
hashRowType should also hash atttypmod field?

also:

bool
equalRowTypes(TupleDesc tupdesc1, TupleDesc tupdesc2)
{
if (tupdesc1->natts != tupdesc2->natts)
return false;
if (tupdesc1->tdtypeid != tupdesc2->tdtypeid)
return false;

for (int i = 0; i < tupdesc1->natts; i++)
{
Form_pg_attribute attr1 = TupleDescAttr(tupdesc1, i);
Form_pg_attribute attr2 = TupleDescAttr(tupdesc2, i);

if (strcmp(NameStr(attr1->attname), NameStr(attr2->attname)) != 0)
return false;
if (attr1->atttypid != attr2->atttypid)
return false;
if (attr1->atttypmod != attr2->atttypmod)
return false;
}

return true;
}

/*
* hashRowType
*
* If two tuple descriptors would be considered equal by equalRowTypes()
* then their hash value will be equal according to this function.
*/
uint32
hashRowType(TupleDesc desc)
{
uint32 s;
int i;

s = hash_combine(0, hash_uint32(desc->natts));
s = hash_combine(s, hash_uint32(desc->tdtypeid));
for (i = 0; i < desc->natts; ++i)
s = hash_combine(s, hash_uint32(TupleDescAttr(desc, i)->atttypid));

return s;
}

from the hashRowType comment, should we also hash attname and atttypmod?