On 2018-08-23 11:04:30 -0400, Tom Lane wrote: > "David G. Johnston" <david.g.johns...@gmail.com> writes: > > On Monday, August 20, 2018, Wu Ivy <ivywu...@gmail.com> wrote: > >> Why are SELECT query never marked nullable? > > > Basically the nullability property is used by the planner for optimization > > during the joining of physical tables. As soon as you try outputting > > columns the ability to enforce not null goes away because of, in > > particular, outer joins. While some changes could maybe be made the > > cost-benefit to do so doesn't seem favorable. > > A further thought on this is that really it's a historical accident that > the elements of tuple descriptors are exactly pg_attribute rows. There > are a *whole lot* of fields in pg_attribute that aren't especially > relevant to tuple sets generated on-the-fly within a query, and typically > won't get filled with anything except default values. The only fields > that really mean a lot for a dynamic tuple set are the data type and > values derived from that, and in some usages the column name.
And arguably there's a fair bit of redundancy in pg_attribute, just because it's convenient for tupledescs. Given that pg_attribute very commonly is the largest catalog table by far, that very well could use some attention. Without tupdescs in mind, there's really not much point for pg_attribute to repeat a good portion of pg_type again, for example, nor is attcacheoff really meaningful. > [ wanders away wondering if it'd be worth our time to design a new, > more compact TupleDesc struct without the meaningless fields ... ] Yes, I think it'd would be. Greetings, Andres Freund