Re: Cluster table based on grand parent?

On 3/28/23 11:28, Dominique Devienne wrote:

On Tue, Mar 28, 2023 at 6:06 PM Ron <ronljohnsonjr@gmail.com> wrote:

You can only get from parent to grandchild via child.id to grandchild.parent, so why not cluster grandchild on grandchild.parent?

Hi. I don't understand your question. Yes, of course, if I want all grand-children of a given parent, I'll do for example:

select p.id, c.id, c.name, gc.*

  from  grandchild gc

   join child c on gc.parent = c.id

   join parent p on c.parent = p.id

where p.name = $1

But w/o clustering on a denormalized grandchild.grandparent FK column, as Peter showed,

and cluster only on grandchild.parent, that's not going to access a mostly continuous range

of pages to fetch those all grandchild rows for that one parent. But probably 10 to 50 "row-clusters",

given the fan-out I mentioned earlier at the child-table level. Or am I missing something?

No, you're not missing something.  If you want to go directly from grandparent to grandchild, then you need to put grandparent_id in the grandchild table.

Rob Sargent is right, too, though: practically it might not make a difference.  You've got to test.