Re: Cluster table based on grand parent?

[Rob Sargent <robjsargent@xxxxxxxxx>]

On 3/28/23 10:28, Dominique Devienne wrote:

On Tue, Mar 28, 2023 at 6:06 PM Ron <ronljohnsonjr@xxxxxxxxx> 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?

    Are you using HDD
      (spinning) or SSD discs?
      Is you world strictly three levels: grand,parent,child?
      What tests have you done so far to compare clustered to
      non-clustered?