Re: Unexpected table size usage for small composite arrays

Thanks Tom for your response!

Yes, I did expect that the first element should take 24+12 bytes and let's
round that to 50 bytes.
If I store another element, I would expect another 12. (or 16 depending on
padding) and take say ~65 bytes. I'm seeing close to 100 bytes.
If I have 3 elements, it's using 150, 4 -> 200, etc all the way up
to around 40 elements as it seems to hit the 2KB limit and starts
compressing the data.

I don't see why it's using 50 bytes per element. There should be just one
24 byte header for the array, not one per element

Anders

On Tue, Oct 22, 2024 at 6:34 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Erik Sjoblom <sjoblom65(at)gmail(dot)com> writes:
> > I’m observing a storage behavior with arrays in a table that differs from
> > my expectations, and I’d appreciate your insights. I was to store key
> value
> > pairs in a very dense data model. I don't haver the requirement of search
> > so that's why I was thinking an array of a composite type would work
> well.
> > I can see that padding might be involved using the int4 and int8
> > combination but there is more overhead. Anyone know where the following
> it
> > coming from?
>
> Composite values use the same 24-byte tuple headers as table rows do.
> So you'd be looking at 40 bytes per array element in this example.
> A large array of them would probably compress pretty well, but
> it's never going to be cheap.
>
> Can you store the int4's and int8's in two parallel arrays?
>
> regards, tom lane
>