Re: Unexpected table size usage for small composite arrays
| От | Erik Sjoblom |
|---|---|
| Тема | Re: Unexpected table size usage for small composite arrays |
| Дата | |
| Msg-id | CAAW=00UjRF8oE-A3BgzNYD_Tj+VLe0v0Hr1oyxhcbg8aXq1=Pg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Unexpected table size usage for small composite arrays (Tom Lane <tgl@sss.pgh.pa.us>) |
| Ответы |
Re: Unexpected table size usage for small composite arrays
Re: Unexpected table size usage for small composite arrays |
| Список | pgsql-hackers |
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@sss.pgh.pa.us> wrote:
Erik Sjoblom <sjoblom65@gmail.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
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