Re: Data Type Size Calculation
| От | Jian He |
|---|---|
| Тема | Re: Data Type Size Calculation |
| Дата | |
| Msg-id | CAMV54g1zQO8HaH97SEaNLkrGAPgdh+bu0BK+maAAYUPy1R1KEQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Data Type Size Calculation (Troy Frericks <troy.frericks@gmail.com>) |
| Список | pgsql-docs |
https://www.depesz.com/2022/02/13/how-much-disk-space-you-can-save-by-using-int4-int-instead-of-int8-bigint/
Hope this link is useful.
create table testb as select 'true'::bool as b from generate_series(1,1000000) i; SELECT 1000000 $ \dt+ testb List of relations Schema │ Name │ Type │ Owner │ Persistence │ Access method │ Size │ Description ────────┼───────┼───────┼────────┼─────────────┼───────────────┼───────┼───────────── public │ testb │ table │ depesz │ permanent │ heap │ 35 MB │
Why is that, though? From what I gather the answer is: performance. I don't know low-level details, but based on what I understand, processors process data in arch-dependent block sizes. 64bit processor works on 64 bits. And this means that if you want to do something on int4 value, that is part of 8 byte block, you have to add operation to zero the other 32 bits.
On Wed, Feb 16, 2022 at 12:26 AM Troy Frericks <troy.frericks@gmail.com> wrote:
For now, yes... I'm suggesting that the documentation be completed by adding a few sentences few extra sentences.Troy.#On Mon, Feb 14, 2022, 12:51 Bruce Momjian <bruce@momjian.us> wrote:On Fri, Feb 11, 2022 at 08:12:08PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/13/datatype-numeric.html
> Description:
>
> > The actual storage requirement is two bytes for each group of four decimal
> digits, plus three to eight bytes overhead.
>
> Please describe what 'overhead' means.
>
> I'd like to be able to calculate the data size of NUMBER(19,4). I can
> calculate 2 bytes per 4 digits... with 19 digits, I have 5 groups of 4
> digits,
>
> so the data length I seek is 5 bytes + overhead... then I'm left hanging.
> :(
Well, you can create it and then call pg_column_size():
CREATE TABLE test (x NUMERIC(19,4));
SELECT pg_column_size('test.x');
pg_column_size
----------------
7
If you want more details, you will need to look at the source code.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.
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