Re: Data Type Size Calculation

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.

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