Re: custom function for converting human readable sizes to bytes - Mailing list pgsql-hackers

On Wed, Dec 30, 2015 at 8:28 PM, Pavel Stehule <pavel.stehule@gmail.com> wrote:

2015-12-30 17:33 GMT+01:00 Robert Haas <robertmhaas@gmail.com>:

On Mon, Dec 28, 2015 at 8:45 AM, Shulgin, Oleksandr
<oleksandr.shulgin@zalando.de> wrote:
> I didn't check out earlier versions of this patch, but the latest one still
> changes pg_size_pretty() to emit PB suffix.
>
> I don't think it is worth it to throw a number of changes together like
> that.  We should focus on adding pg_size_bytes() first and make it
> compatible with both pg_size_pretty() and existing GUC units: that is
> support suffixes up to TB and make sure they have the meaning of powers of
> 2^10, not 10^3.  Re-using the table present in guc.c would be a plus.
>
> Next, we could think about adding handling of PB suffix on input and output,
> but I don't see a big problem if that is emitted as 1024TB or the user has
> to specify it as 1024TB in a GUC or argument to pg_size_bytes(): an minor
> inconvenience only.

+1 to everything in this email.

so I removed support for PB and SI units. Now the memory_unit_conversion_table is shared.

Looks better, thanks.

I'm not sure why the need to touch the regression test for pg_size_pretty():

!                10.5 | 10.5 bytes     | -10.5 bytes

!              1000.5 | 1000.5 bytes   | -1000.5 bytes

!           1000000.5 | 977 kB         | -977 kB

!        1000000000.5 | 954 MB         | -954 MB

!     1000000000000.5 | 931 GB         | -931 GB

!  1000000000000000.5 | 909 TB         | -909 TB

A nitpick, this loop:

+ while (*cp)

+ {

+ if ((isdigit(*cp) || *cp == '.') && ndigits < MAX_DIGITS)

+ digits[ndigits++] = *cp++;

+ else

+ break;

+ }

would be a bit easier to parse if spelled as:

+ while (*cp && (isdigit(*cp) || *cp == '.') && ndigits < MAX_DIGITS)

+ digits[ndigits++] = *cp++;

On the other hand, this seems to truncate the digits silently:

+ digits[ndigits] = '\0';

I don't think we want that, e.g:

postgres=# select pg_size_bytes('9223372036854775807.9');

ERROR:  invalid unit "9"

HINT:  Valid units for this parameter are "kB", "MB", "GB", and "TB".

I think making a mutable copy of the input string and truncating it before passing to numeric_in() would make more sense--no need to hard-code MAX_DIGITS.  The same goes for hard-coding MAX_UNIT_LEN, e.g. compare the following two outputs:

postgres=# select pg_size_bytes('1 KiB');

ERROR:  invalid unit "KiB"

HINT:  Valid units for this parameter are "kB", "MB", "GB", and "TB".

postgres=# select pg_size_bytes('1024 bytes');

ERROR:  invalid format

I believe we should see a similar error message and a hint in the latter case.  (No, I don't think we should add support for 'bytes' as a unit, not even for "compatibility" with pg_size_pretty()--for one, I don't think it would be wise to expect pg_size_bytes() to be able to deparse *every* possible output produced by pg_size_pretty() as it's purpose is human-readable display; also, pg_size_pretty() can easily produce output that doesn't fit into bigint type, or is just negative)

Code comments and doc change need proof-reading by a native English speaker, which I am not.

--

Alex