Re: custom function for converting human readable sizes to bytes

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

>
>
> 2015-12-30 17:33 GMT+01:00 Robert Haas <robertmhaas(at)gmail(dot)com>:
>
>> On Mon, Dec 28, 2015 at 8:45 AM, Shulgin, Oleksandr
>> <oleksandr(dot)shulgin(at)zalando(dot)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