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SI Units for PostgreSQL

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SI Units for PostgreSQL

Christoph Berg [email protected]

postgresql-unit implements a PostgreSQL datatype for SI units, plus byte. The eight base units can be combined to arbitrarily complex derived units using operators defined in the PostgreSQL type system. SI and IEC binary prefixes are used for input and output, and quantities can be converted to arbitrary scale.

Unit and prefix definitions are retrieved from database tables, and new definitions can be added at run time. The extension comes with over 2500 units and over 100 prefixes found in the definitions.units file in GNU Units.

Requires PostgreSQL 9.5 or later (uses HASH_BLOBS), flex, and bison 3 (the pre-built grammar files are used if only bison 2 is available).

Changelog

Build Status

Features

  • Base types: meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd), byte (B)
  • derived units coherent with SI: radian (rad), steradian (sr), hertz (Hz), newton (N), pascal (Pa), joule (J), watt (W), coulomb (C), volt (V), farad (F), ohm (Ω), siemens (S), weber (Wb), tesla (T), henry (H), degree Celsius (°C, support limited), lumen (lm), lux (lx), becquerel (Bq), gray (Gy), sievert (Sv), katal (kat)
  • non-coherent units: minute (min), hour(h), day(d), hectare (ha), litre (l), tonne (t), bar, astronomical unit (au)
  • United States customary units: inch (in), foot (ft), yard (yd), mile (mi), ounce (oz), pound (lb)
  • about 2400 other units imported from GNU Units (including some humorous ones)
  • prefix multiples: da, h, k, M, G, T, P, E, Z, Y
  • prefix fractions: d, c, m, μ, n, p, f, a, z, y
  • IEC binary prefix multiples: Ki, Mi, Gi, Ti, Pi, Ei, Zi, Yi
  • other prefixes imported from GNU Units
  • operators: +, -, *, /, ^, conversion to arbitrary scale (@, @@)
  • aggregate functions
  • range type

Examples

# CREATE extension unit;
CREATE EXTENSION

SELECT '800 m'::unit + '500 m' AS length;

length

1.3 km

SELECT '120 km/h'::unit AS speed;

    speed

33.3333333333333 m/s

SELECT '9.81 N'::unit / 'kg' AS gravity;

gravity

9.81 m/s^2

SELECT '1 kilosecond'::unit AS time;

time

00:16:40 s

SELECT '2 MB/min'::unit @ 'GB/d' AS traffic;

traffic

2.88 GB/d

SELECT '1 hl'::unit @ '0.5 l' AS bottles_of_beer;

bottles_of_beer

200 * 0.5 l

SET unit.byte_output_iec = on;

SELECT '4 TB'::unit AS disk_sold_as_4tb;

disk_sold_as_4tb

3.63797880709171 TiB

SELECT '500 mi'::unit AS walk_500_miles;

walk_500_miles

804.672 km

SELECT unitrange('earthradius_polar', 'earthradius_equatorial') AS earthradius;

           earthradius

["6.35675174834046 Mm","6.37813649 Mm")

Installation

  • Homepage: https://github.com/df7cb/postgresql-unit
  • Debian package: https://tracker.debian.org/pkg/postgresql-unit
  • Arch package: https://aur.archlinux.org/packages/postgresql-unit/

To compile from source, install the PostgreSQL server headers,

flex
, and
bison
. Example on Debian:
sudo apt-get install build-essential postgresql-server-dev-10 flex bison
make PG_CONFIG=/usr/lib/postgresql/10/bin/pg_config
sudo make install PG_CONFIG=/usr/lib/postgresql/10/bin/pg_config

Config

  • unit.byte_output_iec
    : Set to
    on
    to format byte quantities using IEC binary prefixes (Ki, Mi, ...) on output by default. (Default: off)
  • unit.output_base_units
    : Set to
    on
    to format values using only base types without prefixes. (Default: off)
  • unit.output_superscript
    : Set to
    on
    to output unit exponents using Unicode superscripts. (Default: off)
  • unit.time_output_custom
    : Set to on to output time unit values greater or equal to 60 s using minutes, hours, days, and julianyears instead of seconds with SI prefixes. (Default: on)

Internal Representation

Internally,

unit
datums are a 16-byte struct consisting of a double precision floating point number for the unit value, and an array of 8 signed character values to store the exponents of the base units for the unit dimension.
typedef struct Unit {
    double      value;
    signed char units[N_UNITS];
} Unit;

All operators and functions operate with that base-units-only format. Conversion from and to other unit representations is done on input, output, and using the

@
operator only.

This is also the format used in binary mode in the PostgreSQL wire protocol.

Prefixes and Units Lookup Tables

Prefixes are defined in the table

unit_prefixes
:
CREATE TABLE unit_prefixes (
    prefix varchar(32) PRIMARY KEY,
    factor double precision NOT NULL,
    definition text, -- original definition, informational
    dump boolean DEFAULT true
);

Units are defined in the table

unit_units
:
CREATE TABLE unit_units (
    name varchar(32) PRIMARY KEY,
    unit unit NOT NULL,
    shift double precision, -- NULL means 0.0 here
    definition text, -- original definition, informational
    dump boolean DEFAULT true
);

Whenever the unit input lexer encounters a unit name, it queries the

unit_units
table to see if the definition is found. If no match is found, the
unit_prefixes
and
unit_units
tables are cross-joined to see if the concatenation of prefix and unit name matches. At this point, an error is thrown if the prefix-unit combination is ambiguous, e.g. for "dat" which is either a dekatonne (da-t) or a deciatmosphere (d-at). If no match is found, and the unit name is ending with 's' (e.g. "kilometers" or "lbs"), the two lookup steps are repeated with the trailing 's' removed.

If the unit definition could be resolved, the result is stored in a backend-local hash table to speed up the next lookup. (The function

unit_is_hashed()
reports if a given unit name is already cached. The function
unit_reset()
clears the hash table. Use it if unit definitions are changed.)

The

definition
column is only provided for information on how the unit was originally defined.

GNU Units

On installation, the prefixes and units definitions tables are populated with data imported from the

definitions.units
file found in the GNU Units tool. Notable omissions are currency units (we don't have a base type for them, and exchange rates aren't static anyway), and non-linear units such as dBm based on dB and other conversions based on functions and lookup tables.

The definitions.units file is an interesting read on its own due to extensive comments explaining the history and origin of the units covered.

The

unit_load()
function can be used to reload the
unit_prefixes.data
and
unit_units.data
files from disk. Extension upgrades use it to load newly added or changed definitions, but it is also user-callable. User-defined entries are preserved.

User-Defined Prefixes and Units

To create custom prefixes and units, insert new rows into the tables:

# SELECT '1 foobar'::unit;
ERROR:  unit "foobar" is not known
# INSERT INTO unit_prefixes VALUES ('foo', 42);
INSERT 0 1
# SELECT '1 foobar'::unit;
  unit
---------
 4.2 MPa

INSERT INTO unit_units VALUES ('legobrick', '9.6 mm');

SELECT '1 m'::unit @ 'legobricks' AS one_meter;

      one_meter

104.166666666667 legobricks

Note: If user-defined (or built-in) units are changed by updating the tables, call

unit_reset()
to clear the hash table that caches the lookup result. Otherwise, sessions that have already used the unit will continue to use the old definition.

Input Syntax

Unit values allow a fairly complex expression syntax on input.

  • expr expr
    denotes multiplication
  • operators + - * /
  • exponentiation:
    expr^integer
    or Unicode superscripts:
    expr⁺⁻⁰¹²³⁴⁵⁶⁷⁸⁹
  • parentheses ()
  • multiplication binds tighter than division such that
    kg/s^2*A
    can be written without parentheses
  • N|M
    denotes a numeric fraction, e.g.
    3|4
  • use
    hh:mm:ss[.sss]
    for time values, e.g.
    10:05:30 s

Note: This covers the unit input parser for expressions like

'1|2 m / h'::unit
. PostgreSQL operators on type unit values are a separate layer; PostgreSQL's operator precedence applies there.

Output Syntax

Internally, unit values are always stored in base units. On output, values are generally formatted as

+-N x*y/z*w
where
+-N
is a floating point number, and
x*y/z*w
is a set of base units with exponents. Specific combinations of base units are formatted differently (unless
unit.output_base_units
is set):
  • Time (seconds) is formatted as

    N commonyear + hh:mm:ss.sss s
    if the value is at least one minute, and
    unit.time_output_custom
    is set (the default). Otherwise, time is formatted as by the next rules.
  • If the set of base units matches one of the following well-known derived units, output is formatted using SI prefixes and that unit.

| Unit | Name | Dimension | Units | Base Units | | ---- | ------- | ------------------------------------- | ------ | ------------------ | | Hz | hertz | frequency | | s^-1 | | N | newton | force, weight | | kg·m·s^-2 | | Pa | pascal | pressure, stress | N/m^2 | kg·m^-1·s^-2 | | J | joule | energy, work, heat | N·m | kg·m^2·s^-2 | | W | watt | power, radiant flux | J/s | kg·m^2·s^-3 | | C | coulomb | electric charge | | s·A | | V | volt | voltage | W/A | kg·m^2·s^-3·A^-1 | | F | farad | electric capacitance | C/V | kg^-1·m^-2·s^4·A^2 | | Ω | ohm | electric resistance, impedance | V/A | kg·m^2·s^-3·A^-2 | | S | siemens | electrical conductance | A/V | kg^-1·m^-2·s^3·A^2 | | Wb | weber | magnetic flux | V·s | kg·m^2·s^-2·A^-1 | | T | tesla | magnetic flux density | Wb/m^2 | kg·s^-2·A^-1 | | H | henry | inductance | Wb/A | kg·m^2·s^-2·A^-2 | | lx | lux | illuminance | lm/m^2 | m^-2·cd | | Gy | gray | absorbed dose (of ionizing radiation) | J/kg | m^2·s^-2 | | kat | katal | catalytic activity | | mol·s^-1 |

Note: this does not preserve the input dimension, e.g.

N m
(torque) is converted to
J
(energy).
  • If the dimension numerator is exactly a base unit (with exponent 1), output is formatted using SI prefixes and that unit, followed by the denominator units, if any.

    • If that unit is Bytes, and
      unit.byte_output_iec
      is set, IEC prefixes are used instead.

Shifted Units

The

unit_units.shift
column is used for implementing units where 0 is not absolute zero in base units, i.e. most commonly for temperature units (°C, °F).

Shifted units are often used in ambiguous contexts, the intended meaning depending on if an absolute value, or a difference between values is requested. If

20 °C - 15 °C
is requested, the answer
5 K
is clearly correct, but less so
5 °C
, because that is actually 278.15 K. On the other hand, some thermodynamic units are defined based on °C, such as
celsiusheatunit = cal lb (degC) / gram K
where no offset is desired.

This module resolves the ambiguity by only applying the

shift
offset in number name (and plain name) expressions such as
5 °F
. In all other contexts, shifted units behave just like a unit defined on based units without a shift.
# SELECT '5 °F'::unit, '5 * °F'::unit, '5 °F'::unit @ '°C' AS to_celsius, '5 °F'::unit @ '1 * °C' AS celsius_increments;
   unit   |        unit        |      to_celsius      | celsius_increments
----------+--------------------+----------------------+--------------------
 258.15 K | 2.77777777777778 K | -14.9999999999999 °C | 258.15 * 1 * °C

PostgreSQL Operators and Functions

The

unit
extension provides the following objects:
           Objects in extension "unit"
                Object description
--------------------------------------------------
 function ampere(double precision)
 function au(double precision)
 function avg(unit)
 function becquerel(double precision)
 function byte(double precision)
 function candela(double precision)
 function cbrt(unit)
 function celsius(double precision)
 function coulomb(double precision)
 function day(double precision)
 function dbl_unit_div(double precision,unit)
 function dbl_unit_mul(double precision,unit)
 function decibel(double precision)
 function degree_arc(double precision)
 function dimension(unit)
 function farad(double precision)
 function gray(double precision)
 function hectare(double precision)
 function henry(double precision)
 function hertz(double precision)
 function hour(double precision)
 function joule(double precision)
 function katal(double precision)
 function kelvin(double precision)
 function kilogram(double precision)
 function liter(double precision)
 function lumen(double precision)
 function lux(double precision)
 function max(unit)
 function meter(double precision)
 function min(unit)
 function minute_arc(double precision)
 function minute(double precision)
 function mole(double precision)
 function newton(double precision)
 function ohm(double precision)
 function pascal(double precision)
 function radian(double precision)
 function round(unit)
 function second_arc(double precision)
 function second(double precision)
 function siemens(double precision)
 function sievert(double precision)
 function sqrt(unit)
 function stddev_pop(unit)
 function stddev_samp(unit)
 function stddev(unit)
 function steradian(double precision)
 function sum(unit)
 function tesla(double precision)
 function tonne(double precision)
 function unit_accum(unit_accum_t,unit)
 function unit_add(unit,unit)
 function unit_at_double(unit,text)
 function unit_at(unit,text)
 function unit_avg(unit_accum_t)
 function unit_cmp(unit,unit)
 function unit_dbl_div(unit,double precision)
 function unit_dbl_mul(unit,double precision)
 function unit_diff(unit,unit)
 function unit_div(unit,unit)
 function unit(double precision)
 function unit_eq(unit,unit)
 function unit_ge(unit,unit)
 function unit_greatest(unit,unit)
 function unit_gt(unit,unit)
 function unit_in(cstring)
 function unit_is_hashed(cstring)
 function unit_least(unit,unit)
 function unit_le(unit,unit)
 function unit_load()
 function unit_lt(unit,unit)
 function unit_mul(unit,unit)
 function unit_neg(unit)
 function unit_ne(unit,unit)
 function unit_out(unit)
 function unit_pow(unit,integer)
 function unitrange(unit,unit)
 function unitrange(unit,unit,text)
 function unit_recv(internal)
 function unit_reset()
 function unit_send(unit)
 function unit_stddev_pop(unit_accum_t)
 function unit_stddev_samp(unit_accum_t)
 function unit_strict_cmp(unit,unit)
 function unit_strict_eq(unit,unit)
 function unit_strict_ge(unit,unit)
 function unit_strict_gt(unit,unit)
 function unit_strict_le(unit,unit)
 function unit_strict_lt(unit,unit)
 function unit_strict_ne(unit,unit)
 function unit_sub(unit,unit)
 function unit_var_pop(unit_accum_t)
 function unit_var_samp(unit_accum_t)
 function value(unit)
 function variance(unit)
 function var_pop(unit)
 function var_samp(unit)
 function volt(double precision)
 function watt(double precision)
 function weber(double precision)
 operator class unit_ops for access method btree
 operator class unit_strict_ops for access method btree
 operator /(double precision,unit)
 operator *(double precision,unit)
 operator family unit_ops for access method btree
 operator family unit_strict_ops for access method btree
 operator ||/(NONE,unit)
 operator |/(NONE,unit)
 operator -(NONE,unit)
 operator /(unit,double precision)
 operator *(unit,double precision)
 operator ^(unit,integer)
 operator @(unit,text)
 operator @@(unit,text)
 operator <<=(unit,unit)
 operator <<>>(unit,unit)
 operator <(unit,unit)
 operator =(unit,unit)
 operator >>=(unit,unit)
 operator >>(unit,unit)
 operator >(unit,unit)
 operator -(unit,unit)
 operator /(unit,unit)
 operator *(unit,unit)
 operator +(unit,unit)
 table unit_prefixes
 table unit_units
 type unit
 type unit_accum_t
 type unitrange
(136 rows)

Details:

  • dimension(unit): unit

Returns the dimension of a unit value, i.e. its base units with a normalized value of 1.

  • value(unit): double precision

Returns the numeric part of a unit value.

  • round(unit): unit

Rounds a unit value to the nearest integer (in base units).

  • unit @ text: text -- scale conversion, output with unit

Converts a unit value on the left side to the scale of the unit on the right side. The units must have the same dimension. The unit on the right side can include a numeric component (

num unit
), in which case the output is of the form
val * num unit
.
  • unit @@ text: double precision -- scale conversion, value output only

Same as the

@
operator, but returns the value of the scaled unit as double precision number.

Rounding

Besides the

round(unit)
function, output precision is determined by PostgreSQL's
extra_float_digits
GUC. Valid values range from -16 to +3.
# SET extra_float_digits = -12;
SET
# SELECT 'c'::unit AS lightspeed;
 lightspeed
------------
 300 Mm/s

SELECT '25m'::unit @ 'ft' AS feet;

feet

82 ft

Note: Starting with PostgreSQL 12, the default value for

extra_float_digits
is 1, so all printed float values are exact. This makes many converted units (like inch to meters) have a lengthy decimal representation ending in ...0001 or ...9997. In order to restore the "nice" display behavior, eft=1 is internally converted to eft=0 in these PostgreSQL versions. Values returned as floats (like from the
value()
function and the
@@
operator) will still have the "new" representation; set eft=0 to disable. Set eft to 2 or 3 to force the new, exact representation.

References

  • https://en.wikipedia.org/wiki/InternationalSystemof_Units
  • http://physics.nist.gov/cuu/Units/index.html
  • https://en.wikipedia.org/wiki/Binary_prefix
  • https://en.wikipedia.org/wiki/UnitedStatescustomary_units
  • GNU Units: https://www.gnu.org/software/units/
  • Similar library for Ada: http://www.dmitry-kazakov.de/ada/units.htm
  • Similar library for C: https://www.unidata.ucar.edu/software/udunits/

License

Copyright (C) 2016-2019 Christoph Berg

The definitions.units file is Copyright (C) 1996-2018 Free Software Foundation, Inc.

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

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