Relations

A relation is a mapping between dimensions: a generator's bus, a snapshot's period, or the buses a generator may connect to. It is declared as a table, and the data supplies its rows.

relations

dimensions:
  bus: { dtype: str }
  generator: { dtype: str }
  snapshot: { dtype: int }
  period: { dtype: int }
relations:
  gen_bus: { key: generator, values: bus } # each generator on one bus
  period_of: { key: snapshot, values: period }
  connection: { key: [generator, bus] } # no values: a generator may connect to several buses

The key is the combination of dimensions that is unique per row: key: generator says the table has one row per generator. The values are what that row determines: its bus. With no values:, the key is every column, and the table is a bare relation.

Field
key	required. The columns that identify a row
values	the columns the key determines. Omitted, the key is every column	default none
description	free text	default null

A column is named after its dimension. Where two columns share a dimension, the mapping form names them: {bus0: bus, bus1: bus}.

Cardinalities

intention	written	cardinality
each generator has one bus	{key: generator, values: bus}	many-to-one
a bus has several generators	the same table, read the other way	one-to-many
a generator may connect to several buses	{key: [generator, bus]}, no values:	many-to-many
a generator has one zone in each period	{key: [generator, period], values: zone}	many-to-one, keyed by a pair
a snapshot has a month, a week and a weekday	{key: snapshot, values: [month, week, weekday]}	many-to-one, several values
a line has two ends, both buses	{key: line, values: {bus0: bus, bus1: bus}}	many-to-one, two columns over one dimension
a snapshot has a representative snapshot	{key: snapshot, values: {rep: snapshot}}	many-to-one, onto itself
a snapshot has neighbours	{key: {from: snapshot, to: snapshot}}, no values:	many-to-many, onto itself
each generator has one bus, and each bus one generator	not a claim the language has	one-to-one

A key that determines a value holds one column per dimension, so {key: {bus0: bus, bus1: bus}, values: line} is refused. A bare relation may key two columns over one dimension.

The data contract

The data for gen_bus arrives under the key gen_bus, as a table with one column per declared column, named after it.

• One row per key tuple. A generator on two buses is refused when the data binds.
• Every value is a label of its dimension. A value that matches none is refused.
• A partial map is the rows it has. A generator in no row sits on no bus, which is absence.
• A null in any column is refused.
• Row order carries nothing. The order is the dimension's.

How a relation is used

The declaration fixes no direction. A call names the columns it reads, and a key column named at neither end is joined on: the operand carries its dimension, and the result keeps it. A value column named at neither end is not read.

kind	what it does	written as
aggregate	many rows of the operand collapse onto one	sum(x, by=l, over=a, into=b)
read	one row's value becomes a coordinate	at(x, by=l, over=a, into=b)
partition	the frame stays, and its rows are grouped	shift, sum_back, position with by=l, within=c
test	a row's presence keeps or cuts a coordinate	the relation's name in a where

Four rules hold for every use:

1. A call names every column it reads. sum(p, by=gen_bus) is refused.
2. A value column the call does not name is not read. So adding one to the relation changes no call.
3. The key is fixed. To change it, declare a new relation.
4. A dimension the relation does not name passes through to the result.

Aggregates and reads

over= names the columns consumed and into= the columns produced, and either may be a list. With zone_of: { key: [generator, period], values: zone } and p over [generator, period]:

call	consumes	joins on	produces	result
sum(p, by=zone_of, over=generator, into=zone)	generator	period	zone	[zone, period]
sum(p, by=zone_of, over=period, into=zone)	period	generator	zone	[generator, zone]
at(price, by=zone_of, over=zone, into=generator)	zone	period	generator	[generator, period]

• The result is the operand, less the consumed dimensions, plus the produced ones. The operand carries every dimension consumed or joined on, and none that the call lands on. sum(load * p, by=gen_bus, over=generator, into=bus) is refused; write load * sum(p, by=gen_bus, over=generator, into=bus).
• sum consumes at least one key column, and at consumes value columns only. A read finds one row per coordinate, and a sum finds many. Each is refused in the other's case.
• over= and into= name different columns, and neither names two columns over one dimension.

Partitions

shift(x, along=d, by=l, within=c), sum_back(x, along=d, by=l, within=c) and position(d, by=l, within=c) step along the key column over d, join on the other key columns, and group by the value columns within= names. The frame does not change. within= is written whenever by= is. It may name two columns over one dimension, may not name a key column, and a bare relation partitions nothing.

Tests

A where string uses a relation at the frame's own coordinates: it compares a column under values: against a label, compares two columns of one table, or tests that a row exists (where strings).