Combine_data - ObjectVision/GeoDMS GitHub Wiki

Relational functions combine_data

• combine_data (combined_domain, a_rel, b_rel, ...)

combine_data(combined_domain, a_rel, b_rel, ...) results in an attribute with index numbers to the combined_domain argument, combining the values of the data items a_rel, b_rel and further: one relation per factor of combined_domain.

The first argument combined_domain is usually made with a combine function.

The combine_data function results in an attribute with an index to combined_domain for each element of the domain of a_rel, b_rel and further, with as values unit the first argument of the function (combined_domain) and as domain unit the domain unit of the relations.

The last relation varies fastest, as in the numbering of combine: for combined_domain := combine(A, B, C), relations with the index numbers a, b and c give ((a × #B) + b) × #C + c. combine_rel and first_rel(x_rel) to sixteenth_rel(x_rel) compute the relations back.

Since GeoDMS 20.24.0 combine_data is an operator that knows the factors of the combine that made combined_domain, and uses them:

• Each relation is checked against the factor at its position: its values unit must have the value type, metric, projection and spatial reference of that factor. A literal, such as the 0 in combine_data(X, 0, id(P)), passes.
• That check compares value types, not which unit it is. Two factors of the same value type, such as two uint32 domains without a metric, can still be swapped without notice, so keep the relations in the order of the factors.
• The index numbers are taken within the ranges of the factors, so first_rel(combine_data(X, a_rel, b_rel)) is a_rel again.
• A null in any relation gives null, and so does a value outside the range of its factor.

Until then combine_data was a function in the prelude that computed the index from the values units of its arguments alone, without looking at the factors of combined_domain. A relation of another value type than its factor was accepted, and so was a number of relations that differed from the number of factors; both now stop with an error at the item that calls combine_data. A value outside the range of its factor gave the index of another combination, and now gives null.

When combined_domain is not made by a combine function, for instance a unit read from a storage, combine_data still computes as before, from the values units of the relations, without a check.

• unit combined_domain with value type from the group CanBeDomainUnit.
• attributes a_rel, b_rel, ... with value type from group CanBeDomainUnit.

• The domain units of the relations must match (or be void, as that of a parameter).
• One relation per factor of combined_domain. When some of the factors are themselves combines, one relation per factor of those is also accepted, see below.
• The values unit of each relation must match the factor at its position in the combine with which combined_domain has been defined.

7.131

O(n) where n = number of elements in the domain of the relations. Single pass computing the combined index from the input relations.

Since GeoDMS 20.24.0 combine_data takes one relation per factor directly:

unit<uint32>   ABC              := combine_unit(A, B, C);
attribute<ABC> ABC_rel (domain) := combine_data(ABC, domain/A_rel, domain/B_rel, domain/C_rel);

A combine of combines takes one relation per factor, or one per factor of the inner combines; both give the same result:

unit<uint32>    BC                := combine_unit(B,  C);
unit<uint32>    A_BC              := combine_unit(A, BC);
attribute<A_BC> A_BC_rel (domain) := combine_data(A_BC, domain/A_rel, combine_data(BC, domain/B_rel, domain/C_rel));
attribute<A_BC> same_rel (domain) := combine_data(A_BC, domain/A_rel, domain/B_rel, domain/C_rel);

unit<uint32> NHCityYear := combine(NHCity, YearDomain);
attribute<NHCityYear> NHCityYear_rel (Adomain) := combine_data(NHCityYear, NHCity_rel, YearDomain_rel);

ADomain, nr of rows = 5

domain NHCityYear, nr of rows = 6

domain NHCity, nr of rows = 3

Yeardomain, nr of rows = 2

Until October 2026 the column NHCityYear_rel of the first table read 0, 3, 4, 2, 5, a numbering in which the first factor varies fastest. combine_data has the last factor varying fastest, as the table of NHCityYear shows.

• combine_rel, for the relations back to the factors
• full configuration example: Overlay versus Combine data