Queries on Multidimensional Data Enriched with Geographic Information"

In geomultistar: Multidimensional Queries Enriched with Geographic Data

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

library(geomultistar)

Introduction

The multidimensional data model was defined with the aim of supporting data analysis. In multidimensional systems, data is structured in facts and dimensions[^1].

[^1]: Basic concepts of dimensional modelling and star schemas are presented in `rolap`` vignettes.

The star model is widely accepted, it is recommended for use in widely distributed end-user tools. In it we have a table for facts and a table for each dimension. Dimensions provide factual views for easy querying.

The geographical dimension plays a fundamental role in multidimensional systems. In a multidimensional schema, there can be more than one geographic dimension.

These dimensions allow us to associate places of different levels of detail with the factual data. For example, we can record data at the city level but later we may be interested in studying them grouped at the zone or nation level.

It is very interesting to have the possibility of representing the reports obtained from multidimensional systems, using their geographic dimensions, on a map, or performing spatial analysis on them. Thus, the goal of this package is to enrich multidimensional queries with geographic data. In other words, it is not a question of making spatial queries but of generating a spatial layer with the result of the multidimensional queries and that this generation is done automatically, once the configuration of the geographical dimensions has been made.

The rest of this document is structured as follows: First we show various ways to obtain a multistar object. Afterwards, the functions that allow us to define multidimensional queries are presented. The following section shows how to add geographic information to the model and also how to include it in the results of multidimensional queries. Then, the document ends with conclusions.

Get a multistar object

To perform multidimensional queries, the multistar class was defined in this package. A multistar implements star schemas: it has a table for each dimension and a table for the facts; however, it can contain multiple fact tables with some dimensions in common.

Using the functions defined in the rolap package, starting from a flat table implemented by means of a tibble, we can generate a star database from which we can directly obtain a multistar object.

If we already have a star schema, geomultistar package offers functions to generate a multistar object from the fact and dimension tables in tibble format.

Get a multistar object from the rolap package

The rolap package allows us to obtain a multidimensional database from one or more flat tables.

Using the as_multistar() function of this package, we obtain an object of class multistar that we can work with directly using the geomultistar package.

Generate a multistar object

In this case, we are going to suppose that we have tables of facts and dimensions in tibble format, which we have imported into R. In particular, we have the tables mrs_fact_age, mrs_fact_cause, mrs_where, mrs_when and mrs_who: Two fact tables that share two of the three dimensions.

Add fact tables

We create an empty object, to which we will add elements. First we have to add a fact table, later we will add dimension tables or other fact tables.

ms <- multistar() |>
  add_facts(
    fact_name = "mrs_age",
    fact_table = mrs_fact_age,
    measures = "n_deaths",
    nrow_agg = "count"
  ) 

For the facts we indicate a name, the table that contains its data and the names of the columns that contain the measures.

We can indicate an aggregation function associated with each measure. This parameter should be defined only if some measure is not additive. In this case it is not necessary.

Finally, we can indicate the name of a field that represents the number of rows grouped in each query: We can indicate the name of an existing column in the table for that purpose, or the name that you want to give to the column to be added if none exists. In this case, the name of a column in the table is assigned.

Next we add another table of facts with characteristics similar to the previous one.

ms <- ms |>
  add_facts(
    fact_name = "mrs_cause",
    fact_table = mrs_fact_cause,
    measures = c("pneumonia_and_influenza_deaths", "other_deaths"),
    nrow_agg = "nrow_agg"
  )

In this case, the column that contains the number of grouped rows precisely has the name that is assigned by default.

Add dimension tables

Once we have at least one fact table, we can add dimension tables.

ms <- ms |>
  add_dimension(
    dimension_name = "where",
    dimension_table = mrs_where,
    dimension_key = "where_pk",
    fact_name = "mrs_age",
    fact_key = "where_fk"
  )

For each dimension we define its name, the table that contains the data, the name of the primary key and, for the table of facts with which we are going to relate it, its name and the name of the corresponding foreign key.

To establish the relationship successfully, it is verified that there is referential integrity between the tables using the indicated columns. The columns corresponding to the primary and foreign keys are renamed and are no longer available for queries. If you want to keep the field in the dimension, it can be indicated by a parameter, as is shown below by parameter key_as_data.

ms <- ms |>
  add_dimension(
    dimension_name = "when",
    dimension_table = mrs_when,
    dimension_key = "when_pk",
    fact_name = "mrs_age",
    fact_key = "when_fk",
    key_as_data = TRUE
  ) |>
  add_dimension(
    dimension_name = "who",
    dimension_table = mrs_who,
    dimension_key = "who_pk",
    fact_name = "mrs_age",
    fact_key = "who_fk"
  )

If a dimension is related to more than one fact table, when it is added, its relationship to only one can be defined. Later, additional relationships can be defined, as we will see next.

Relate dimensions

Once a dimension is included in the multistar object, we can relate it to other fact tables.

ms <- ms |>
  relate_dimension(dimension_name = "where",
                   fact_name = "mrs_cause",
                   fact_key = "where_fk") |>
  relate_dimension(dimension_name = "when",
                   fact_name = "mrs_cause",
                   fact_key = "when_fk")

In this case, to specify the dimension we only have to indicate its name.

Additional operations on the multistar object

Through the previous operations, we generate a multistar object to which we can apply the operations defined for this class. At this moment we can export it as a flat table, using multistar_as_flat_table, or define multidimensional queries, as we will later use dimensional_query.

geomultistar query functions

A query is defined on a multistar object and the result is another multistar object.

This section presents the functions available to define queries.

dimensional_query()

From a multistar object, an empty dimensional_query object is created where we can select fact measures, dimension attributes and filter dimension rows.

Example:

dq <- dimensional_query(ms)

select_fact()

To define the fact table to be consulted, its name is indicated, optionally, a vector of names of selected measures and another of aggregation functions are also indicated. If the name of any measure is not indicated, only the one corresponding to the number of aggregated rows is included, which is always included. If no aggregation function is included, those defined for the measures are considered.

Examples:

dq_1 <- dq |>
  select_fact(
    name = "mrs_age",
    measures = "n_deaths",
    agg_functions = "MAX"
  )

The measure is considered with the indicated aggregation function. In addition, the measure corresponding to the number of grouped records that make up the result is automatically included.

dq_2 <- dq |>
  select_fact(name = "mrs_age",
              measures = "n_deaths")

The measure is considered with the aggregation function defined in the multidimensional scheme.

dq_3 <- dq |>
  select_fact(name = "mrs_age")

Only the measure corresponding to the number of grouped records is included.

dq_4 <- dq |>
  select_fact(name = "mrs_age",
              measures = "n_deaths") |>
  select_fact(name = "mrs_cause")

More than one fact table can be selected in a query.

select_dimension()

To include a dimension in a dimensional_query object, we have to define its name and a subset of the dimension attributes. If only the name of the dimension is indicated, it is considered that all its attributes should be added.

Example:

dq_1 <- dq |>
  select_dimension(name = "where",
                   attributes = c("city", "state"))

Only the indicated attributes of the dimension will be included.

dq_2 <- dq |>
  select_dimension(name = "where")

All attributes of the dimension will be included.

filter_dimension()

Allows us to define selection conditions for dimension rows. Conditions can be defined on any attribute of the dimension, not only on attributes selected in the query for the dimension. The selection is made based on the function dplyr::filter(). Conditions are defined in exactly the same way as in that function.

Example:

dq <- dq |>
  filter_dimension(name = "when", week <= "03") |>
  filter_dimension(name = "where", city == "Bridgeport")

run_query()

Once we have selected the facts, dimensions and defined the conditions on the instances, we can execute the query to obtain the result.

Example:

dq <- dimensional_query(ms) |>
  select_dimension(name = "where",
                   attributes = c("division_name", "region_name")) |>
  select_dimension(name = "when",
                   attributes = c("year", "week")) |>
  select_fact(name = "mrs_age",
              measures = "n_deaths") |>
  filter_dimension(name = "when", week <= "03")

ms_2 <- dq |>
  run_query()

class(ms_2)

The result of a query is an object of the multistar class that meets the defined conditions. Other queries can continue to be defined on this object.

In this case we transform it into a flat table to more easily show the result.

ft <- ms_2 |>
  multistar_as_flat_table()

Below are the first rows of the result.

pander::pandoc.table(head(ft), split.table = Inf)

Add geographic information

Both the multidimensional data model and multidimensional queries can be enriched with geographic information. This is what we are going to do in this section.

Define geographic dimensions and attributes

To define the dimensions and geographic attributes of a multistar object, we must define a geomultistar specialization from it, which allows to store this information.

We create a geomultistar object from a multistar one defining the names of the dimensions that contain geographic information. In the example only one dimension.

gms <-
  geomultistar(ms, geodimension = "where")

For each attribute of a geographic dimension that we want to use in queries, we can define a vector geographic data layer with which a relationship can be established using one or more attributes of the dimension.

gms <- gms |>
  define_geoattribute(
    attribute = "city",
    from_layer = usa_cities,
    by = c("city" = "city", "state" = "state")
  ) 

For the city attribute, a relationship is defined with a vector geographic data layer in sf format (usa_cities), using the city and state attributes[^2] that have the same name in the layer.

[^2]: It is appropriate to use city and state to establish the relationship because the granularity of the data is city and there can be repeated city names in different states.

Sometimes there may be problems establishing the correspondence between the geographic attributes and the vector layer: Instances may remain unrelated. To detect these situations, we can query the rows that do not have associated geometry using the following function.

empty_city <- gms |>
  get_empty_geoinstances(attribute = "city")

The result obtained is shown below.

pander::pandoc.table(empty_city, split.table = Inf)

In this case, for the unknown cities, their location has not been determined. There may be several because other geographic data of less granularity may be known.

In the same way, the relationship for county with the corresponding layer (usa_counties) is defined.

gms <- gms |>
  define_geoattribute(
    attribute = "county",
    from_layer = usa_counties,
    by = c("county" = "county", "state" = "state")
  )  

We check if they have all been related.

empty_county <- gms |>
  get_empty_geoinstances(attribute = "county")

And the result obtained is shown below.

pander::pandoc.table(empty_county, split.table = Inf)

It also happens for the same instances. In this case we can see that the associated geometry is of a different type.

In the case of state the definition is carried out by associating the code to the corresponding one in the layer (usa_states).

gms <- gms |>
  define_geoattribute(
    attribute = c("state"),
    from_layer = usa_states,
    by = c("state" = "state")
  ) 

Additionally, for an attribute we can generate its layer from the one associated with another related attribute of the dimension. This is what has been done below for division.

gms <- gms |>
  define_geoattribute(
    attribute = "division",
    from_attribute = "state"
  ) 

In this case, the vector layer is generated from the data available in the layer under consideration. Sometimes this is precisely what is desired. If not, look for a vector layer at that level of detail.

If no attribute name is indicated, this operation is applied to the rest of the attributes of the dimension that do not have an associated vector layer by any of the methods presented, as shown below.

gms <- gms |>
  define_geoattribute(from_attribute = "state")

With this we have all the attributes of the dimension with an associated layer, defined at its level of granularity[^3]. On the other hand, we can change the layer of any attribute at any time, independently of the rest.

[^3]: If we are not going to use them in queries it is not necessary that they have the associated layer.

Run queries adding geographic information

Next we define the same query as before but on the data model enriched with geographic information.

gdq <- dimensional_query(gms) |>
  select_dimension(name = "where",
                   attributes = c("division_name", "region_name")) |>
  select_dimension(name = "when",
                   attributes = c("year", "week")) |>
  select_fact(name = "mrs_age",
              measures = "n_deaths") |>
  filter_dimension(name = "when", week <= "03")

gms_2 <- gdq |>
  run_query()

class(gms_2)

If instead of executing the standard query, we execute run_geoquery() function, we automatically obtain a vector geographic data layer at the finest possible level of detail, depending on the definition of the query.

vl_sf <- gdq |>
  run_geoquery()

class(vl_sf)

The first rows of the result can be seen below in table form.

pander::pandoc.table(head(vl_sf), split.table = Inf)

The result is a vector geographic data layer that we can save, perform spatial analysis or queries on it, or we can see it as a map, using the functions associated with the sf class.

plot(vl_sf[,"n_deaths"])

Although we have indicated in the query the attributes division_name and region_name, as can be seen in the figure, the result obtained is at the finest granularity level, in this case at the division_name level.

Only the parts of the divisions made up of states where there is recorded data are shown. If we wanted to show the full extent of each division, we should have explicitly associated a layer at that level.

Get wide tables

In geographic layers, geographic objects usually are not repeated. The tables are wide: for each object the rest of the attributes are defined as columns. By means of the parameter wider we can indicate that we want a result of this type.

vl_sf_w <- gdq |>
  run_geoquery(wider = TRUE)

The first rows of the result can be seen below in table form.

pander::pandoc.table(head(vl_sf_w$sf), split.table = Inf)

We can see that the attributes that are multivalued for each geographic object have been eliminated from the result table, and new measurement columns have been generated: one for each combination of values of these attributes with the original measurements.

The meaning of the name of the columns of the measurements is part of the result obtained, also in table format, as can be seen below.

pander::pandoc.table(head(vl_sf_w$variables), split.table = Inf)

In this case there was only one variable with a multiplicity greater than one. If there were more variables in this situation, they would be added to this table in the same way.

This data dictionary table and layer structure can be saved in GeoPackage format using the save_as_geopackage() function.

filepath <- tempdir()
l <- save_as_geopackage(vl_sf_w, "division", filepath = filepath)

file <- paste0(filepath, "/division.gpkg")
sf::st_layers(file)

The GeoPackages thus obtained can be used directly, for example in QGIS.

Conclusions

To generate the multidimensional structure on which to define queries, we can use the functions of the rolap package or, if we already have the multidimensional design implemented, the import functions included in this package.

The geomultistar package allows generating vector geographic data layers in sf format as a result of multidimensional queries. To do this, all we need is to associate vector geographic data layers with some of the attributes of the geographic dimensions, so that the layers associated with the rest of the attributes can be obtained automatically. Queries do not present any additional difficulties due to the fact of returning geographic data.

The data obtained can be processed with the sf package to define spatial queries or analysis, be presented in maps or saved as a file to be used by a GIS (Geographical Information System) tool.

geomultistar documentation built on Sept. 11, 2024, 6:43 p.m.