Summary of the rPref Package

Description

rPref contains routines to select and visualize the maxima for a given strict partial order. This especially includes the computation of the Pareto frontier, also known as (Top-k) Skyline operator, and some generalizations (database preferences).

Preference Composition/Selection

• Preferences are primarily composed from base preferences (see base_pref) and complex preferences (see complex_pref), where especially the Pareto operator for Skylines is such a complex preference.

• Some utility functions for preferences are collected in general_pref.

• Additionally some base preference macros are provided in base_pref_macros.

• The (top(-level)-k) preference selection psel allows to retrieve the maxima of a preference (or Pareto frontier, Skyline), constructed with the functions above, on a given data set.

Visualization and Analysis of Preferences

• The visualization of the preference order in a Better-Than-Graph (Hasse diagram) is possible via plot_btg.

• The adjacency list of the Hasse diagram can be accessed via get_hasse_diag.

• Predecessors/successors in the Hasse diagram are calculated with the pred_succ functions.

• The Pareto frontier can be plotted using the plot_front function.

String Output of Preferences

• The preference query for some preference-supporting DBMS can be given by show.query.

• A preference is partially evaluated and printed with show.pref.

Vignettes

To learn the basics of rPref, start with the vignettes:

• A general introduction and some examples are given in

  vignette("introduction", package = "rPref")

• The visualization of preferences is explained in

  vignette("visualization", package = "rPref")

Further Information

The rPref website is http://www.p-roocks.de/rpref/. To submit bugs, feature requests or other comments, feel free to write a mail to me.

Author(s)

Patrick Roocks, mail@p-roocks.de

See Also

Useful links:

• https://www.p-roocks.de/rpref/

Base Preferences

Description

Base preferences are used to describe the different objectives (also called dimensions, in the context of a Skyline query) of a preference query.

Usage

low(expr, df = NULL)

low_(expr, df = NULL)

high(expr, df = NULL)

high_(expr, df = NULL)

true(expr, df = NULL)

true_(expr, df = NULL)

is.base_pref(x)

Arguments

Details

Mathematically, all base preferences are strict weak orders (irreflexive, transitive and negative transitive).

The three fundamental base preferences are:

low(a), high(a)

Search for minimal/maximal values of a, i.e., the induced order is the "smaller than" or "greater than" order on the values of a. The values of a must be numeric values.

true(a)

Search for true values in logical expressions, i.e., TRUE is considered to be better than FALSE. The values of a must be logical values. For a tuplewise evaluation of a complex logical expression one has to use the & and | operators for logical AND/OR (and not the && and || operators).

The term expr may be just a single attribute or may contain an arbitrary expression, depending on more than one attribute, e.g., low(a+2*b+f(c)). There a, b and c are columns of the addressed data set and f has to be a previously defined function.

Functions contained in expr are evaluated over the entire data set, i.e., it is possible to use aggregate functions (min, mean, etc.). Note that all functions (and also variables which are not columns of the data set, where expr will be evaluated on) must be defined in the same environment (e.g., environment of a function or global environment) as the base preference is defined.

The function is.base_pref returns TRUE if x is a preference object and FALSE otherwise.

Using Expressions in Preferences

The low_, high_ and true_ preferences have the same functionality as low, high and true but expect an expression, a call or a string as argument. For example, low(a) is equivalent to low_(expression(a)) or low_("a"). Lazy expressions (see the lazyeval package) are also possible.

This is helpful for developing your own base preferences. Assume you want to define a base Preference false as the dual of true. A definition like false <- function(x) -true(x) is the wrong approach, as psel(data.frame(a = c(1,2)), false(a == 1)) will result in the error "object 'a' not found". This is because a is considered as a variable and not as an (abstract) symbol to be evaluated later. By defining

false <- function(x, ...) -true_(substitute(x), ...)

one gets a preference which behaves like a "built-in" preference. Additional optional parameters (like df) are bypassed. The object false(a == 1) will output [Preference] -true(a == 1) on the console and psel(data.frame(a = c(1,2)), false(a==1)) returns correctly the second tuple with a==2.

There is a special symbol df__ which can be used in preference expression to access the given data set df, when psel is called on this data set. For example, on a data set where the first column has the name A the preference low(df__[[1]]) is equivalent to low(A).

Partial Evaluation and Associated Data Frames

If the optional parameter df is given, then the expression is evaluated at the time of definition as far as possible. All variables occurring as columns in df remain untouched. For example, consider

f <- function(x) 2 * x
p <- true(cyl == f(1), mtcars)

Then p is equivalent to the preference true(cyl == 2) as the variable cyl is a column in mtcars. Additionally the data set mtcars is associated with the preference p, implying that the preference selection can be done with peval. See assoc.df for details on associated data sets.

The preference selection, i.e., psel(mtcars, p) can be invoked without the partial evaluation. But this results in an error, if the function f has meanwhile removed from the current environment. Hence it is safer to do an early partial evaluation of all preferences, as far as they contain user defined functions.

The partial evaluation can be done manually by partial.eval.pref.

See Also

See complex_pref how to compose complex preferences to retrieve e.g., the Skyline. See general_pref for functions applying to all kind of preferences. See base_pref_macros for more base preferences.

Examples

# Defines a preference with a score value combining mpg and hp.
p1 <- high(4 * mpg + hp)
# Perform the preference selection:
psel(mtcars, p1)

# Defines a preference with a given function.
f <- function(x, y) (abs(x - mean(x)) / max(x) + abs(y - mean(y)) / max(y))
p2 <- low(f(mpg, hp))
psel(mtcars, p2)

# Use partial evaluation for weighted scoring.
p3 <- high(mpg / sum(mtcars$mpg) + hp / sum(mtcars$hp), df = mtcars)
p3
# Select Pareto optima.
peval(p3)

Useful Base Preference Macros

Description

In addition to the base preferences, rPref offers some macros to define preferences where a given interval or point is preferred.

Usage

around(expr, center, df = NULL)

between(expr, left, right, df = NULL)

pos(expr, pos_value, df = NULL)

layered(expr, ..., df = NULL)

Arguments

Definition of the Preference Macros

between(expr, left, right)

Those tuples are preferred where expr evaluates to a value between left and right. For values not in this interval, the values nearest to the interval are preferred.

around(expr, center)

Same as between(expr, center, center).

pos(expr, pos_value)

Those tuples are preferred, where expr evaluates to a value which is contained in pos_value.

layered(expr, layer1, layer2, ..., layerN)

For the most preferred tuples expr must evaluate to a value in layer1. The second-best tuples are those where expr evaluates to a value in layer2 and so forth. Values occurring in none of the layers are considered worse than those in layerN. Technically, this is realized by a prioritization chain (lexicographical order) of true preferences.

Note that only the argument expr may contain columns from the data frame, all other variables must evaluate to explicit values. For example around(mpg, mean(mpg)) is not allowed. In this case, one can use around(mpg, mean(mtcars$mpg)) instead. Or alternatively, without using the base preference macros, low(abs(mpg - mean(mpg))) does the same. There, the actual mean value of mpg is calculated just when the preference selection via psel is called.

Examples

# Search for cars where mpg is near to 25.
psel(mtcars, around(mpg, 25))

# Consider cyl = 2 and cyl = 4 as equally good, while cyl = 6 is worse.
psel(mtcars, layered(cyl, c(2, 4), 6))

Complex Preferences

Description

Complex preferences are used to compose different preference orders. For example the Pareto composition (via operator *) is the usual operator to compose the preference for a Skyline query. The Skyline is also known as Pareto frontier. All complex preferences are mathematically strict partial orders (irreflexive and transitive).

Usage

## S4 method for signature 'preference,preference'
e1 * e2

## S4 method for signature 'preference,preference'
e1 & e2

## S4 method for signature 'preference,preference'
e1 | e2

## S4 method for signature 'preference,preference'
e1 + e2

reverse(p)

is.complex_pref(x)

Arguments

Skylines

The most important preference composition operator is the Pareto operator (p1 * p2) to formulate a Skyline query. A tuple t1 is better than t2 w.r.t. p1 * p2 if it is strictly better w.r.t. one of the preferences p1, p2 and is better or equal w.r.t. the other preference.

The syntactical correspondence to other query languages supporting Skylines/preferences to rPref is given as follows:

• A query in the syntax from Börzsönyi et. al (2001) like

  "... SKYLINE OF a MAX, b MIN, c MAX"

  corresponds in rPref to the preference

  high(a) * low(b) * high(c).

• A query in the syntax from Kießling (2002) like

  "... PREFERRING a LOWEST AND (b HIGHEST PRIOR TO c LOWEST)"

  corresponds in rPref to

  low(a) * (high(b) & low(c)).

• A query in the syntax of the "Skyline" feature of the commercial database "EXASOL EXASolution 5" like

  "... PREFERRING LOW a PLUS (b = 1 PRIOR TO LOW c))"

  corresponds in rPref to

  low(a) * (true(b == 1) & low(c)).

Note that preferences in rPref can be translated to some of this query dialects by show.query.

Definition of Additional Preference Operators

Additionally, rPref supports the following preference composition operators:

p1 & p2

Prioritization (lexicographical order): A tuple t1 is better than t2 w.r.t. p1 & p2 if it is strictly better w.r.t. p1 or is equal w.r.t. p1 and is better w.r.t. p2.

p1 | p2

Intersection preference: A tuple t1 is better than t2 w.r.t. p1 | p2 if it is strictly better w.r.t. both preferences. This is a stricter variant of the Pareto operator. The evaluation of psel(df, p1 | p2) is always a subset of psel(df, p1 * p2).

p1 + p2

Union preference: A tuple t1 is better than t2 w.r.t. p1 + p2 if it is strictly better w.r.t. to one of the preferences. Note that this can violate the strict partial order property, if the domains (the tuples on which p1 and p2 define better-than-relationships) of the preferences are not disjoint.

reverse(p1) or -p1

Reverse preference (converse relation): A tuple t1 is better than t2 w.r.t. -p1 if t2 is better than t1 w.r.t. p1. The unary minus operator, i.e. -p1, is a short hand notation for reverse(p1).

The function is.complex_pref returns TRUE if x is a complex preference object (i.e., was constructed by one of these binary operators or the unary operator reverse) and FALSE otherwise.

Associated Data Sets

If one of the preferences for a binary operator are associated with a data set (see base_pref), then this association is propagated. For example, the preference

p <- high(mpg, df = mtcars) * high(hp)

as well as

p <- high(mpg) * high(hp, df = mtcars)

both result in the same complex preference which is associated with mtcars. A partial evaluation is also invoked for all preferences which are added. For example, using this p,

p <- p * true(cyl == max(mtcars$cyl))

generates the following console output:

[Preference] high(mpg) * high(hp) * true(cyl == 8)
* associated data source: data.frame "mtcars" [32 x 11]

We see that the association with the data set is propagated and max(mtcars$cyl) is partially evaluated.

References

S. Börzsönyi, D. Kossmann, K. Stocker (2001): The Skyline Operator. In Data Engineering (ICDE '01), pages 421-430.

W. Kießling (2002): Foundations of Preferences in Database Systems. In Very Large Data Bases (VLDB '02), pages 311-322.

S. Mandl, O. Kozachuk, M. Endres, W. Kießling (2015): Preference Analytics in EXASolution. 16th Conference on Database Systems for Business, Technology, and Web.

See Also

See base_pref for the construction of base preferences. See general_pref for functions applicable to all kind of preferences. See psel for the evaluation of preferences.

Examples

# Defines a preference for cars with low consumption (high mpg-value)
# and simultaneously high horsepower.
p1 <- high(mpg) * high(hp)

# Performs the preference search.
psel(mtcars, p1)

# Alternative way: create preference with associated data set.
p2 <- high(mpg, df = mtcars) * high(hp)
peval(p2)

Utility Functions for Preferences

Description

Collection of some useful functions which are applicable to all preference objects.

Usage

empty()

is.empty_pref(x)

## S4 method for signature 'preference'
length(x)

is.preference(x)

## S4 method for signature 'preference'
as.expression(x, ...)

## S4 method for signature 'preference'
assoc.df(x)

## S4 replacement method for signature 'preference'
assoc.df(x) <- value

Arguments

Details

The empty preference empty() is a neutral element for the complex preference compositions {*, &, +}. It holds that empty() * p and empty() & p is equal to p for all preferences p.

The function length(p) returns the term length of the preference term p which is defined as the number of base preferences in a complex preference term. The empty preference empty() has length 0, and all base preferences have length 1.

With as.expression(p) for a preference p the call to the preference is constructed. This means, eval(as.expression(p)) returns the preference p, evaluated in the current environment.

The function is.empty_pref returns TRUE if x is the empty preference object empty() and FALSE otherwise.

With assoc.df the associated data frame of a preference can be retrieved or set. Setting the associated data frame means that a partial evaluation based on this data frame is done. See show.pref for details on partial evaluation of preferences. Next, the preference is linked to that data frame, such that peval(p) can be used instead of psel(df, p). It returns NULL if no data frame is associated. Use set.assoc.df(NULL) to delete an associated data frame.

See Also

See base_pref for the construction of base preferences, and complex_pref for the construction of complex preferences. See show.pref for string output and partial evaluation of preference terms.

Examples

# Same as low(a) * low(b).
p <- low(a) * low(b) * empty()

# Returns 2, as empty() does not count.
length(p)

# The preference expression (without empty()).
as.expression(p)

Adjacency List of Hasse diagram

Description

Returns the adjacency list of the Hasse diagram of a preference as an (n x 2) matrix. This is the transitive reduction of the preference relation.

Usage

get_hasse_diag(df, pref)

Arguments

Details

A row (i, j) in the resulting matrix means that df[i,] is better than df[j,] with regard to the preference p. The matrix is the transitive reduction (Hasse diagram) of the induced relations, i.e., if (1,2) and (2,3) occur in the result, then (1,3) will not be contained. The number of rows in the result depends on the number of non-transitive Better-Than-Relationships in df w.r.t. p.

See Also

get_btg to plot the Hasse diagram.

Examples

get_hasse_diag(mtcars, low(mpg))

Better-Than-Graphs

Description

Returns or plots a Hasse diagram of a preference order (also called the Better-Than-Graph, short BTG) on a given data set. Plotting within R relies on the igraph package or the Rgraphviz package. Alternatively, a dot file for an external Graphviz/dot interpreter can be generated.

Usage

plot_btg(
  df,
  pref,
  labels = 1:nrow(df),
  flip.edges = FALSE,
  levelwise = TRUE,
  use_dot = "Rgraphviz" %in% rownames(installed.packages())
)

get_btg(
  df,
  pref,
  flip.edges = FALSE,
  use_dot = "Rgraphviz" %in% rownames(installed.packages())
)

get_btg_dot(
  df,
  pref,
  labels = 1:nrow(df),
  flip.edges = FALSE,
  levelwise = TRUE,
  file = NULL
)

Arguments

Details

The Hasse diagram of a preference visualizes all the better-than-relationships on a given data set. All edges which can be retrieved by transitivity of the order are omitted in the graph.

The functions get_btg and plot_btg either use the igraph package (if use_dot = FALSE) or the dot layouter from the Rgraphviz package (if use_dot = TRUE). If Rgraphviz is available it is used by default, otherwise the igraph Package is used. Note that Rgraphviz is only available on BioConductor and not on CRAN.

The dot layouter from Rgraphviz is more appropriate for Better-Than-Graphs than the igraph layouter, as all edges will be directed in the same direction (rank based ordering). Using levelwise = TRUE (the default), all tuples of the same level are placed on the same row.

BTGs with igraph

If used with use_dot = FALSE, the function get_btg returns a list l with the following list entries:

l$graph

An igraph object, created with the igraph package.

l$layout

A typical Hasse diagram layout for plotting the graph, also created with igraph.

To plot the resulting graph returned from get_btg, use the plot function as follows:

plot(l$graph, layout = l$layout)

For more details, see plot.igraph.

BTGs with Rgraphviz

If used with use_dot = FALSE, the function get_btg returns a graphNEL object from the graph-package (Rgraphviz is build on top of that package). This object can also be plotted using plot(...).

Direct Plotting

In both cases (whether Rgraphviz is used or not), the function plot_btg directly plots the Better-Than-Graph. There is an additional parameter labels, specifying the node labels. The default are the row numbers (not the rownames of the data frame), ranging from "1" to as.character(nrow(df)).

Dot (Graphviz) String Output

The function get_btg_dot produces the source code of the Better-Than-Graph in the dot language of the Graphviz software. This is useful for an external dot interpreter. Depending on the file parameter the output is either written to a file (if a file path is given) or returned as a string (if file = NULL).

Additional Parameters

By default, the directed edges in the diagram point from better to worse nodes w.r.t. the preference. This means an arrow can be read as "is better than". If flip.edges = TRUE is set, then the arrows point from worse nodes to better nodes ("is worse than"). In any case, the better nodes are plotted at the top and the worse nodes at the bottom of the diagram.

If Rgraphviz is used for plot_btg and for get_btg_dot, the option levelwise controls if all nodes of the same level are placed in one row. If this parameter is FALSE, then the vertical arrangement is subject to the dot layouter.

Examples

# Pick a small data set and create preference and BTG.
df <- mtcars[1:10, ]
pref <- high(mpg) * low(wt)

# Directly plot the BTG with row numbers as labels.
# This uses Rgraphviz if available and igraph otherwise.
plot_btg(df, pref)

# Plot the graph with labels with relevant values.
labels <- paste0(df$mpg, "; ", df$wt)
plot_btg(df, pref, labels)

# Show lattice structure of a 3-dimensional Pareto preference.
df <- merge(merge(data.frame(x = 1:3), data.frame(y = 1:3)), data.frame(z = 1:2))
labels <- paste0(df$x, ",", df$y, ",", df$z)
plot_btg(df, low(x) * low(y) * low(z), labels)

# Create a graph with external Graphviz (requires installed Graphviz).
## Not run: 
# Vreates tmpgraph.dot in the current working directoy
get_btg_dot(df, pref, labels, file = "tmpgraph.dot")
# Convert to diagram tmpgraph.png using Graphviz
shell(paste0(
  '"C:/Program Files (x86)/Graphviz2.38/bin/dot.exe"',
  " -Tpng tmpgraph.dot -o tmpgraph.png"
))
# Open resulting image
shell("tmpgraph.png")

## End(Not run)

Pareto Front Plot

Description

Connects the points of a Pareto front (also known as Pareto frontier) and hence visualizes the dominance region of a Skyline.

Usage

plot_front(df, pref, ...)

Arguments

Details

plot_front assumes that there is an existing plot, where the value of the first preference was plotted as x-coordinate and the value of the second preference as y-coordinate.

Note that plot_front is only recommended if you want to use the plotting functionality from base R. If you prefer to use ggplot2, we recommend using geom_step for plotting the Pareto front. See vignette("visualization", package = "rPref") for examples.

Examples

# plots Pareto fronts for the hp/mpg values of mtcars
show_front <- function(pref) {
  plot(mtcars$hp, mtcars$mpg)
  sky <- psel(mtcars, pref)
  plot_front(mtcars, pref, col = rgb(0, 0, 1))
  points(sky$hp, sky$mpg, lwd = 3)
}

# do this for all four combinations of Pareto compositions
show_front(low(hp) * low(mpg))
show_front(low(hp) * high(mpg))
show_front(high(hp) * low(mpg))
show_front(high(hp) * high(mpg))

# compare this to the front of a intersection preference
show_front(high(hp) | high(mpg))

Predecessor and Successor Functions

Description

Functions for traversing the BTG (Better-Than-Graph or Hasse diagram) of a preference.

Usage

init_pred_succ(p, df = NULL)

hasse_pred(p, v, intersect = FALSE)

hasse_succ(p, v, intersect = FALSE)

all_pred(p, v, intersect = FALSE)

all_succ(p, v, intersect = FALSE)

Arguments

Details

These functions return the predecessors and successors in the Better-Than-Graph of a preference. Note that the successors/predecessors can can be plotted via get_btg. Before any of the successor/predecessor functions can be used the initialization has to be called as follows:

init_pred_succ(p, df)

There p is a preference object and df a data frame. When this done, the data frame df is associated with p, i.e., implicitly assoc.df is called. If the preference has already an associated data frame, df can be omitted. For example

p <- low(mpg, df = mtcars)
init_pred_succ(p)

does the initialization of the preference low(mpg) on the data set mtcars.

The init_pred_succ function calculates the Better-Than-Relation on df w.r.t. p. Afterwards the predecessor and successor functions, as subsequently described, can be called. The value of v is a numeric vector within 1:nrow(df) and characterizes a subset of tuples in df. The return value of these functions is again a numeric vector referring to the row numbers in df and it is always ordered ascending, independently of the order of the indices in v.

all_pred(p, v)

Returns all predecessors of v, i.e., indices of better tuples than v.

all_succ(p, v)

Returns all successors of v, i.e., indices of worse tuples than v.

hasse_pred(p, v)

Returns the direct predecessors of v, i.e., indices of better tuples than v where the better-than-relation is contained in the transitive reduction.

hasse_succ(p, v)

Returns the direct successors of v, i.e., indices of worse tuples than v where the better-than-relation is contained in the transitive reduction.

If v has length 1, then the value of intersect does not matter, as there is nothing to intersect or join. For scalar values x and y the following identities hold, where f is one of the predecessor/successor functions:

f(p, c(x, y), intersect = FALSE) == union(f(p, x), f(p, y))

f(p, c(x, y), intersect = TRUE) == intersect(f(p, x), f(p, y))

Examples

# Preference on mtcars for high mpg and low weight
p <- high(mpg) * low(wt)
init_pred_succ(p, mtcars)

# Helper to show mpg/hp values
show_vals <- function(x) mtcars[x, c("mpg", "wt")]

# Pick some tuple "in the middle":
show_vals(10)

# Show (direct) predecessors/successors of tuple 10:
show_vals(hasse_pred(p, 10)) # Next better car
show_vals(hasse_succ(p, 10)) # Next worse car
show_vals(all_pred(p, 10)) # All better cars
show_vals(all_succ(p, 10)) # All worse cars

Preference Selection

Description

Evaluates a preference on a given data set, i.e., returns the maximal elements of a data set for a given preference order.

Usage

psel(df, pref, ...)

psel.indices(df, pref, ...)

peval(pref, ...)

Arguments

Details

The difference between the three variants of the preference selection is:

• psel: Returns a subset of the data set containing the maxima according to the given preference.

• psel.indices: Returns just the row indices of the maxima (except top-k queries with show_level = TRUE, see top-k preference selection). Hence, psel(df, pref) is equivalent to df[psel.indices(df, pref), ] for non-grouped data frames.

• peval: Does the same as psel, but assumes that pref has an associated data frame which is used for the preference selection. See base_pref for details, or use assoc.df to explicitly associate a preference with a data frame.

Top-k Preference Selection

For a given top value of k the k best elements and their level values (column .level) are returned. The elements of level k are also called the n-th stratum in the literature. The level values are determined as follows:

• All the maxima of a data set w.r.t. a preference have level 1.

• The maxima of the remainder, i.e., the data set without the level 1 maxima, have level 2.

• The n-th iteration of "Take the maxima from the remainder" returns tuples of level n.

By default, psel.indices does not return the level values. By setting show_level = TRUE this function returns a data frame with the columns '.index' and '.level'. Note that, if none of the top-k values {top, at_least, top_level} is set, then all level values are equal to 1.

By definition, a top-k preference selection is non-deterministic. A top-1 query of two equivalent tuples (equivalence according to pref) can return both of these tuples.

Consider the following example:

df <- data.frame(a = c(1, 1, 3), b = c(1, 2, 3))

The query psel(df, low(a)) returns:

    a b
  1 1 1
  2 1 2

The top-1 psel(df, low(a), top = 1) selection returns:

    a b .level
  1 1 1      1

Theoretically, the b=2 could also be returned in the above query. On the contrary, a preference selection using at_least is deterministic by adding all tuples having the same level as the worst level of the corresponding top-k query. This means, the result is filled with all tuples being not worse than the top-k result. A preference selection with top-level-k returns all tuples having level k or better.

If the top or at_least value is greater than the number of elements in df (i.e., nrow(df)), or top_level is greater than the highest level in df, then all elements of df will be returned without further warning. In addition, the we can set top = Inf to return all tuples, i.e., psel(df, low(a), top = Inf) returns:

    a b .level
  1 1 1      1
  2 1 2      1
  3 3 3      2

By setting top_level = 2 we return the first two levels, i.e., all tuples in this case.

If multiple top-k parameters are specified, their interaction is controlled by and_connected. Let cond1 and cond2 be top-conditions like top=2 or top_level=3, then psel([...], cond1, cond2) is equivalent to the intersection of psel([...], cond1) and psel([...], cond2). If we have and_connected = FALSE, these conditions are or-connected. This corresponds to the union of psel([...], cond1) and psel([...], cond2).

Grouped Preference Selection

Using psel it is also possible to perform a preference selection where the maxima are calculated for every group separately. The groups have to be created with group_by from the dplyr package. The preference selection preserves the grouping, i.e., the groups are restored after the preference selection.

For example, if the summarize function from dplyr is applied to psel(group_by(...), pref), the summarizing is done for the set of maxima of each group. This can be used to e.g., calculate the number of maxima in each group, see the examples below.

A {top, at_least, top_level} preference selection is applied to each group separately. A top=k selection returns the k best tuples for each group. Hence if there are 3 groups in df, each containing at least 2 elements, and we have top = 2, then 6 tuples will be returned.

Parallel Computation

On multi-core machines the preference selection can be run in parallel using a divide-and-conquer approach. Depending on the data set, this may be faster than a single-threaded computation. To activate parallel computation within rPref the following option has to be set:

options(rPref.parallel = TRUE)

If this option is not set, rPref will use single-threaded computation by default. With the option rPref.parallel.threads the maximum number of threads can be specified. The default is the number of cores on your machine. To set the number of threads to the value of 4, use:

options(rPref.parallel.threads = 4)

See Also

See complex_pref on how to construct a Skyline preference.

Examples

# Skyline and top-k/at-least Skyline
psel(mtcars, low(mpg) * low(hp))
psel(mtcars, low(mpg) * low(hp), top = 5)
psel(mtcars, low(mpg) * low(hp), at_least = 5, show_index = TRUE)

# Preference with associated data frame and evaluation
p <- low(mpg, df = mtcars) * (high(cyl) & high(gear))
peval(p)

# Visualizes the Skyline in a plot.
sky1 <- psel(mtcars, high(mpg) * high(hp))
plot(mtcars$mpg, mtcars$hp)
points(sky1$mpg, sky1$hp, lwd = 3)

# Grouped preference with dplyr.
library(dplyr)
psel(group_by(mtcars, cyl), low(mpg))

# Returns the size of each maxima group.
summarise(psel(group_by(mtcars, cyl), low(mpg)), n())

Partial Evaluation and String Output of Preferences

Description

Functions to substitute variables and functions in preferences which can be calculated before the preference is evaluated on a data frame and character output of preferences.

Usage

show.pref(p, df = NULL)

## S4 method for signature 'preference'
as.character(x, ...)

pref.str(p, df = NULL)

partial.eval.pref(p, df = NULL)

Arguments

Details

The function pref.str (or as.character(p) for a preference p) returns the preference string while show.pref outputs it directly to the console, preceded by '[Preference]'. If df is specified, then a partial evaluation of the preference is done before converting it to a string.

The function partial.eval.pref (with given data frame df) partially evaluates the internal preference expression and returns again a preference object. All expressions in p are evaluated in the environment where p was defined, except the column names in df (which are potential attributes in p) and except the special variable df__, which accesses the entire data set (see psel). The content of the data frame df does not matter; only names(df) is used to get the "free variables" in p.

If p has already an associated data frame (see assoc.df), then a partial evaluation was already done when the data frame was associated. In this case, the df parameter should not be used. The association will not be changed if one of these function are called with a given data frame on a preference object having an associated data frame.

Partial Evaluation Before String Output

The functions show.pref and pref.str have the optional parameter df. If this parameter is given, these functions call partial.eval.pref before they output or return the preference string. The following equalities hold:

• as.character(partial.eval.pref(p, df)) == pref.str(p, df)

• show(partial.eval.pref(p, df)) produces the same console output as show.pref(p, df)

See Also

See general_pref for more utility functions for preferences.

Examples

f <- function(x) 2 * x
p <- true(cyl == f(1))

# prints 'true(cyl == f(1))'
p

# prints 'true(cyl == 2)'
show.pref(p, mtcars)
partial.eval.pref(p, mtcars)

Show Preferences in Database Query Languages

Description

For a given preference this shows the PREFERRING clause of a database query in different SQL dialects which support preferences.

Usage

show.query(p, dialect = "EXASOL", df = NULL)

Arguments

Details

There are few database systems supporting Skyline queries. A Skyline query consists of a usual SQL query followed by a PREFERRING-clause (in some rarely used dialects also SKYLINE OF). For example consider a database table r(a,b). The preference selection psel(r, low(a) * high(b)) can be expressed by (in the Exasol dialect):

SELECT * FROM r PREFERRING LOW a PLUS HIGH b

The show.query function generates just the PREFERRING-clause, i.e. show.query(low(a) * high(b)) returns

PREFERRING LOW a PLUS HIGH b

As usual in SQL queries, all keywords are not case sensitive, i.e., PLUS or plus does not make any difference.

References

W. Kießling, M. Endres, F. Wenzel (2011): The Preference SQL System - An Overview. IEEE Data Engineering Bulletin, Vol. 34 No. 3, pages 12-19.

S. Mandl, O. Kozachuk, M. Endres, W. Kießling (2015): Preference Analytics in EXASolution. 16th Conference on Database Systems for Business, Technology, and Web.

Examples

show.query(low(a) * high(b))

show.query(low(a) * high(b), dialect = "Preference SQL")