8.55 The group module
This module contains the primitives to construct, derive, and perform statistical operations on BATs representing groups. The default scheme in Monet is to assume the head to represent the group identifier and the tail an element in the group.
Groups play an important role in datamining, where they are used to construct cross-tables. Such cross tables over a single BAT are already supported by the histogram function. This module provides extensions to support identification of groups in a (multi-)dimensional space.
The module implementation has a long history. The first implementation provided several alternatives to produce/derive the grouping. A more complete (and complex) scheme was derived during its extensive use in the context of the Data Distilleries product. The current implementation is partly a cleanup of this code-base, but also enables provides better access to the intermediate structures produced in the process, i.e. the histogram and the sub-group mapping. They can be used for various optimization schemes at the MAL level.
The prime limitation of the current implementation is that an underlying database of
oid->any BATs is assumed. This enables representation of each group using an oid, and the value representation of the group can be accordingly be retrieved easily. An optimized implementation in which we use positional integer id's (as embodied by Monet's void type) is also available.
This limitation on (v)oid-headers is marginal. The primitive GRPsplit produces for any BAT two copies with both a (v)oid header.
There are several approaches to build a cross table. The one chosen here is aimed at incremental construction, such that re-use of intermediates becomes possible. Starting with the first dimension, a BAT is derived to represent the various groups, called a GRP BAT or cross-table BAT.
8.55.2 Cross Table (GRP)
A cross table is an <oid,oid> BAT where the first (head) denotes a tuple in the cross table and the second (tail) marks all identical lists. The tail-oids contain group identifiers; that is, this value is equal iff two tuples belong to the same group. The group identifiers are chosen from the domain of the tuple-identifiers. This simplifies getting back to the original tuples, when talking about a group. If the tuple-oid of 'John' is chosen as a group-id, you might view this as saying that each member of the group is 'like John' with respect to the grouping-criterion.
Successively the subgroups can be identified by modifying the GRP BAT or to derive a new GRP BAT for the subgroups. After all groups have been identified this way, a BAT histogram operation can be used to obtain the counts of each data cube. Other aggregation operations using the MIL set aggregate construct
(bat) can be used as well; note for instance that
histogram == (b.reverse()).
The Monet interface module specification is shown below. Ideally we should defined stronger type constraints, e.g. command group.new(attr:bat[,:any_1]
The group macro is split along three dimensions:
|[type:]||Type specific implementation for selecting the right hash function and data size etc.;|
|[clustered:]||The select the appropriate algorithm, i.e., with or without taking advantage of an order of values in the parent groups;|
|[physical properties:]||Values , choosing between a fixed predefined and a custom hashmask. Custom allows the user to determine the size of the hashmask (and indirectly the estimated size of the result). The hashmask is 2^n - 1 where n is given by the user, or 1023 otherwise, and the derived result size is 4 ... 2^n.|
Further research should point out whether fitting a simple statistical model (possibly a simple mixture model) can help choose these parameters automatically; the current idea is that the user (which could be a domain-specific extension of the higher-level language) knows the properties of the data, especially for IR in which the standard grouping settings differ significantly from the original datamining application.