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Re: What is the relationship between transforming, mapping,and

• From: "Ghislain Fourny" <gfourny@inf.ethz.ch>
• To: Michael Kay <mike@saxonica.com>
• Date: Wed, 1 Sep 2021 08:37:48 +0000

Dear Roger and Mike,

In the context of large scale query processing, for example Apache Spark, "map" and "filter" are two kinds of transformations, in other words, "transformation" is a generic term for "atoms" of lazily and "parallely" evaluated, intermediate-level queries that act on RDDs (intermediate data that is partitioned over a cluster). There are many other transformations such as flatMap, sort, group, etc and even binary ones (join, etc).

In XQuery, they "kind of" correspond to FLWOR clauses (let "maps" input tuples to output tuples, where "filters", etc) with a more complete correspondence described in [1].

Kind regards,
Ghislain
[1] https://arxiv.org/pdf/1910.11582.pdf


> On 1 Sep 2021, at 00:45, Michael Kay <mike@saxonica.com> wrote:
> 
> The term "mapping" is generally used to mean an operation in which the same function f(x) is applied to every item x in some set (or sequence) of items X.
> 
> The term "filtering" is generall used to mean an operation in which some predicate (boolean function) f(x) is applied to every item x in some set of items X, such that x is included in the result if f(x) is true, and excluded if it is false. Filtering using a function p(x) is equivalent to mapping using the function (if (p(x)) then x else ()).
> 
> I'm not aware of any mathematically precise use of the term "transformation". An XSLT transformation is of course a function from input documents to output documents.
> 
> Michael Kay
> Saxonica
> 
>> On 31 Aug 2021, at 18:53, Roger L Costello <costello@mitre.org> wrote:
>> 
>> Hi Folks,
>> 
>> How do you define those terms? Are they synonymous? Is one a subset of another?
>> 
>> By "mapping" I mean populating XML instances of one data standard with data from another data standard. For example, I am currently mapping a military air navigation data standard to a civilian air navigation data standard.
>> 
>> By "filtering" I mean performing actions on data such as redacting, fuzzing, and zeroing. One could think of these actions as mapping: if I fuzz a lat/long location, I am essentially mapping the location to another location.
>> 
>> By "transforming" I mean all the kinds of things you can do with XSLT.
>> 
>> Is filtering a subset of mapping? Is mapping a subset of transforming?
>> 
>> /Roger
>> 
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