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Date: Thu, 25 May 2023 08:59:55 +0200 (CEST)
From: Erik Wienhold <ewie@ewie.name>
To: Laurenz Albe <laurenz.albe@cybertec.at>, pgsql-docs@lists.postgresql.org
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Subject: Re: Cross-Product JOIN?
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> On 25/05/2023 06:52 CEST Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>
> On Wed, 2023-05-24 at 21:10 +0200, Erik Wienhold wrote:
> > > On 24/05/2023 15:46 CEST Erik Wienhold <ewie@ewie.name> wrote:
> > >
> > > Personally, I think it should read cartesian product because cross product is
> > > an overloaded term and cartesian product is used more often in the documentation
> > > overall.
> > >
> > > But the same page [0] also uses cross product when talking about grouping sets.
> > >
> > > [0] https://www.postgresql.org/docs/15/queries-table-expressions.html
> >
> > Here's a patch that fixes those two places.
>
> +1
>
> "Cross product" seems to be a misbegotten hybrid of "cross join" and
> "Cartesian product".

Cross product can mean Cartesian product depending on the context. [0]

> Since we are talking about Cartesian products: is the term "Cartesian join"
> used anywhere?

Not in the docs:

	$ git grep -iw cartesian -- doc
	doc/src/sgml/cube.sgml:    the lower-dimensional one is assumed to be a Cartesian projection, i. e., having zeroes
	doc/src/sgml/perform.sgml:   would be inefficient, since the full Cartesian product of A and C
	doc/src/sgml/planstats.sgml:   Cartesian product of the two inputs, multiplied by the
	doc/src/sgml/queries.sgml:    (that is, the Cartesian product of their rows is formed; see below).
	doc/src/sgml/queries.sgml:        <replaceable>T2</replaceable> (i.e., a Cartesian product),
	doc/src/sgml/ref/select.sgml:    specified, the result is the Cartesian product (cross join) of all
	doc/src/sgml/ref/select.sgml:    Cartesian product.
	doc/src/sgml/ref/select.sgml:        Cartesian product (i.e., all combined rows that pass its join
	doc/src/sgml/ref/select.sgml:        They produce a simple Cartesian product, the same result as you get from

but in a few comments:

	$ git grep -i 'cartesian\(\s\+join\|\s*$\)'
	src/backend/optimizer/geqo/geqo_eval.c: * a cartesian join to be performed.  When force is false, do only
	src/backend/optimizer/path/joinrels.c:   * upper level, we must be willing to make a cartesian join of a and b;
	src/backend/optimizer/path/joinrels.c:   * cartesian joins in this case (no bushy).
	src/test/regress/expected/join.out:-- test for ability to use a cartesian join when necessary
	src/test/regress/sql/join.sql:-- test for ability to use a cartesian join when necessary

[0] https://mathworld.wolfram.com/CartesianProduct.html

--
Erik