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Glossary: improve definition of "relation"
Define the more general term first, then the Postgres-specific meaning. Wording from Tom Lane. Discussion: https://postgr.es/m/CACJufxEZ48toGH0Em_6vdsT57Y3L8pLF=DZCQ_gCii6=C3MeXw@mail.gmail.com
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@@ -1419,11 +1419,15 @@
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<glossterm>Relation</glossterm>
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<glossdef>
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<para>
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The generic term for all objects in a
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<glossterm linkend="glossary-database">database</glossterm>
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that have a name and a list of
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<glossterm linkend="glossary-attribute">attributes</glossterm>
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defined in a specific order.
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Mathematically, a <firstterm>relation</firstterm> is a set of
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<glossterm linkend="glossary-tuple">tuples</glossterm>;
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this is the sense meant in the term "relational database".
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</para>
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<para>
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In <productname>PostgreSQL</productname>, "relation" is commonly used to
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mean an <glossterm linkend="glossary-sql-object">SQL object</glossterm>
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that has a name and a list of attributes defined in a specific order.
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<glossterm linkend="glossary-table">Tables</glossterm>,
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<glossterm linkend="glossary-sequence">sequences</glossterm>,
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<glossterm linkend="glossary-view">views</glossterm>,
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@@ -1431,15 +1435,14 @@
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<glossterm linkend="glossary-materialized-view">materialized views</glossterm>,
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composite types, and
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<glossterm linkend="glossary-index">indexes</glossterm> are all relations.
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A relation in this sense is a container or a descriptor for a set of tuples.
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</para>
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<para>
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More generically, a relation is a set of tuples; for example,
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the result of a query is also a relation.
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</para>
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<para>
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In <productname>PostgreSQL</productname>,
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<firstterm>Class</firstterm> is an archaic synonym for
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<firstterm>relation</firstterm>.
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<firstterm>Class</firstterm> is an alternative but archaic term.
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The system catalog
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<link linkend="catalog-pg-class"><structname>pg_class</structname></link>
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holds an entry for each <productname>PostgreSQL</productname> relation.
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</para>
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</glossdef>
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</glossentry>
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