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>2.2. Concepts</A
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>    
    
    
    
    

    <SPAN
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>PostgreSQL</SPAN
> is a <I
CLASS="FIRSTTERM"
>relational
    database management system</I
> (<ACRONYM
CLASS="ACRONYM"
>RDBMS</ACRONYM
>).
    That means it is a system for managing data stored in
    <I
CLASS="FIRSTTERM"
>relations</I
>.  Relation is essentially a
    mathematical term for <I
CLASS="FIRSTTERM"
>table</I
>.  The notion of
    storing data in tables is so commonplace today that it might
    seem inherently obvious, but there are a number of other ways of
    organizing databases.  Files and directories on Unix-like
    operating systems form an example of a hierarchical database.  A
    more modern development is the object-oriented database.
   </P
><P
>    
    

    Each table is a named collection of <I
CLASS="FIRSTTERM"
>rows</I
>.
    Each row of a given table has the same set of named
    <I
CLASS="FIRSTTERM"
>columns</I
>,
    and each column is of a specific data type.  Whereas columns have
    a fixed order in each row, it is important to remember that SQL
    does not guarantee the order of the rows within the table in any
    way (although they can be explicitly sorted for display).
   </P
><P
>    
    

    Tables are grouped into databases, and a collection of databases
    managed by a single <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server
    instance constitutes a database <I
CLASS="FIRSTTERM"
>cluster</I
>.
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