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20th chunk of `doc/src/sgml/glossary.sgml`
6f54407afd94507cede5597cb614cf68513f97aea592aecc0000000100000fa2
 <glossdef>
    <para>
     More generically, the term <firstterm>schema</firstterm> is used to mean
     all data descriptions (<glossterm linkend="glossary-table">table</glossterm> definitions,
     <glossterm linkend="glossary-constraint">constraints</glossterm>, comments, etc.)
     for a given <glossterm linkend="glossary-database">database</glossterm> or
     subset thereof.
    </para>
    <para>
     For more information, see
     <xref linkend="ddl-schemas"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry>
   <glossterm>Segment</glossterm>
   <glosssee otherterm="glossary-file-segment" />
  </glossentry>

  <glossentry id="glossary-select">
   <glossterm>Select</glossterm>
   <glossdef>
    <para>
     The <acronym>SQL</acronym> command used to request data from a
     <glossterm linkend="glossary-database">database</glossterm>.
     Normally, <command>SELECT</command> commands are not expected to modify the
     <glossterm linkend="glossary-database">database</glossterm> in any way,
     but it is possible that
     <glossterm linkend="glossary-function">functions</glossterm> invoked within
     the query could have side effects that do modify data.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-select"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-sequence">
   <glossterm>Sequence (relation)</glossterm>
   <glossdef>
    <para>
     A type of relation that is used to generate values.
     Typically the generated values are sequential non-repeating numbers.
     They are commonly used to generate surrogate
     <glossterm linkend="glossary-primary-key">primary key</glossterm>
     values.
    </para>
   </glossdef>
  </glossentry>

<!--  XXX should define all other isolation levels (and improve this definition)
  <glossentry id="glossary-serializable">
   <glossterm>Serializable (isolation level)</glossterm>
   <glossdef>
    <para>
     Transactions defined as <literal>SERIALIZABLE</literal> are unable to
     see changes made within other transactions. In effect, for the
     initializing session the entire <glossterm linkend="glossary-database">database</glossterm>
     appears to be frozen for the duration of the
     <glossterm linkend="glossary-transaction">Transaction</glossterm>.
    </para>
   </glossdef>
  </glossentry>
-->

  <glossentry id="glossary-server">
   <glossterm>Server</glossterm>
   <glossdef>
    <para>
     A computer on which <productname>PostgreSQL</productname>
     <glossterm linkend="glossary-instance">instances</glossterm> run.
     The term <firstterm>server</firstterm> denotes real hardware, a
     container, or a <firstterm>virtual machine</firstterm>.
    </para>
    <para>
     This term is sometimes used to refer to an instance or to a host.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-session">
   <glossterm>Session</glossterm>
   <glossdef>
    <para>
     A state that allows a client and a backend to interact,
     communicating over a <glossterm linkend="glossary-connection">connection</glossterm>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-shared-memory">
   <glossterm>Shared memory</glossterm>
   <glossdef>
    <para>
     <acronym>RAM</acronym> which is used by the processes common to an
     <glossterm linkend="glossary-instance">instance</glossterm>.
     It mirrors parts of <glossterm linkend="glossary-database">database</glossterm>
     files, provides a transient area for
     <glossterm linkend="glossary-wal-record">WAL records</glossterm>,
     and stores additional common information.
     Note that shared memory belongs to the complete instance, not to a single
     database.
    </para>
    <para>
     The largest part of shared memory is known as <firstterm>shared buffers</firstterm>
     and is used to mirror part of data files, organized into pages.
     When a page is modified, it is called a dirty page until it is
     written back to the
Title: Database Concepts: Schema, Select, and Server
Summary
This section elaborates on several database concepts. It expands on the definition of schema, describing it as all data descriptions for a database or its subset, including table definitions, constraints, and comments. The SELECT command is explained as an SQL command used to request data from a database, typically without modifying data, though functions within the query might have side effects. The text also introduces the concept of a sequence relation, used to generate sequential non-repeating numbers, often for primary keys. Finally, it defines a server in the context of PostgreSQL as a computer (physical or virtual) on which PostgreSQL instances run, noting that the term can sometimes refer to an instance or host.