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$Header: /cvsroot/pgsql/doc/src/sgml/ref/declare.sgml,v 1.19 2003/03/10 03:53:48 tgl Exp $
PostgreSQL documentation
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<refentry id="SQL-DECLARE">
 <refmeta>
  <refentrytitle id="SQL-DECLARE-TITLE">DECLARE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   DECLARE
  </refname>
  <refpurpose>
   define a cursor
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INSENSITIVE ] [ SCROLL ]
    CURSOR FOR <replaceable class="parameter">query</replaceable>
    [ FOR { READ ONLY | UPDATE [ OF <replaceable class="parameter">column</replaceable> [, ...] ] ]
  </synopsis>
  <refsect2 id="R2-SQL-DECLARE-1">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>
    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">cursorname</replaceable></term>
      <listitem>
       <para>
	The name of the cursor to be used in subsequent FETCH operations.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>BINARY</term>
      <listitem>
       <para>
	Causes the cursor to return data in binary rather than in text format.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>INSENSITIVE</term>
      <listitem>
       <para>
	<acronym>SQL92</acronym> keyword indicating that data retrieved
	from the cursor should be unaffected by updates from other processes or cursors.
	Since cursor operations occur within transactions
	in <productname>PostgreSQL</productname> this is always the case.
	This keyword has no effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SCROLL</term>
      <listitem>
       <para>
	Specifies that the cursor may be used to retrieve rows
	in a nonsequential fashion (e.g., backwards).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">query</replaceable></term>
      <listitem>
       <para>
	A <command>SELECT</> query which will provide the rows to be
	returned by the cursor.
	Refer to <xref linkend="sql-select" endterm="sql-select-title">
	for further information about valid arguments.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>READ ONLY</term>
      <listitem>
       <para>
	<acronym>SQL92</acronym> keyword indicating that the cursor will be used
	in a read only mode. Since this is the only cursor access mode
	available in <productname>PostgreSQL</productname> this keyword has no effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>UPDATE</term>
      <listitem>
       <para>
	<acronym>SQL92</acronym> keyword indicating that the cursor will be used
	to update tables. Since cursor updates are not currently
	supported in <productname>PostgreSQL</productname> this keyword
	provokes an informational error message.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">column</replaceable></term>
      <listitem>
       <para>
	Column(s) to be updated.
	Since cursor updates are not currently
	supported in <productname>PostgreSQL</productname> the UPDATE clause
	provokes an informational error message.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <para>
    The BINARY, INSENSITIVE, and SCROLL keywords may appear in any order.
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-DECLARE-2">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
DECLARE CURSOR
       </computeroutput></term>
      <listitem>
       <para>
	The message returned if the SELECT is run successfully.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
WARNING:  Closing pre-existing portal "<replaceable class="parameter">cursorname</replaceable>"
       </computeroutput></term>
      <listitem>
       <para>
	This message is reported if the same cursor name was already declared
	in the current transaction block.  The previous definition is
	discarded.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
ERROR:  DECLARE CURSOR may only be used in begin/end transaction blocks
       </computeroutput></term>
      <listitem>
       <para>
	This error occurs if the cursor is not declared within a transaction block.
       </para>
      </listitem>
     </varlistentry>     
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-DECLARE-1">
  <refsect1info>
   <date>1998-09-04</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>DECLARE</command> allows a user to create cursors, which
   can be used to retrieve
   a small number of rows at a time out of a larger query. Cursors can
   return data either in text or in binary format using
   <xref linkend="sql-fetch" endterm="sql-fetch-title">.
  </para>

  <para>
   Normal cursors return data in text format, the same as a <command>SELECT</>
   would produce.  Since
   data is stored natively in binary format, the system must
   do a conversion to produce the text format. In addition,
   text formats are often larger in size than the corresponding binary format.
   Once the information comes back in text form,  the client
   application may need to convert it to a binary format to
   manipulate it.
   BINARY cursors give you back the data in the native binary
   representation.
  </para>

  <para>
   As an example, if a query returns a value of one from an integer column,
   you would get a string of <literal>1</> with a default cursor
   whereas with a binary cursor you would get
   a 4-byte value equal to control-A (<literal>^A</literal>).
  </para>

  <para>
   BINARY cursors should be used carefully. User applications such
   as <application>psql</application> are not aware of binary cursors
   and expect data to come back in a text format.
  </para>

  <para>
   String representation is architecture-neutral whereas binary
   representation can differ between different machine architectures.
   <emphasis><productname>PostgreSQL</productname> does not resolve
    byte ordering or representation issues for binary cursors</emphasis>.
   Therefore, if your client machine and server machine use different
   representations (e.g., <quote>big-endian</quote> versus <quote>little-endian</quote>),
   you will probably not want your data returned in
   binary format.

   <tip>
    <para>
     If you intend to display the data as text, retrieving it in text form
     will save you some effort on the client side.
    </para>
   </tip>
  </para>

  <refsect2 id="R2-SQL-DECLARE-3">
   <refsect2info>
    <date>1998-09-04</date>
   </refsect2info>
   <title>
    Notes
   </title>

   <para>
    Cursors are only available within transactions. Use
    <xref linkend="sql-begin" endterm="sql-begin-title">,
    <xref linkend="sql-commit" endterm="sql-commit-title">
    and
    <xref linkend="sql-rollback" endterm="sql-rollback-title">
    to define a transaction block.
   </para>

   <para>
    The <literal>SCROLL</> option should be specified when defining a cursor 
    that will be used to fetch backwards.  This is required by
    <acronym>SQL92</acronym>.  However, for compatibility with
    earlier versions, <productname>PostgreSQL</productname> will allow
    backward fetches without <literal>SCROLL</>, if the cursor's query plan
    is simple enough that no extra overhead is needed to support it.
   </para>

   <para>
    In <acronym>SQL92</acronym> cursors are only available in
    embedded <acronym>SQL</acronym> (<acronym>ESQL</acronym>) applications. 
    The <productname>PostgreSQL</productname> backend
    does not implement an explicit <command>OPEN cursor</command>
    statement; a cursor is considered to be open when it is declared.
    However, <application>ecpg</application>, the
    embedded SQL preprocessor for <productname>PostgreSQL</productname>,
    supports the <acronym>SQL92</acronym> cursor conventions, including those
    involving DECLARE and OPEN statements.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-DECLARESTATEMENT-2">
  <title>
   Usage
  </title>
  <para>
   To declare a cursor:

   <programlisting>
DECLARE liahona CURSOR
    FOR SELECT * FROM films;
   </programlisting>
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-DECLARESTATEMENT-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-DECLARESTATEMENT-4">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    <acronym>SQL92</acronym> allows cursors only in embedded <acronym>SQL</acronym>
    and in modules. <productname>PostgreSQL</productname> permits cursors to be used
    interactively.
    <acronym>SQL92</acronym> allows embedded or modular cursors to
    update database information.
    All <productname>PostgreSQL</productname> cursors are read only.
    The BINARY keyword is a <productname>PostgreSQL</productname> extension.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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