postgresql/doc/src/sgml/ref/declare.sgml

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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 ] [ [ NO ] SCROLL ]
    CURSOR [ { WITH | WITHOUT } HOLD ] 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
	<command>FETCH</command> 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.  By default, all cursors are insensitive.
	This keyword currently has no effect and is present for
	compatibility with the SQL standard.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NO SCROLL</term>
      <listitem>
       <para>
        Specifies that the cursor cannot be used to retrieve rows in a
        nonsequential fashion (e.g., backward).
       </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., backward). Depending upon the
	complexity of the query's execution plan, specifying
	<literal>SCROLL</literal> may impose a performance penalty
	on the query's execution time.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>WITHOUT HOLD</term>
      <listitem>
       <para>
        Specifies that the cursor cannot be used outside of the
        transaction that created it. If neither <literal>WITHOUT
        HOLD</literal> nor <literal>WITH HOLD</literal> is specified,
        <literal>WITHOUT HOLD</literal> is the default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>WITH HOLD</term>
      <listitem>
       <para>
        Specifies that the cursor may continue to be used after the
	transaction that creates it successfully commits.
       </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 <literal>BINARY</literal>, <literal>INSENSITIVE</literal>,
    and <literal>SCROLL</literal> 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 <command>SELECT</command> 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 a cursor with the same name already
	exists. 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, and <literal>WITH HOLD</literal> is not
	specified.
       </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>
    If <literal>WITH HOLD</literal> is not specified, the cursor
    created by this command can only be used within the current
    transaction. 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>
    If <literal>WITH HOLD</literal> is specified and the transaction
    that created the cursor successfully commits, the cursor can be
    continue to be accessed by subsequent transactions in the same session.
    (But if the creating
    transaction is aborted, the cursor is removed.) A cursor created
    with <literal>WITH HOLD</literal> is closed when an explicit
    <command>CLOSE</command> command is issued on it, or when the client
    connection is terminated.  In the current implementation, the rows
    represented by a held cursor are copied into a temporary file or
    memory area so that they remain available for subsequent transactions.
   </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. However, application developers are advised not to rely on
    using backward fetches from a cursor that has not been created
    with <literal>SCROLL</literal>.  If <literal>NO SCROLL</> is specified,
    then backward fetches are disallowed in any case.
   </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 <command>DECLARE</command> and <command>OPEN</command> 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</>
    permits cursors to be used interactively.
   </para>

   <para>
    <acronym>SQL92</acronym> allows embedded or modular cursors to
    update database information. All <productname>PostgreSQL</>
    cursors are read only.
   </para>

   <para>
    The <literal>BINARY</literal> keyword is a
    <productname>PostgreSQL</productname> extension.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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