declare.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/declare.sgml,v 1.18 2002/05/18 15:44:47 petere 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 fetch 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>
	<acronym>SQL92</acronym> keyword indicating that data may be retrieved
	in multiple rows per FETCH operation. Since this is allowed at all times
	by <productname>PostgreSQL</productname> this keyword has no effect.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">query</replaceable></term>
      <listitem>
       <para>
	An SQL query which will provide the rows to be governed by the
	cursor.
	Refer to the SELECT statement 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>
  </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, either ASCII or another
   encoding scheme depending on how the <productname>PostgreSQL</productname>
   backend was built. 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.
   However, binary cursors may be a
   little more efficient since there is less conversion overhead in
   the server to client data transfer.

   <tip>
    <para>
     If you intend to display the data in
     ASCII,  getting it back in ASCII 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 in transactions. Use to
    <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>
    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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