codekingpro/portable-devtools

1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>DECLARE</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-deallocate.html" title="DEALLOCATE" /><link rel="next" href="sql-delete.html" title="DELETE" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">DECLARE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-deallocate.html" title="DEALLOCATE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-delete.html" title="DELETE">Next</a></td></tr></table><hr /></div><div class="refentry" id="SQL-DECLARE"><div class="titlepage"></div><a id="id-1.9.3.99.1" class="indexterm"></a><a id="id-1.9.3.99.2" class="indexterm"></a><a id="id-1.9.3.99.3" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">DECLARE</span></h2><p>DECLARE — define a cursor</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">3DECLARE <em class="replaceable"><code>name</code></em> [ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ]4    CURSOR [ { WITH | WITHOUT } HOLD ] FOR <em class="replaceable"><code>query</code></em>5</pre></div><div class="refsect1" id="id-1.9.3.99.7"><h2>Description</h2><p>6   <code class="command">DECLARE</code> allows a user to create cursors, which7   can be used to retrieve8   a small number of rows at a time out of a larger query.9   After the cursor is created, rows are fetched from it using10   <a class="link" href="sql-fetch.html" title="FETCH"><code class="command">FETCH</code></a>.11  </p><div class="note"><h3 class="title">Note</h3><p>12    This page describes usage of cursors at the SQL command level.13    If you are trying to use cursors inside a <span class="application">PL/pgSQL</span>14    function, the rules are different —15    see <a class="xref" href="plpgsql-cursors.html" title="43.7. Cursors">Section 43.7</a>.16   </p></div></div><div class="refsect1" id="id-1.9.3.99.8"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>17      The name of the cursor to be created.18      This must be different from any other active cursor name in the19      session.20     </p></dd><dt><span class="term"><code class="literal">BINARY</code></span></dt><dd><p>21      Causes the cursor to return data in binary rather than in text format.22     </p></dd><dt><span class="term"><code class="literal">ASENSITIVE</code><br /></span><span class="term"><code class="literal">INSENSITIVE</code></span></dt><dd><p>23      Cursor sensitivity determines whether changes to the data underlying the24      cursor, done in the same transaction, after the cursor has been25      declared, are visible in the cursor.  <code class="literal">INSENSITIVE</code>26      means they are not visible, <code class="literal">ASENSITIVE</code> means the27      behavior is implementation-dependent.  A third behavior,28      <code class="literal">SENSITIVE</code>, meaning that such changes are visible in29      the cursor, is not available in <span class="productname">PostgreSQL</span>.30      In <span class="productname">PostgreSQL</span>, all cursors are insensitive;31      so these key words have no effect and are only accepted for32      compatibility with the SQL standard.33     </p><p>34      Specifying <code class="literal">INSENSITIVE</code> together with <code class="literal">FOR35      UPDATE</code> or <code class="literal">FOR SHARE</code> is an error.36     </p></dd><dt><span class="term"><code class="literal">SCROLL</code><br /></span><span class="term"><code class="literal">NO SCROLL</code></span></dt><dd><p><code class="literal">SCROLL</code> specifies that the cursor can be used37      to retrieve rows in a nonsequential fashion (e.g.,38      backward). Depending upon the complexity of the query's39      execution plan, specifying <code class="literal">SCROLL</code> might impose40      a performance penalty on the query's execution time.41      <code class="literal">NO SCROLL</code> specifies that the cursor cannot be42      used to retrieve rows in a nonsequential fashion.  The default is to43      allow scrolling in some cases; this is not the same as specifying44      <code class="literal">SCROLL</code>. See <a class="xref" href="sql-declare.html#SQL-DECLARE-NOTES" title="Notes">Notes</a>45      below for details.46     </p></dd><dt><span class="term"><code class="literal">WITH HOLD</code><br /></span><span class="term"><code class="literal">WITHOUT HOLD</code></span></dt><dd><p><code class="literal">WITH HOLD</code> specifies that the cursor can47      continue to be used after the transaction that created it48      successfully commits.  <code class="literal">WITHOUT HOLD</code> specifies49      that the cursor cannot be used outside of the transaction that50      created it. If neither <code class="literal">WITHOUT HOLD</code> nor51      <code class="literal">WITH HOLD</code> is specified, <code class="literal">WITHOUT52      HOLD</code> is the default.53     </p></dd><dt><span class="term"><em class="replaceable"><code>query</code></em></span></dt><dd><p>54      A <a class="link" href="sql-select.html" title="SELECT"><code class="command">SELECT</code></a> or55      <a class="link" href="sql-values.html" title="VALUES"><code class="command">VALUES</code></a> command56      which will provide the rows to be returned by the cursor.57     </p></dd></dl></div><p>58   The key words <code class="literal">ASENSITIVE</code>, <code class="literal">BINARY</code>,59   <code class="literal">INSENSITIVE</code>, and <code class="literal">SCROLL</code> can60   appear in any order.61  </p></div><div class="refsect1" id="SQL-DECLARE-NOTES"><h2>Notes</h2><p>62   Normal cursors return data in text format, the same as a63   <code class="command">SELECT</code> would produce.  The <code class="literal">BINARY</code> option64   specifies that the cursor should return data in binary format.65   This reduces conversion effort for both the server and client,66   at the cost of more programmer effort to deal with platform-dependent67   binary data formats.68   As an example, if a query returns a value of one from an integer column,69   you would get a string of <code class="literal">1</code> with a default cursor,70   whereas with a binary cursor you would get71   a 4-byte field containing the internal representation of the value72   (in big-endian byte order).73  </p><p>74   Binary cursors should be used carefully.  Many applications,75   including <span class="application">psql</span>, are not prepared to76   handle binary cursors and expect data to come back in the text77   format.78  </p><div class="note"><h3 class="title">Note</h3><p>79    When the client application uses the <span class="quote">“<span class="quote">extended query</span>”</span> protocol80    to issue a <code class="command">FETCH</code> command, the Bind protocol message81    specifies whether data is to be retrieved in text or binary format.82    This choice overrides the way that the cursor is defined.  The concept83    of a binary cursor as such is thus obsolete when using extended query84    protocol — any cursor can be treated as either text or binary.85   </p></div><p>86    Unless <code class="literal">WITH HOLD</code> is specified, the cursor87    created by this command can only be used within the current88    transaction.  Thus, <code class="command">DECLARE</code> without <code class="literal">WITH89    HOLD</code> is useless outside a transaction block: the cursor would90    survive only to the completion of the statement.  Therefore91    <span class="productname">PostgreSQL</span> reports an error if such a92    command is used outside a transaction block.93    Use94    <a class="link" href="sql-begin.html" title="BEGIN"><code class="command">BEGIN</code></a> and95    <a class="link" href="sql-commit.html" title="COMMIT"><code class="command">COMMIT</code></a>96    (or <a class="link" href="sql-rollback.html" title="ROLLBACK"><code class="command">ROLLBACK</code></a>)97    to define a transaction block.98   </p><p>99    If <code class="literal">WITH HOLD</code> is specified and the transaction100    that created the cursor successfully commits, the cursor can101    continue to be accessed by subsequent transactions in the same102    session.  (But if the creating transaction is aborted, the cursor103    is removed.)  A cursor created with <code class="literal">WITH HOLD</code>104    is closed when an explicit <code class="command">CLOSE</code> command is105    issued on it, or the session ends.  In the current implementation,106    the rows represented by a held cursor are copied into a temporary107    file or memory area so that they remain available for subsequent108    transactions.109   </p><p>110    <code class="literal">WITH HOLD</code> may not be specified when the query111    includes <code class="literal">FOR UPDATE</code> or <code class="literal">FOR SHARE</code>.112   </p><p>113    The <code class="literal">SCROLL</code> option should be specified when defining a114    cursor that will be used to fetch backwards.  This is required by115    the SQL standard.  However, for compatibility with earlier116    versions, <span class="productname">PostgreSQL</span> will allow117    backward fetches without <code class="literal">SCROLL</code>, if the cursor's query118    plan is simple enough that no extra overhead is needed to support119    it. However, application developers are advised not to rely on120    using backward fetches from a cursor that has not been created121    with <code class="literal">SCROLL</code>.  If <code class="literal">NO SCROLL</code> is122    specified, then backward fetches are disallowed in any case.123   </p><p>124    Backward fetches are also disallowed when the query125    includes <code class="literal">FOR UPDATE</code> or <code class="literal">FOR SHARE</code>; therefore126    <code class="literal">SCROLL</code> may not be specified in this case.127   </p><div class="caution"><h3 class="title">Caution</h3><p>128     Scrollable cursors may give unexpected129     results if they invoke any volatile functions (see <a class="xref" href="xfunc-volatility.html" title="38.7. Function Volatility Categories">Section 38.7</a>).  When a previously fetched row is130     re-fetched, the functions might be re-executed, perhaps leading to131     results different from the first time.  It's best to132     specify <code class="literal">NO SCROLL</code> for a query involving volatile133     functions.  If that is not practical, one workaround134     is to declare the cursor <code class="literal">SCROLL WITH HOLD</code> and commit the135     transaction before reading any rows from it.  This will force the136     entire output of the cursor to be materialized in temporary storage,137     so that volatile functions are executed exactly once for each row.138    </p></div><p>139    If the cursor's query includes <code class="literal">FOR UPDATE</code> or <code class="literal">FOR140    SHARE</code>, then returned rows are locked at the time they are first141    fetched, in the same way as for a regular142    <a class="link" href="sql-select.html" title="SELECT"><code class="command">SELECT</code></a> command with143    these options.144    In addition, the returned rows will be the most up-to-date versions.145   </p><div class="caution"><h3 class="title">Caution</h3><p>146     It is generally recommended to use <code class="literal">FOR UPDATE</code> if the cursor147     is intended to be used with <code class="command">UPDATE ... WHERE CURRENT OF</code> or148     <code class="command">DELETE ... WHERE CURRENT OF</code>.  Using <code class="literal">FOR UPDATE</code>149     prevents other sessions from changing the rows between the time they are150     fetched and the time they are updated.  Without <code class="literal">FOR UPDATE</code>,151     a subsequent <code class="literal">WHERE CURRENT OF</code> command will have no effect if152     the row was changed since the cursor was created.153    </p><p>154     Another reason to use <code class="literal">FOR UPDATE</code> is that without it, a155     subsequent <code class="literal">WHERE CURRENT OF</code> might fail if the cursor query156     does not meet the SQL standard's rules for being <span class="quote">“<span class="quote">simply157     updatable</span>”</span> (in particular, the cursor must reference just one table158     and not use grouping or <code class="literal">ORDER BY</code>).  Cursors159     that are not simply updatable might work, or might not, depending on plan160     choice details; so in the worst case, an application might work in testing161     and then fail in production.  If <code class="literal">FOR UPDATE</code> is162     specified, the cursor is guaranteed to be updatable.163    </p><p>164     The main reason not to use <code class="literal">FOR UPDATE</code> with <code class="literal">WHERE165     CURRENT OF</code> is if you need the cursor to be scrollable, or to be166     isolated from concurrent updates (that is, continue to show the old167     data).  If this is a requirement, pay close heed to the caveats shown168     above.169    </p></div><p>170    The SQL standard only makes provisions for cursors in embedded171    <acronym class="acronym">SQL</acronym>.  The <span class="productname">PostgreSQL</span>172    server does not implement an <code class="command">OPEN</code> statement for173    cursors; a cursor is considered to be open when it is declared.174    However, <span class="application">ECPG</span>, the embedded SQL175    preprocessor for <span class="productname">PostgreSQL</span>, supports176    the standard SQL cursor conventions, including those involving177    <code class="command">DECLARE</code> and <code class="command">OPEN</code> statements.178   </p><p>179    The server data structure underlying an open cursor is called a180    <em class="firstterm">portal</em>.  Portal names are exposed in the181    client protocol: a client can fetch rows directly from an open182    portal, if it knows the portal name.  When creating a cursor with183    <code class="command">DECLARE</code>, the portal name is the same as the184    cursor name.185   </p><p>186    You can see all available cursors by querying the <a class="link" href="view-pg-cursors.html" title="54.6. pg_cursors"><code class="structname">pg_cursors</code></a>187    system view.188   </p></div><div class="refsect1" id="id-1.9.3.99.10"><h2>Examples</h2><p>189   To declare a cursor:190</p><pre class="programlisting">191DECLARE liahona CURSOR FOR SELECT * FROM films;192</pre><p>193   See <a class="xref" href="sql-fetch.html" title="FETCH"><span class="refentrytitle">FETCH</span></a> for more194   examples of cursor usage.195  </p></div><div class="refsect1" id="id-1.9.3.99.11"><h2>Compatibility</h2><p>196   The SQL standard allows cursors only in embedded197   <acronym class="acronym">SQL</acronym> and in modules. <span class="productname">PostgreSQL</span>198   permits cursors to be used interactively.199  </p><p>200   According to the SQL standard, changes made to insensitive cursors by201   <code class="literal">UPDATE ... WHERE CURRENT OF</code> and <code class="literal">DELETE202   ... WHERE CURRENT OF</code> statements are visible in that same203   cursor.  <span class="productname">PostgreSQL</span> treats these statements like204   all other data changing statements in that they are not visible in205   insensitive cursors.206  </p><p>207   Binary cursors are a <span class="productname">PostgreSQL</span>208   extension.209  </p></div><div class="refsect1" id="id-1.9.3.99.12"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-close.html" title="CLOSE"><span class="refentrytitle">CLOSE</span></a>, <a class="xref" href="sql-fetch.html" title="FETCH"><span class="refentrytitle">FETCH</span></a>, <a class="xref" href="sql-move.html" title="MOVE"><span class="refentrytitle">MOVE</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-deallocate.html" title="DEALLOCATE">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-delete.html" title="DELETE">Next</a></td></tr><tr><td width="40%" align="left" valign="top">DEALLOCATE </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> DELETE</td></tr></table></div></body></html>