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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-16 19:46:48 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-16 19:46:48 +0000 |
| commit | 311bcfc6b3acdd6fd152798c7f287ddf74fa2a98 (patch) | |
| tree | 0ec307299b1dada3701e42f4ca6eda57d708261e /doc/src/sgml/html/sql-declare.html | |
| parent | Initial commit. (diff) | |
| download | postgresql-15-upstream.tar.xz postgresql-15-upstream.zip | |
Adding upstream version 15.4.upstream/15.4upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'doc/src/sgml/html/sql-declare.html')
| -rw-r--r-- | doc/src/sgml/html/sql-declare.html | 200 |
1 files changed, 200 insertions, 0 deletions
diff --git a/doc/src/sgml/html/sql-declare.html b/doc/src/sgml/html/sql-declare.html new file mode 100644 index 0000000..d600b61 --- /dev/null +++ b/doc/src/sgml/html/sql-declare.html @@ -0,0 +1,200 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 15.4 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><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"> +DECLARE <em class="replaceable"><code>name</code></em> [ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ] + CURSOR [ { WITH | WITHOUT } HOLD ] FOR <em class="replaceable"><code>query</code></em> +</pre></div><div class="refsect1" id="id-1.9.3.99.6"><h2>Description</h2><p> + <code class="command">DECLARE</code> 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. + After the cursor is created, rows are fetched from it using + <a class="link" href="sql-fetch.html" title="FETCH"><code class="command">FETCH</code></a>. + </p><div class="note"><h3 class="title">Note</h3><p> + This page describes usage of cursors at the SQL command level. + If you are trying to use cursors inside a <span class="application">PL/pgSQL</span> + function, the rules are different — + see <a class="xref" href="plpgsql-cursors.html" title="43.7. Cursors">Section 43.7</a>. + </p></div></div><div class="refsect1" id="id-1.9.3.99.7"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p> + The name of the cursor to be created. + </p></dd><dt><span class="term"><code class="literal">BINARY</code></span></dt><dd><p> + Causes the cursor to return data in binary rather than in text format. + </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> + Cursor sensitivity determines whether changes to the data underlying the + cursor, done in the same transaction, after the cursor has been + declared, are visible in the cursor. <code class="literal">INSENSITIVE</code> + means they are not visible, <code class="literal">ASENSITIVE</code> means the + behavior is implementation-dependent. A third behavior, + <code class="literal">SENSITIVE</code>, meaning that such changes are visible in + the cursor, is not available in <span class="productname">PostgreSQL</span>. + In <span class="productname">PostgreSQL</span>, all cursors are insensitive; + so these key words have no effect and are only accepted for + compatibility with the SQL standard. + </p><p> + Specifying <code class="literal">INSENSITIVE</code> together with <code class="literal">FOR + UPDATE</code> or <code class="literal">FOR SHARE</code> is an error. + </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 used + to retrieve rows in a nonsequential fashion (e.g., + backward). Depending upon the complexity of the query's + execution plan, specifying <code class="literal">SCROLL</code> might impose + a performance penalty on the query's execution time. + <code class="literal">NO SCROLL</code> specifies that the cursor cannot be + used to retrieve rows in a nonsequential fashion. The default is to + allow scrolling in some cases; this is not the same as specifying + <code class="literal">SCROLL</code>. See <a class="xref" href="sql-declare.html#SQL-DECLARE-NOTES" title="Notes">Notes</a> + below for details. + </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 can + continue to be used after the transaction that created it + successfully commits. <code class="literal">WITHOUT HOLD</code> specifies + that the cursor cannot be used outside of the transaction that + created it. If neither <code class="literal">WITHOUT HOLD</code> nor + <code class="literal">WITH HOLD</code> is specified, <code class="literal">WITHOUT + HOLD</code> is the default. + </p></dd><dt><span class="term"><em class="replaceable"><code>query</code></em></span></dt><dd><p> + A <a class="link" href="sql-select.html" title="SELECT"><code class="command">SELECT</code></a> or + <a class="link" href="sql-values.html" title="VALUES"><code class="command">VALUES</code></a> command + which will provide the rows to be returned by the cursor. + </p></dd></dl></div><p> + The key words <code class="literal">ASENSITIVE</code>, <code class="literal">BINARY</code>, + <code class="literal">INSENSITIVE</code>, and <code class="literal">SCROLL</code> can + appear in any order. + </p></div><div class="refsect1" id="SQL-DECLARE-NOTES"><h2>Notes</h2><p> + Normal cursors return data in text format, the same as a + <code class="command">SELECT</code> would produce. The <code class="literal">BINARY</code> option + specifies that the cursor should return data in binary format. + This reduces conversion effort for both the server and client, + at the cost of more programmer effort to deal with platform-dependent + binary data formats. + As an example, if a query returns a value of one from an integer column, + you would get a string of <code class="literal">1</code> with a default cursor, + whereas with a binary cursor you would get + a 4-byte field containing the internal representation of the value + (in big-endian byte order). + </p><p> + Binary cursors should be used carefully. Many applications, + including <span class="application">psql</span>, are not prepared to + handle binary cursors and expect data to come back in the text + format. + </p><div class="note"><h3 class="title">Note</h3><p> + When the client application uses the <span class="quote">“<span class="quote">extended query</span>”</span> protocol + to issue a <code class="command">FETCH</code> command, the Bind protocol message + specifies whether data is to be retrieved in text or binary format. + This choice overrides the way that the cursor is defined. The concept + of a binary cursor as such is thus obsolete when using extended query + protocol — any cursor can be treated as either text or binary. + </p></div><p> + Unless <code class="literal">WITH HOLD</code> is specified, the cursor + created by this command can only be used within the current + transaction. Thus, <code class="command">DECLARE</code> without <code class="literal">WITH + HOLD</code> is useless outside a transaction block: the cursor would + survive only to the completion of the statement. Therefore + <span class="productname">PostgreSQL</span> reports an error if such a + command is used outside a transaction block. + Use + <a class="link" href="sql-begin.html" title="BEGIN"><code class="command">BEGIN</code></a> and + <a class="link" href="sql-commit.html" title="COMMIT"><code class="command">COMMIT</code></a> + (or <a class="link" href="sql-rollback.html" title="ROLLBACK"><code class="command">ROLLBACK</code></a>) + to define a transaction block. + </p><p> + If <code class="literal">WITH HOLD</code> is specified and the transaction + that created the cursor successfully commits, the cursor can + 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 <code class="literal">WITH HOLD</code> + is closed when an explicit <code class="command">CLOSE</code> command is + issued on it, or the session ends. 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. + </p><p> + <code class="literal">WITH HOLD</code> may not be specified when the query + includes <code class="literal">FOR UPDATE</code> or <code class="literal">FOR SHARE</code>. + </p><p> + The <code class="literal">SCROLL</code> option should be specified when defining a + cursor that will be used to fetch backwards. This is required by + the SQL standard. However, for compatibility with earlier + versions, <span class="productname">PostgreSQL</span> will allow + backward fetches without <code class="literal">SCROLL</code>, 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 <code class="literal">SCROLL</code>. If <code class="literal">NO SCROLL</code> is + specified, then backward fetches are disallowed in any case. + </p><p> + Backward fetches are also disallowed when the query + includes <code class="literal">FOR UPDATE</code> or <code class="literal">FOR SHARE</code>; therefore + <code class="literal">SCROLL</code> may not be specified in this case. + </p><div class="caution"><h3 class="title">Caution</h3><p> + Scrollable cursors may give unexpected + 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 is + re-fetched, the functions might be re-executed, perhaps leading to + results different from the first time. It's best to + specify <code class="literal">NO SCROLL</code> for a query involving volatile + functions. If that is not practical, one workaround + is to declare the cursor <code class="literal">SCROLL WITH HOLD</code> and commit the + transaction before reading any rows from it. This will force the + entire output of the cursor to be materialized in temporary storage, + so that volatile functions are executed exactly once for each row. + </p></div><p> + If the cursor's query includes <code class="literal">FOR UPDATE</code> or <code class="literal">FOR + SHARE</code>, then returned rows are locked at the time they are first + fetched, in the same way as for a regular + <a class="link" href="sql-select.html" title="SELECT"><code class="command">SELECT</code></a> command with + these options. + In addition, the returned rows will be the most up-to-date versions. + </p><div class="caution"><h3 class="title">Caution</h3><p> + It is generally recommended to use <code class="literal">FOR UPDATE</code> if the cursor + is intended to be used with <code class="command">UPDATE ... WHERE CURRENT OF</code> or + <code class="command">DELETE ... WHERE CURRENT OF</code>. Using <code class="literal">FOR UPDATE</code> + prevents other sessions from changing the rows between the time they are + fetched and the time they are updated. Without <code class="literal">FOR UPDATE</code>, + a subsequent <code class="literal">WHERE CURRENT OF</code> command will have no effect if + the row was changed since the cursor was created. + </p><p> + Another reason to use <code class="literal">FOR UPDATE</code> is that without it, a + subsequent <code class="literal">WHERE CURRENT OF</code> might fail if the cursor query + does not meet the SQL standard's rules for being <span class="quote">“<span class="quote">simply + updatable</span>”</span> (in particular, the cursor must reference just one table + and not use grouping or <code class="literal">ORDER BY</code>). Cursors + that are not simply updatable might work, or might not, depending on plan + choice details; so in the worst case, an application might work in testing + and then fail in production. If <code class="literal">FOR UPDATE</code> is + specified, the cursor is guaranteed to be updatable. + </p><p> + The main reason not to use <code class="literal">FOR UPDATE</code> with <code class="literal">WHERE + CURRENT OF</code> is if you need the cursor to be scrollable, or to be + isolated from concurrent updates (that is, continue to show the old + data). If this is a requirement, pay close heed to the caveats shown + above. + </p></div><p> + The SQL standard only makes provisions for cursors in embedded + <acronym class="acronym">SQL</acronym>. The <span class="productname">PostgreSQL</span> + server does not implement an <code class="command">OPEN</code> statement for + cursors; a cursor is considered to be open when it is declared. + However, <span class="application">ECPG</span>, the embedded SQL + preprocessor for <span class="productname">PostgreSQL</span>, supports + the standard SQL cursor conventions, including those involving + <code class="command">DECLARE</code> and <code class="command">OPEN</code> statements. + </p><p> + 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> + system view. + </p></div><div class="refsect1" id="id-1.9.3.99.9"><h2>Examples</h2><p> + To declare a cursor: +</p><pre class="programlisting"> +DECLARE liahona CURSOR FOR SELECT * FROM films; +</pre><p> + See <a class="xref" href="sql-fetch.html" title="FETCH"><span class="refentrytitle">FETCH</span></a> for more + examples of cursor usage. + </p></div><div class="refsect1" id="id-1.9.3.99.10"><h2>Compatibility</h2><p> + The SQL standard allows cursors only in embedded + <acronym class="acronym">SQL</acronym> and in modules. <span class="productname">PostgreSQL</span> + permits cursors to be used interactively. + </p><p> + According to the SQL standard, changes made to insensitive cursors by + <code class="literal">UPDATE ... WHERE CURRENT OF</code> and <code class="literal">DELETE + ... WHERE CURRENT OF</code> statements are visible in that same + cursor. <span class="productname">PostgreSQL</span> treats these statements like + all other data changing statements in that they are not visible in + insensitive cursors. + </p><p> + Binary cursors are a <span class="productname">PostgreSQL</span> + extension. + </p></div><div class="refsect1" id="id-1.9.3.99.11"><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 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> DELETE</td></tr></table></div></body></html>
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