1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
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 15.7 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 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> DELETE</td></tr></table></div></body></html>
|
