Current File : //opt/alt/postgresql11/usr/share/man/man7/DECLARE.7
'\" t
.\" Title: DECLARE
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\" Date: 2017-11-06
.\" Manual: PostgreSQL 9.2.24 Documentation
.\" Source: PostgreSQL 9.2.24
.\" Language: English
.\"
.TH "DECLARE" "7" "2017-11-06" "PostgreSQL 9.2.24" "PostgreSQL 9.2.24 Documentation"
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.SH "NAME"
DECLARE \- define a cursor
.\" DECLARE
.\" cursor: DECLARE
.SH "SYNOPSIS"
.sp
.nf
DECLARE \fIname\fR [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
CURSOR [ { WITH | WITHOUT } HOLD ] FOR \fIquery\fR
.fi
.SH "DESCRIPTION"
.PP
\fBDECLARE\fR
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
\fBFETCH\fR(7)\&.
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBNote\fR
.ps -1
.br
.PP
This page describes usage of cursors at the SQL command level\&. If you are trying to use cursors inside a
PL/pgSQL
function, the rules are different \(em see
Section 39.7, \(lqCursors\(rq, in the documentation\&.
.sp .5v
.RE
.SH "PARAMETERS"
.PP
\fIname\fR
.RS 4
The name of the cursor to be created\&.
.RE
.PP
BINARY
.RS 4
Causes the cursor to return data in binary rather than in text format\&.
.RE
.PP
INSENSITIVE
.RS 4
Indicates that data retrieved from the cursor should be unaffected by updates to the table(s) underlying the cursor that occur after the cursor is created\&. In
PostgreSQL, this is the default behavior; so this key word has no effect and is only accepted for compatibility with the SQL standard\&.
.RE
.PP
SCROLL, NO SCROLL
.RS 4
SCROLL
specifies that the cursor can be used to retrieve rows in a nonsequential fashion (e\&.g\&., backward)\&. Depending upon the complexity of the query\*(Aqs execution plan, specifying
SCROLL
might impose a performance penalty on the query\*(Aqs execution time\&.
NO SCROLL
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
SCROLL\&. See
NOTES
for details\&.
.RE
.PP
WITH HOLD, WITHOUT HOLD
.RS 4
WITH HOLD
specifies that the cursor can continue to be used after the transaction that created it successfully commits\&.
WITHOUT HOLD
specifies that the cursor cannot be used outside of the transaction that created it\&. If neither
WITHOUT HOLD
nor
WITH HOLD
is specified,
WITHOUT HOLD
is the default\&.
.RE
.PP
\fIquery\fR
.RS 4
A
\fBSELECT\fR(7)
or
\fBVALUES\fR(7)
command which will provide the rows to be returned by the cursor\&.
.RE
.PP
The key words
BINARY,
INSENSITIVE, and
SCROLL
can appear in any order\&.
.SH "NOTES"
.PP
Normal cursors return data in text format, the same as a
\fBSELECT\fR
would produce\&. The
BINARY
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
1
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)\&.
.PP
Binary cursors should be used carefully\&. Many applications, including
psql, are not prepared to handle binary cursors and expect data to come back in the text format\&.
.if n \{\
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.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
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.ps +1
\fBNote\fR
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.br
.PP
When the client application uses the
\(lqextended query\(rq
protocol to issue a
\fBFETCH\fR
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 \(em any cursor can be treated as either text or binary\&.
.sp .5v
.RE
.PP
Unless
WITH HOLD
is specified, the cursor created by this command can only be used within the current transaction\&. Thus,
\fBDECLARE\fR
without
WITH HOLD
is useless outside a transaction block: the cursor would survive only to the completion of the statement\&. Therefore
PostgreSQL
reports an error if such a command is used outside a transaction block\&. Use
\fBBEGIN\fR(7)
and
\fBCOMMIT\fR(7)
(or
\fBROLLBACK\fR(7)) to define a transaction block\&.
.PP
If
WITH HOLD
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
WITH HOLD
is closed when an explicit
\fBCLOSE\fR
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\&.
.PP
WITH HOLD
may not be specified when the query includes
FOR UPDATE
or
FOR SHARE\&.
.PP
The
SCROLL
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,
PostgreSQL
will allow backward fetches without
SCROLL, if the cursor\*(Aqs 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
SCROLL\&. If
NO SCROLL
is specified, then backward fetches are disallowed in any case\&.
.PP
Backward fetches are also disallowed when the query includes
FOR UPDATE
or
FOR SHARE; therefore
SCROLL
may not be specified in this case\&.
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBCaution\fR
.ps -1
.br
.PP
Scrollable and
WITH HOLD
cursors may give unexpected results if they invoke any volatile functions (see
Section 35.6, \(lqFunction Volatility Categories\(rq, in the documentation)\&. When a previously fetched row is re\-fetched, the functions might be re\-executed, perhaps leading to results different from the first time\&. One workaround for such cases is to declare the cursor
WITH HOLD
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\&.
.sp .5v
.RE
.PP
If the cursor\*(Aqs query includes
FOR UPDATE
or
FOR SHARE, then returned rows are locked at the time they are first fetched, in the same way as for a regular
\fBSELECT\fR(7)
command with these options\&. In addition, the returned rows will be the most up\-to\-date versions; therefore these options provide the equivalent of what the SQL standard calls a
\(lqsensitive cursor\(rq\&. (Specifying
INSENSITIVE
together with
FOR UPDATE
or
FOR SHARE
is an error\&.)
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBCaution\fR
.ps -1
.br
.PP
It is generally recommended to use
FOR UPDATE
if the cursor is intended to be used with
\fBUPDATE \&.\&.\&. WHERE CURRENT OF\fR
or
\fBDELETE \&.\&.\&. WHERE CURRENT OF\fR\&. Using
FOR UPDATE
prevents other sessions from changing the rows between the time they are fetched and the time they are updated\&. Without
FOR UPDATE, a subsequent
WHERE CURRENT OF
command will have no effect if the row was changed since the cursor was created\&.
.PP
Another reason to use
FOR UPDATE
is that without it, a subsequent
WHERE CURRENT OF
might fail if the cursor query does not meet the SQL standard\*(Aqs rules for being
\(lqsimply updatable\(rq
(in particular, the cursor must reference just one table and not use grouping or
ORDER BY)\&. 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\&.
.PP
The main reason not to use
FOR UPDATE
with
WHERE CURRENT OF
is if you need the cursor to be scrollable, or to be insensitive to the subsequent updates (that is, continue to show the old data)\&. If this is a requirement, pay close heed to the caveats shown above\&.
.sp .5v
.RE
.PP
The SQL standard only makes provisions for cursors in embedded
SQL\&. The
PostgreSQL
server does not implement an
\fBOPEN\fR
statement for cursors; a cursor is considered to be open when it is declared\&. However,
ECPG, the embedded SQL preprocessor for
PostgreSQL, supports the standard SQL cursor conventions, including those involving
\fBDECLARE\fR
and
\fBOPEN\fR
statements\&.
.PP
You can see all available cursors by querying the
pg_cursors
system view\&.
.SH "EXAMPLES"
.PP
To declare a cursor:
.sp
.if n \{\
.RS 4
.\}
.nf
DECLARE liahona CURSOR FOR SELECT * FROM films;
.fi
.if n \{\
.RE
.\}
.sp
See
\fBFETCH\fR(7)
for more examples of cursor usage\&.
.SH "COMPATIBILITY"
.PP
The SQL standard says that it is implementation\-dependent whether cursors are sensitive to concurrent updates of the underlying data by default\&. In
PostgreSQL, cursors are insensitive by default, and can be made sensitive by specifying
FOR UPDATE\&. Other products may work differently\&.
.PP
The SQL standard allows cursors only in embedded
SQL
and in modules\&.
PostgreSQL
permits cursors to be used interactively\&.
.PP
Binary cursors are a
PostgreSQL
extension\&.
.SH "SEE ALSO"
\fBCLOSE\fR(7), \fBFETCH\fR(7), \fBMOVE\fR(7)
Mini Shell