declare(5)
NAME
DECLARE - define a cursor
SYNOPSIS
DECLARE name [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
CURSOR [ { WITH | WITHOUT } HOLD ] FOR query
[ FOR { READ ONLY | UPDATE [ OF column [, ...] ] } ]
DESCRIPTION
DECLARE 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 FETCH [fetch(5)].
Normal cursors return data in text format, the same as a SELECT would
produce. Since data is stored natively in binary format, the system
must do a conversion to produce the text format. Once the information
comes back in text form, the client application may need to convert it
to a binary format to manipulate it. In addition, data in the text for-
mat is often larger in size than in the binary format. Binary cursors
return the data in a binary representation that may be more easily
manipulated. Nevertheless, if you intend to display the data as text
anyway, retrieving it in text form will save you some effort on the
client side.
As an example, if a query returns a value of one from an integer col-
umn, 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 rep-
resentation of the value (in big-endian byte order).
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.
Note: When the client application uses the ``extended query''
protocol to issue a FETCH command, the Bind protocol message
specifies whether data is to be retrieved in text or binary for-
mat. 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.
PARAMETERS
name The name of the cursor to be created.
BINARY Causes the cursor to return data in binary rather than in text
format.
INSENSITIVE
Indicates that data retrieved from the cursor should be unaf-
fected by updates to the tables underlying the cursor while the
cursor exists. In PostgreSQL, all cursors are insensitive; this
key word currently has no effect and is present for compatibil-
ity with the SQL standard.
SCROLL
NO SCROLL
SCROLL specifies that the cursor may be used to retrieve rows in
a nonsequential fashion (e.g., backward). Depending upon the
complexity of the query's execution plan, specifying SCROLL may
impose a performance penalty on the query's 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
[declare(5)] for details.
WITH HOLD
WITHOUT HOLD
WITH HOLD specifies that the cursor may 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.
query A SELECT [select(5)] or VALUES [values(5)] command which will
provide the rows to be returned by the cursor.
FOR READ ONLY
FOR UPDATE
FOR READ ONLY indicates that the cursor will be used in a read-
only mode. FOR UPDATE indicates that the cursor will be used to
update tables. Since cursor updates are not currently supported
in PostgreSQL, specifying FOR UPDATE will cause an error message
and specifying FOR READ ONLY has no effect.
column Column(s) to be updated by the cursor. Since cursor updates are
not currently supported in PostgreSQL, the FOR UPDATE clause
provokes an error message.
The key words BINARY, INSENSITIVE, and SCROLL may appear in any order.
NOTES
Unless WITH HOLD is specified, the cursor created by this command can
only be used within the current transaction. Thus, DECLARE 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 this command is used outside a transaction block. Use
BEGIN [begin(5)], COMMIT [commit(5)] and ROLLBACK [rollback(5)] to
define a transaction block.
If WITH HOLD is specified and the transaction that created the cursor
successfully commits, the cursor can continue to be accessed by subse-
quent transactions in the same session. (But if the creating transac-
tion is aborted, the cursor is removed.) A cursor created with WITH
HOLD is closed when an explicit CLOSE 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.
The SCROLL option should be specified when defining a cursor that will
be used to fetch backwards. This is required by the SQL standard. How-
ever, for compatibility with earlier versions, PostgreSQL will allow
backward fetches without SCROLL, if the cursor's query plan is simple
enough that no extra overhead is needed to support it. However, appli-
cation 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.
The SQL standard only makes provisions for cursors in embedded SQL. The
PostgreSQL server does not implement an OPEN 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 DECLARE and OPEN state-
ments.
You can see all available cursors by querying the pg_cursors system
view.
EXAMPLES
To declare a cursor:
DECLARE liahona CURSOR FOR SELECT * FROM films;
See FETCH [fetch(5)] for more examples of cursor usage.
COMPATIBILITY
The SQL standard allows cursors only in embedded SQL and in modules.
PostgreSQL permits cursors to be used interactively.
The SQL standard allows cursors to update table data. All PostgreSQL
cursors are read only.
Binary cursors are a PostgreSQL extension.
SEE ALSO
CLOSE [close(5)], FETCH [fetch(l)], MOVE [move(l)]
SQL - Language Statements 2008-06-08 DECLARE()
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