Defines a cursor.
DECLARE <name> [BINARY] [INSENSITIVE] [NO SCROLL] CURSOR
[{WITH | WITHOUT} HOLD]
FOR <query> [FOR READ ONLY]
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.
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 format 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 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).
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 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.
A cursor can be specified in the WHERE CURRENT OF
clause of the UPDATE or DELETE statement to update or delete table data. The UPDATE
or DELETE
statement can only be executed on the server, for example in an interactive psql session or a script. Language extensions such as PL/pgSQL do not have support for updatable cursors.
SCROLL
) are not supported.
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. WITHOUT HOLD
is the default.
WITH HOLD
cannot not be specified when the
query
includes a
FOR UPDATE
or
FOR SHARE
clause.
A SELECT or VALUES command which will provide the rows to be returned by the cursor.
If the cursor is used in the WHERE CURRENT OF
clause of the UPDATE or DELETE command, the SELECT
command must satisfy the following conditions:
The table must be updatable. For example, the following are not updatable: table functions, set-returning functions, append-only tables, columnar tables.
- A grouping clause
- A set operation such as `UNION ALL` or `UNION DISTINCT`
- A sorting clause
- A windowing clause
- A join or a self-join
Specifying the `FOR UPDATE` clause in the `SELECT` command prevents other sessions from changing the rows between the time they are fetched and the time they are updated. Without the `FOR UPDATE` clause, a subsequent use of the `UPDATE` or `DELETE` command with the `WHERE CURRENT OF` clause has no effect if the row was changed since the cursor was created.
**Note:** Specifying the `FOR UPDATE` clause in the `SELECT` command locks the entire table, not just the selected rows.
FOR READ ONLY
indicates that the cursor is used in a read-only mode.
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 Greenplum Database reports an error if this command is used outside a transaction block. Use BEGIN
, COMMIT
and ROLLBACK
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 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 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.
If you create a cursor with the DECLARE
command in a transaction, you cannot use the SET
command in the transaction until you close the cursor with the CLOSE
command.
Scrollable cursors are not currently supported in Greenplum Database. You can only use FETCH
to move the cursor position forward, not backwards.
DECLARE...FOR UPDATE
is not supported with append-optimized tables.
You can see all available cursors by querying the pg_cursors
system view.
Declare a cursor:
DECLARE mycursor CURSOR FOR SELECT * FROM mytable;
SQL standard allows cursors only in embedded SQL and in modules. Greenplum Database permits cursors to be used interactively.
Greenplum Database does not implement an OPEN
statement for cursors. A cursor is considered to be open when it is declared.
The SQL standard allows cursors to move both forward and backward. All Greenplum Database cursors are forward moving only (not scrollable).
Binary cursors are a Greenplum Database extension.
CLOSE, DELETE, FETCH, MOVE, SELECT, UPDATE
Parent topic: SQL Command Reference