DECLARE

Defines a cursor.

Synopsis

  1. DECLARE <name> [BINARY] [INSENSITIVE] [NO SCROLL] CURSOR
  2.      [{WITH | WITHOUT} HOLD]
  3.      FOR <query> [FOR READ ONLY]

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.

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.

Parameters

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 unaffected by updates to the tables underlying the cursor while the cursor exists. In HAWQ, all cursors are insensitive. This key word currently has no effect and is present for compatibility with the SQL standard.

NO SCROLL

A cursor cannot be used to retrieve rows in a nonsequential fashion. This is the default behavior in HAWQ, since scrollable cursors (SCROLL) are not supported.

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. WITHOUT HOLD is the default.

A SELECT command which will provide the rows to be returned by the cursor.

FOR READ ONLY

FOR READ ONLY indicates that the cursor is used in a read-only mode. Cursors can only be used in a read-only mode in HAWQ. HAWQ does not support updatable cursors (FOR UPDATE), so this is the default behavior.

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 HAWQ 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.

Scrollable cursors are not currently supported in HAWQ. You can only use FETCH to move the cursor position forward, not backwards.

You can see all available cursors by querying the pg_cursors system view.

Examples

Declare a cursor:

  1. DECLARE mycursor CURSOR FOR SELECT * FROM mytable;

Compatibility

SQL standard allows cursors only in embedded SQL and in modules. HAWQ permits cursors to be used interactively.

HAWQ 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 HAWQ cursors are forward moving only (not scrollable).

Binary cursors are a HAWQ extension.

See Also

CLOSE, FETCH, SELECT