| PostgreSQL 7.4 Documentation | ||||
|---|---|---|---|---|
| Prev | Fast Backward | Fast Forward | Next | |
DECLARE name [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
CURSOR [ { WITH | WITHOUT } HOLD ] FOR query
[ FOR { READ ONLY | UPDATE [ OF column [, ...] ] } ]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.
The name of the cursor to be created.
Causes the cursor to return data in binary rather than in text format.
Indicates that data retrieved from the cursor should be unaffected 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 compatibility with the SQL standard.
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.
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.
A SELECT command that will provide the rows to be returned by the cursor. Refer to SELECT for further information about valid queries.
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(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.
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, 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.
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'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 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 statements.
To declare a cursor:
DECLARE liahona CURSOR FOR SELECT * FROM films;
See FETCH for more examples of cursor usage.