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DECLARESynopsisDECLARE name [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
CURSOR [ { WITH | WITHOUT } HOLD ] FOR query
[ FOR { READ ONLY | UPDATE [ OF column [, ...] ] } ]DescriptionDECLARE 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.
Parameters
The key words BINARY, INSENSITIVE, and SCROLL may appear in any order. NotesUnless 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 EnterpriseDB 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, EnterpriseDB 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 EnterpriseDB 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 EnterpriseDB, supports the standard SQL cursor conventions, including those involving DECLARE and OPEN statements. ExamplesTo declare a cursor: DECLARE liahona CURSOR FOR SELECT * FROM films; See FETCH for more examples of cursor usage. |