DECLARE
DECLARE
Synopsis
DECLARE cursorname [ BINARY ] [ INSENSITIVE ] [ SCROLL ]
CURSOR FOR query
[ FOR { READ ONLY | UPDATE [ OF column [, ...] ] ]
Inputs
- cursorname
The name of the cursor to be used in subsequent FETCH operations.
- BINARY
Causes the cursor to fetch data in binary rather than in text format.
- INSENSITIVE
SQL92 keyword indicating that data retrieved from the cursor should be unaffected by updates from other processes or cursors. Since cursor operations occur within transactions in PostgreSQL this is always the case. This keyword has no effect.
- SCROLL
SQL92 keyword indicating that data may be retrieved in multiple rows per FETCH operation. Since this is allowed at all times by PostgreSQL this keyword has no effect.
- query
An SQL query which will provide the rows to be governed by the cursor. Refer to the SELECT statement for further information about valid arguments.
- READ ONLY
SQL92 keyword indicating that the cursor will be used in a read only mode. Since this is the only cursor access mode available in PostgreSQL this keyword has no effect.
- UPDATE
SQL92 keyword indicating that the cursor will be used to update tables. Since cursor updates are not currently supported in PostgreSQL this keyword provokes an informational error message.
- column
Column(s) to be updated. Since cursor updates are not currently supported in PostgreSQL the UPDATE clause provokes an informational error message.
Outputs
- DECLARE CURSOR
The message returned if the SELECT is run successfully.
- WARNING: Closing pre-existing portal "cursorname"
This message is reported if the same cursor name was already declared in the current transaction block. The previous definition is discarded.
- ERROR: DECLARE CURSOR may only be used in begin/end transaction blocks
This error occurs if the cursor is not declared within a transaction block.
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, either ASCII or another encoding scheme depending on how the PostgreSQL backend was built. Since data is stored natively in binary format, the system must do a conversion to produce the text format. In addition, text formats are often larger in size than the corresponding binary format. Once the information comes back in text form, the client application may need to convert it to a binary format to manipulate it. BINARY cursors give you back the data in the native binary representation.
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 value equal to control-A (^A).
BINARY cursors should be used carefully. User applications such as psql are not aware of binary cursors and expect data to come back in a text format.
String representation is architecture-neutral whereas binary representation can differ between different machine architectures. PostgreSQL does not resolve byte ordering or representation issues for binary cursors. Therefore, if your client machine and server machine use different representations (e.g., "big-endian" versus "little-endian"), you will probably not want your data returned in binary format. However, binary cursors may be a little more efficient since there is less conversion overhead in the server to client data transfer.
Tip: If you intend to display the data in ASCII, getting it back in ASCII will save you some effort on the client side.
Notes
Cursors are only available in transactions. Use to BEGIN, COMMIT and ROLLBACK to define a transaction block.
In SQL92 cursors are only available in embedded SQL (ESQL) applications. The PostgreSQL backend does not implement an explicit OPEN cursor statement; a cursor is considered to be open when it is declared. However, ecpg, the embedded SQL preprocessor for PostgreSQL, supports the SQL92 cursor conventions, including those involving DECLARE and OPEN statements.