DECLARE

PostgreSQL
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DECLARE

Name

DECLARE — Defines a cursor for table access

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 Postgres 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 Postgres 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 readonly mode. Since this is the only cursor access mode available in Postgres 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 Postgres this keyword provokes an informational error message.
column: Column(s) to be updated. Since cursor updates are not currently supported in Postgres the UPDATE clause provokes an informational error message.

Outputs

SELECT: The message returned if the SELECT is run successfully.
NOTICE BlankPortalAssignName: portal "cursorname" already exists: This error occurs if cursorname is already declared.
ERROR: Named portals 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 Postgres 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 and Postgres 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 Postgres 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 Postgres, supports the SQL92 cursor conventions, including those involving DECLARE and OPEN statements.

Usage

To declare a cursor:

DECLARE liahona CURSOR
    FOR SELECT * FROM films;

Compatibility

SQL92

SQL92 allows cursors only in embedded SQL and in modules. Postgres permits cursors to be used interactively. SQL92 allows embedded or modular cursors to update database information. All Postgres cursors are readonly. The BINARY keyword is a Postgres extension.

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