DECLARE() SQL Commands DECLARE()NAME
DECLARE - Defines a cursor for table access
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 opera‐
tions.
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 read
only 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 informa‐
tional error message.
OUTPUTS
SELECT The message returned if the SELECT is run successfully.
NOTICE: Closing pre-existing portal "cursorname"
This message is reported if the same cursor name was already
declared in the current transaction block. The previous defini‐
tion 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 trans‐
action 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 [fetch(l)].
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 conver‐
sion to produce the text format. In addition, text formats are often
larger in size than the corresponding binary format. Once the informa‐
tion comes back in text form, the client application may need to con‐
vert 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 col‐
umn, 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 represen‐
tation can differ between different machine architectures. Postgres
does not resolve byte ordering or representation issues for binary cur‐
sors. Therefore, if your client machine and server machine use differ‐
ent 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 con‐
version 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 [begin(l)],
COMMIT [commit(l)] and ROLLBACK [rollback(l)] to define a transaction
block.
In SQL92 cursors are only available in embedded SQL (ESQL) applica‐
tions. The Postgres backend does not implement an explicit OPEN cursor
statement; a cursor is considered to be open when it is declared. How‐
ever, 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 per‐
mits cursors to be used interactively. SQL92 allows embedded or modu‐
lar cursors to update database information. All Postgres cursors are
read only. The BINARY keyword is a Postgres extension.
SQL - Language Statements 29 March 2001 DECLARE()
