FETCH
FETCH
Synopsis
FETCH [ direction ] [ count ] { IN | FROM } cursor
FETCH [ FORWARD | BACKWARD | RELATIVE ] [ # | ALL | NEXT | PRIOR ]
{ IN | FROM } cursor
Inputs
- direction
selector defines the fetch direction. It can be one of the following:
- FORWARD
fetch next row(s). This is the default if selector is omitted.
- BACKWARD
fetch previous row(s).
- RELATIVE
Noise word for SQL92 compatibility.
- count
count determines how many rows to fetch. It can be one of the following:
- #
A signed integer that specifies how many rows to fetch. Note that a negative integer is equivalent to changing the sense of FORWARD and BACKWARD.
- ALL
Retrieve all remaining rows.
- NEXT
Equivalent to specifying a count of 1.
- PRIOR
Equivalent to specifying a count of -1.
- cursor
An open cursor's name.
Outputs
FETCH returns the results of the query defined by the specified cursor. The following messages will be returned if the query fails:
- WARNING: PerformPortalFetch: portal "cursor" not found
If cursor is not previously declared. The cursor must be declared within a transaction block.
- WARNING: FETCH/ABSOLUTE not supported, using RELATIVE
PostgreSQL does not support absolute positioning of cursors.
- ERROR: FETCH/RELATIVE at current position is not supported
SQL92 allows one to repetitively retrieve the cursor at its "current position" using the syntax
FETCH RELATIVE 0 FROM cursor.
PostgreSQL does not currently support this notion; in fact the value zero is reserved to indicate that all rows should be retrieved and is equivalent to specifying the ALL keyword. If the RELATIVE keyword has been used, PostgreSQL assumes that the user intended SQL92 behavior and returns this error message.
Description
FETCH allows a user to retrieve rows using a cursor. The number of rows retrieved is specified by #. If the number of rows remaining in the cursor is less than #, then only those available are fetched. Substituting the keyword ALL in place of a number will cause all remaining rows in the cursor to be retrieved. Instances may be fetched in both FORWARD and BACKWARD directions. The default direction is FORWARD.
Tip: Negative numbers are allowed to be specified for the row count. A negative number is equivalent to reversing the sense of the FORWARD and BACKWARD keywords. For example, FORWARD -1 is the same as BACKWARD 1.
Notes
Note that the FORWARD and BACKWARD keywords are PostgreSQL extensions. The SQL92 syntax is also supported, specified in the second form of the command. See below for details on compatibility issues.
Updating data in a cursor is not supported by PostgreSQL, because mapping cursor updates back to base tables is not generally possible, as is also the case with VIEW updates. Consequently, users must issue explicit UPDATE commands to replace data.
Cursors may only be used inside of transactions because the data that they store spans multiple user queries.
Use MOVE to change cursor position. DECLARE will define a cursor. Refer to BEGIN, COMMIT, and ROLLBACK for further information about transactions.
Usage
The following examples traverses a table using a cursor.
-- Set up and use a cursor: BEGIN WORK; DECLARE liahona CURSOR FOR SELECT * FROM films; -- Fetch first 5 rows in the cursor liahona: FETCH FORWARD 5 IN liahona; code | title | did | date_prod | kind | len -------+-------------------------+-----+------------+----------+------- BL101 | The Third Man | 101 | 1949-12-23 | Drama | 01:44 BL102 | The African Queen | 101 | 1951-08-11 | Romantic | 01:43 JL201 | Une Femme est une Femme | 102 | 1961-03-12 | Romantic | 01:25 P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08 P_302 | Becket | 103 | 1964-02-03 | Drama | 02:28 -- Fetch previous row: FETCH BACKWARD 1 IN liahona; code | title | did | date_prod | kind | len -------+---------+-----+------------+--------+------- P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08 -- close the cursor and commit work: CLOSE liahona; COMMIT WORK;
Compatibility
SQL92
Note: The non-embedded use of cursors is a PostgreSQL extension. The syntax and usage of cursors is being compared against the embedded form of cursors defined in SQL92.
SQL92 allows absolute positioning of the cursor for FETCH, and allows placing the results into explicit variables:
FETCH ABSOLUTE #
FROM cursor
INTO :variable [, ...]
- ABSOLUTE
The cursor should be positioned to the specified absolute row number. All row numbers in PostgreSQL are relative numbers so this capability is not supported.
- :variable
Target host variable(s).