FETCH
Retrieves rows from a query using a cursor.
Synopsis
FETCH [ <forward_direction> { FROM | IN } ] <cursorname>
where forward_direction can be empty or one of:
NEXT
FIRST
LAST
ABSOLUTE <count>
RELATIVE <count>
<count>
ALL
FORWARD
FORWARD <count>
FORWARD ALL
Description
FETCH retrieves rows using a previously-created cursor.
A cursor has an associated position, which is used by FETCH. The cursor position can be before the first row of the query result, on any particular row of the result, or after the last row of the result. When created, a cursor is positioned before the first row. After fetching some rows, the cursor is positioned on the row most recently retrieved. If FETCH runs off the end of the available rows then the cursor is left positioned after the last row. FETCH ALL will always leave the cursor positioned after the last row.
The forms NEXT, FIRST, LAST, ABSOLUTE, RELATIVE fetch a single row after moving the cursor appropriately. If there is no such row, an empty result is returned, and the cursor is left positioned before the first row or after the last row as appropriate.
The forms using FORWARD retrieve the indicated number of rows moving in the forward direction, leaving the cursor positioned on the last-returned row (or after all rows, if the count exceeds the number of rows available). Note that it is not possible to move a cursor position backwards in HAWQ, since scrollable cursors are not supported. You can only move a cursor forward in position using FETCH.
RELATIVE 0 and FORWARD 0 request fetching the current row without moving the cursor, that is, re-fetching the most recently fetched row. This will succeed unless the cursor is positioned before the first row or after the last row, in which case no row is returned.
Outputs
On successful completion, a FETCH command returns a command tag of the form
FETCH count
The count is the number of rows fetched (possibly zero). Note that in psql, the command tag will not actually be displayed, since psql displays the fetched rows instead.
Parameters
ABSOLUTE 1). Only allowed if it is the first FETCH operation using this cursor.ABSOLUTE -1).RELATIVE 0 re-fetches the current row, if any. Only allowed if count moves the cursor position forward.FORWARD count).FORWARD ALL).NEXT).FORWARD 0 re-fetches the current row.Notes
HAWQ does not support scrollable cursors, so you can only use FETCH to move the cursor position forward.
ABSOLUTE fetches are not any faster than navigating to the desired row with a relative move: the underlying implementation must traverse all the intermediate rows anyway.
Updating data via a cursor is currently not supported by HAWQ.
DECLARE is used to define a cursor. Use MOVE to change cursor position without retrieving data.
Examples
– Start the transaction:
BEGIN;
– Set up a cursor:
DECLARE mycursor CURSOR FOR SELECT * FROM films;
– Fetch the first 5 rows in the cursor mycursor:
FETCH FORWARD 5 FROM mycursor;
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
– Close the cursor and end the transaction:
CLOSE mycursor;
COMMIT;
Compatibility
SQL standard allows cursors only in embedded SQL and in modules. HAWQ permits cursors to be used interactively.
The variant of FETCH described here returns the data as if it were a SELECT result rather than placing it in host variables. Other than this point, FETCH is fully upward-compatible with the SQL standard.
The FETCH forms involving FORWARD, as well as the forms FETCH count and FETCH ALL, in which FORWARD is implicit, are HAWQ extensions. BACKWARD is not supported.
The SQL standard allows only FROM preceding the cursor name; the option to use IN is an extension.