Name
FETCH -- retrieve rows from a query using a cursorSynopsis
FETCH [ direction [ FROM | IN ] ] cursor_name where direction can be empty or one of: NEXT PRIOR FIRST LAST ABSOLUTE count RELATIVE count count ALL FORWARD FORWARD count FORWARD ALL BACKWARD BACKWARD count BACKWARD 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, or before the first row if fetching backward. FETCH ALL
or FETCH BACKWARD ALL
will always leave the cursor positioned after the last row or before the first row.
The forms NEXT
, PRIOR
, 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
and BACKWARD
retrieve the indicated number of rows moving in the forward or backward direction, leaving the cursor positioned on the last-returned row (or after/before all rows, if the count
exceeds the number of rows available).
RELATIVE 0
, FORWARD 0
, and BACKWARD 0
all 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.
Note: This page describes usage of cursors at the SQL command level. If you are trying to use cursors inside a PL/pgSQL function, the rules are different — see Section 41.7.
Parameters
direction
-
direction
defines the fetch direction and number of rows to fetch. It can be one of the following:NEXT
-
Fetch the next row. This is the default if
direction
is omitted. PRIOR
-
Fetch the prior row.
FIRST
-
Fetch the first row of the query (same as
ABSOLUTE 1
). LAST
-
Fetch the last row of the query (same as
ABSOLUTE -1
). ABSOLUTE
count
-
Fetch the
count
'th row of the query, or theabs(
'th row from the end ifcount
)count
is negative. Position before first row or after last row ifcount
is out of range; in particular,ABSOLUTE 0
positions before the first row. RELATIVE
count
-
Fetch the
count
'th succeeding row, or theabs(
'th prior row ifcount
)count
is negative.RELATIVE 0
re-fetches the current row, if any. count
-
Fetch the next
count
rows (same asFORWARD
).count
ALL
-
Fetch all remaining rows (same as
FORWARD ALL
). FORWARD
-
Fetch the next row (same as
NEXT
). FORWARD
count
-
Fetch the next
count
rows.FORWARD 0
re-fetches the current row. FORWARD ALL
-
Fetch all remaining rows.
BACKWARD
-
Fetch the prior row (same as
PRIOR
). BACKWARD
count
-
Fetch the prior
count
rows (scanning backwards).BACKWARD 0
re-fetches the current row. BACKWARD ALL
-
Fetch all prior rows (scanning backwards).
count
-
count
is a possibly-signed integer constant, determining the location or number of rows to fetch. ForFORWARD
andBACKWARD
cases, specifying a negativecount
is equivalent to changing the sense ofFORWARD
andBACKWARD
. cursor_name
-
An open cursor's name.
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.
Notes
The cursor should be declared with the SCROLL
option if one intends to use any variants of FETCH
other than FETCH NEXT
or FETCH FORWARD
with a positive count. For simple queries PostgreSQL will allow backwards fetch from cursors not declared with SCROLL
, but this behavior is best not relied on. If the cursor is declared with NO SCROLL
, no backward fetches are allowed.
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. Negative absolute fetches are even worse: the query must be read to the end to find the last row, and then traversed backward from there. However, rewinding to the start of the query (as with FETCH ABSOLUTE 0
) is fast.
DECLARE is used to define a cursor. Use MOVE to change cursor position without retrieving data.
Examples
The following example traverses a table using a cursor:
BEGIN WORK; -- Set up a cursor: DECLARE liahona SCROLL CURSOR FOR SELECT * FROM films; -- Fetch the first 5 rows in the cursor liahona: FETCH FORWARD 5 FROM 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 the previous row: FETCH PRIOR FROM liahona; code | title | did | date_prod | kind | len -------+---------+-----+------------+--------+------- P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08 -- Close the cursor and end the transaction: CLOSE liahona; COMMIT WORK;
Compatibility
The SQL standard defines FETCH
for use in embedded SQL only. 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
and BACKWARD
, as well as the forms FETCH
and count
FETCH ALL
, in which FORWARD
is implicit, are PostgreSQL extensions.
The SQL standard allows only FROM
preceding the cursor name; the option to use IN
, or to leave them out altogether, is an extension.
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