FETCH — retrieve rows from a query using a cursor
FETCH [direction
[ FROM | IN ] ]cursor_name
wheredirection
can be empty or one of: NEXT PRIOR FIRST LAST ABSOLUTEcount
RELATIVEcount
count
ALL FORWARD FORWARDcount
FORWARD ALL BACKWARD BACKWARDcount
BACKWARD ALL
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.
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 42.7.3절.
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 the abs(
'th row from
the end if count
)count
is negative. Position
before first row or after last row if count
is out of range; in
particular, ABSOLUTE 0
positions before
the first row.
RELATIVE count
Fetch the count
'th succeeding row, or
the abs(
'th prior
row if count
)count
is
negative. RELATIVE 0
re-fetches the
current row, if any.
count
Fetch the next count
rows (same as
FORWARD
).
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. For FORWARD
and
BACKWARD
cases, specifying a negative count
is equivalent to changing
the sense of FORWARD
and BACKWARD
.
cursor_name
An open cursor's name.
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.
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.
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;
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.