FETCH

FETCH
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FETCH — retrieve rows from a query using a cursor

Synopsis

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 41.7.3.

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 the abs(count)'th row from the end if 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(count)'th prior row if 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.

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 ltsql, the command tag will not actually be displayed, since ltsql 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 LightDB 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 count and FETCH ALL, in which FORWARD is implicit, are LightDB 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.

Add GUC parameter lightdb_cursor_after_commit, When the parameter value is on, the cursor can continue after the transaction commits. When the parameter value is off, the cursor cannot proceed after the transaction is committed. The default value of this parameter is off.

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