Title: FETCH Author: The PostgreSQL Global Development Group Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/> Date: 2018 Manual: PostgreSQL 9.6.8 Documentation Source: PostgreSQL 9.6.8 Language: English
NAMEFETCH - retrieve rows from a query using a cursor
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
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 Section 41.7, lqCursorsrq, in the documentation.
defines the fetch direction and number of rows to fetch. It can be one of the following:
- Fetch the next row. This is the default if direction is omitted.
- Fetch the prior row.
- Fetch the first row of the query (same as ABSOLUTE 1).
- Fetch the last row of the query (same as ABSOLUTE -1).
- 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.
- 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.
- Fetch the next count rows (same as FORWARD count).
- Fetch all remaining rows (same as FORWARD ALL).
- Fetch the next row (same as NEXT).
- Fetch the next count rows. FORWARD 0 re-fetches the current row.
- Fetch all remaining rows.
- Fetch the prior row (same as PRIOR).
- Fetch the prior count rows (scanning backwards). BACKWARD 0 re-fetches the current row.
- Fetch all prior rows (scanning backwards).
- 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.
- An open cursor's name.
On successful completion, a FETCH command returns a command tag of the form
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.
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 count and 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.