FETCH
Synopsis
FETCH [direction [ FROM | IN ] ]cursor_namewheredirection can be empty or one of: NEXT PRIOR FIRST LAST ABSOLUTEcount RELATIVEcountcount ALL FORWARD FORWARDcount FORWARD ALL BACKWARD BACKWARDcount BACKWARD ALLDescription
FETCH retrieves rows using a previously-created cursor.
A cursor has an associated position, which is used byFETCH. 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. IfFETCH 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 orFETCH BACKWARD ALL will always leave the cursor positioned after the last row or before the first row.
The formsNEXT,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 usingFORWARD andBACKWARD 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 thecount exceeds the number of rows available).
RELATIVE 0,FORWARD 0, andBACKWARD 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 aPL/pgSQL function, the rules are different — seeSection 40.7.
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 ifdirection is omitted.
- PRIOR
Fetch the prior row.
- FIRST
Fetch the first row of the query (same asABSOLUTE 1).
- LAST
Fetch the last row of the query (same asABSOLUTE -1).
- ABSOLUTEcount
Fetch thecount'th row of the query, or theabs(count)'th row from the end ifcount is negative. Position before first row or after last row ifcount is out of range; in particular,ABSOLUTE 0 positions before the first row.
- RELATIVEcount
Fetch thecount'th succeeding row, or theabs(count)'th prior row ifcount is negative.RELATIVE 0 re-fetches the current row, if any.
- count
Fetch the nextcount rows (same asFORWARDcount).
- ALL
Fetch all remaining rows (same asFORWARD ALL).
- FORWARD
Fetch the next row (same asNEXT).
- FORWARDcount
Fetch the nextcount rows.FORWARD 0 re-fetches the current row.
- FORWARD ALL
Fetch all remaining rows.
- BACKWARD
Fetch the prior row (same asPRIOR).
- BACKWARDcount
Fetch the priorcount 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, aFETCH command returns a command tag of the form
FETCHcount
Thecount is the number of rows fetched (possibly zero). Note that inpsql, the command tag will not actually be displayed, sincepsql displays the fetched rows instead.
Notes
The cursor should be declared with theSCROLL option if one intends to use any variants ofFETCH other thanFETCH NEXT orFETCH FORWARD with a positive count. For simple queriesPostgreSQL will allow backwards fetch from cursors not declared withSCROLL, but this behavior is best not relied on. If the cursor is declared withNO 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 withFETCH ABSOLUTE 0) is fast.
DECLARE is used to define a cursor. UseMOVE 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 definesFETCH for use in embedded SQL only. The variant ofFETCH described here returns the data as if it were aSELECT result rather than placing it in host variables. Other than this point,FETCH is fully upward-compatible with the SQL standard.
TheFETCH forms involvingFORWARD andBACKWARD, as well as the formsFETCHcount andFETCH ALL, in whichFORWARD is implicit, arePostgreSQL extensions.
The SQL standard allows onlyFROM preceding the cursor name; the option to useIN, or to leave them out altogether, is an extension.