postgreSQL PL/SQL编程学习笔记（三）——游标(Cursors)

Cursors

Rather than executing a whole query at once, it is possible to set up a cursor that encapsulates the query, and then read the query result a few rows at a time. A more interesting usage is to return a reference to a cursor that a function has created, allowing the caller to read the rows.

1. Declaring Cursor Variables

All access to cursors in PL/pgSQL goes through cursor variables, which are always of the special data type refcursor. One way to create a cursor variable is just to declare it as a variable of type refcursor. Another way is to use the cursor declaration syntax, which in general is:

name [ [ NO ] SCROLL ] CURSOR [ ( arguments ) ] FOR query;

If SCROLL is specified, the cursor will be capable of scrolling backward;

If NO SCROLL is specified, backward fetches will be rejected;

If neither specification appears, it is query-dependent whether backward fetches will be allowed.

Arguments, if specified, is a comma-separated list of pairs name datatype that define names to be replaced by parameter values in the given query.

Some examples:

DECLARE
curs1 refcursor;
curs2 CURSOR FOR SELECT * FROM tenk1;
curs3 CURSOR (key integer) FOR SELECT * FROM tenk1 WHERE unique1 = key;

The variable curs1 is said to be unbound since it is not bound to any particular query. while the second has a fully specified query already bound to it, and the last has a parameterized query bound to it.

2. Opening Cursors

Before a cursor can be used to retrieve rows, it must be opened. (equal to the SQL command DECLARE CURSOR.) PL/pgSQL has three forms of the OPEN statement, two of which use unbound cursor variables while the third uses a bound cursor variable.

2.1. OPEN FOR query

OPEN unbound_cursorvar [ [ NO ] SCROLL ] FOR query;

The cursor variable here should be a simple refcursor variable that has not been opened.

The query must be a SELECT, or something else that returns rows (such as EXPLAIN). The query is treated in the same way as other SQL commands in PL/pgSQL: PL/pgSQL variable names are substituted, and the query plan is cached for possible reuse.

When a PL/pgSQL variable is substituted into the cursor query, the value that is substituted is the one it has at the time of the OPEN; subsequent changes to the variable will not affect the cursor’s behavior.

The SCROLL and NO SCROLL options have the same meanings as for a bound cursor.

An example:

OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;

2.2. OPEN FOR EXECUTE

OPEN unbound_cursorvar [ [ NO ] SCROLL ] FOR EXECUTE query_string
[ USING expression [, ... ] ];

The cursor variable is opened and given the specified query to execute. The cursor variable here should be a simple refcursor variable that has not been opened.

The query is specified as a string expression, in the same way as in the EXECUTE command.

As usual, this gives flexibility so the query plan can vary from one run to the next, and it also means that variable substitution is not done on the command string.

As with EXECUTE, parameter values can be inserted into the dynamic command via format() and USING.

The SCROLL and NO SCROLL options have the same meanings as for a bound cursor.

An example:

OPEN curs1 FOR EXECUTE format(’SELECT * FROM %I WHERE col1 = $1’,tabname) USING keyvalue;

In this example, the table name is inserted into the query via format(). The comparison value for col1 is inserted via a USING parameter, so it needs no quoting.

2.3. Opening a Bound Cursor

OPEN bound_cursorvar [ ( [ argument_name := ] argument_value [, ...] ) ];

This form of OPEN is used to open a cursor variable whose query was bound to it when it was declared. The cursor cannot be open already. A list of actual argument value expressions must appear if and only if the cursor was declared to take arguments. These values will be substituted in the query.

The query plan for a bound cursor is always considered cacheable; there is no equivalent of EXECUTE in this case.

Notice that SCROLL and NO SCROLL cannot be specified in OPEN, as the cursor’s scrolling behavior was already determined.

Argument values can be passed using either positional or named notation.

In positional notation, all arguments are specified in order.

In named notation, each argument’s name is specified using := to separate it from the argument expression.

Similar to calling functions, it is also allowed to mix positional and named notation. Examples (these use the cursor declaration examples above):

OPEN curs2;
OPEN curs3(42);
OPEN curs3(key := 42);

Because variable substitution is done on a bound cursor’s query, there are really two ways to pass values into the cursor: either with an explicit argument to OPEN, or implicitly by referencing a PL/pgSQL variable in the query. For example, another way to get the same effect as the curs3 example above is

DECLARE
key integer;
curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
BEGIN
key := 42;
OPEN curs4;

3. Using Cursors

Once a cursor has been opened, it can be manipulated with the statements described here. These manipulations need not occur in the same function that opened the cursor to begin with. You can return a refcursor value out of a function and let the caller operate on the cursor.

3.1. FETCH

FETCH [ direction { FROM | IN } ] cursor INTO target;

FETCH retrieves the next row from the cursor into a target, which might be a row variable, a record variable, or a comma-separated list of simple variables, just like SELECT INTO.

If there is no next row, the target is set to NULL(s). As with SELECT INTO, the special variable FOUND can be checked to see whether a row was obtained or not.

The direction clause can be any of the variants allowed in the SQL FETCH command except the ones that can fetch more than one row; namely, it can be NEXT, PRIOR, FIRST, LAST, ABSOLUTE count, RELATIVE count, FORWARD, or BACKWARD. Omitting direction is the same as specifying NEXT.

direction values that require moving backward are likely to fail unless the cursor was declared or opened with the SCROLL option.

cursor must be the name of a refcursor variable that references an open cursor portal.

Examples:

FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;
FETCH LAST FROM curs3 INTO x, y;
FETCH RELATIVE -2 FROM curs4 INTO x;

3.2. MOVE

MOVE [ direction { FROM | IN } ] cursor;

MOVE repositions a cursor without retrieving any data. MOVE works exactly like the FETCH command, except it only repositions the cursor and does not return the row moved to.

As with SELECT INTO, the special variable FOUND can be checked to see whether there was a next row to move to.

The direction clause can be any of the variants allowed in the SQL FETCH command. Omitting direction is the same as specifying NEXT.

direction values that require moving backward are likely to fail unless the cursor was declared or opened with the SCROLL option.

Examples:

MOVE curs1;
MOVE LAST FROM curs3;
MOVE RELATIVE -2 FROM curs4;
MOVE FORWARD 2 FROM curs4;

3.3. UPDATE/DELETE WHERE CURRENT OF

UPDATE table SET ... WHERE CURRENT OF cursor;
DELETE FROM table WHERE CURRENT OF cursor;

When a cursor is positioned on a table row, that row can be updated or deleted using the cursor to identify the row.

There are restrictions on what the cursor’s query can be (in particular, no grouping) and it’s best to use FOR UPDATE in the cursor. For more information see the DECLARE reference page.

An example:

UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;

3.4. CLOSE

CLOSE cursor;

CLOSE closes the portal underlying an open cursor. This can be used to release resources earlier than end of transaction, or to free up the cursor variable to be opened again.

An example:

CLOSE curs1;

3.5. Returning Cursors

PL/pgSQL functions can return cursors to the caller. This is useful to return multiple rows or columns, especially with very large result sets.

To do this, the function opens the cursor and returns the cursor name to the caller (or simply opens the cursor using a portal name specified by or otherwise known to the caller).

The caller can then fetch rows from the cursor. The cursor can be closed by the caller, or it will be closed automatically when the transaction closes.

The portal name used for a cursor can be specified by the programmer or automatically generated. To specify a portal name, simply assign a string to the refcursor variable before opening it. The string value of the refcursor variable will be used by OPEN as the name of the underlying portal. However, if the refcursor variable is null, OPEN automatically generates a name that does not conflict with any existing portal, and assigns it to the refcursor variable.

Note: A bound cursor variable is initialized to the string value representing its name, so that the portal name is the same as the cursor variable name, unless the programmer overrides it by assignment before opening the cursor. But an unbound cursor variable defaults to the null value initially, so it will receive an automatically-generated unique name, unless overridden.

The following example shows one way a cursor name can be supplied by the caller:

CREATE TABLE test (col text);
INSERT INTO test VALUES (’123’);
CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS ’
BEGIN
OPEN $1 FOR SELECT col FROM test;
RETURN $1;
END;
’ LANGUAGE plpgsql;
BEGIN;
SELECT reffunc(’funccursor’);
FETCH ALL IN funccursor;
COMMIT;

The following example uses automatic cursor name generation:

CREATE FUNCTION reffunc2() RETURNS refcursor AS ’
DECLARE
ref refcursor;
BEGIN
OPEN ref FOR SELECT col FROM test;
RETURN ref;
END;
’ LANGUAGE plpgsql;
-- need to be in a transaction to use cursors.
BEGIN;
SELECT reffunc2();
reffunc2
--------------------
<unnamed cursor 1>
(1 row)
FETCH ALL IN "<unnamed cursor 1>";
COMMIT;

The following example shows one way to return multiple cursors from a single function:

CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
BEGIN
OPEN $1 FOR SELECT * FROM table_1;
RETURN NEXT $1;
OPEN $2 FOR SELECT * FROM table_2;
RETURN NEXT $2;
END;
$$ LANGUAGE plpgsql;
-- need to be in a transaction to use cursors.
BEGIN;
SELECT * FROM myfunc(’a’, ’b’);
FETCH ALL FROM a;

4. Looping Through a Cursor’s Result

There is a variant of the FOR statement that allows iterating through the rows returned by a cursor.

The syntax is:

[ <<label>> ]
FOR recordvar IN bound_cursorvar [ ( [ argument_name := ] argument_value [, ...] ) ] LOOP
statements
END LOOP [ label ];

The cursor variable must have been bound to some query when it was declared, and it cannot be open already. The FOR statement automatically opens the cursor, and it closes the cursor again when the loop exits.

A list of actual argument value expressions must appear if and only if the cursor was declared to take arguments. These values will be substituted in the query, in just the same way as during an OPEN.

The variable recordvar is automatically defined as type record and exists only inside the loop (any existing definition of the variable name is ignored within the loop). Each row returned by the cursor is successively assigned to this record variable and the loop body is executed.