Cursors

PostgreSQL 7.4.18 Documentation
Prev | Fast Backward | Chapter 37. PL/pgSQL - SQL Procedural Language | Fast Forward | Next

# 37.8. 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. One reason for doing this is to avoid memory overrun when the result contains a large number of rows. (However, PL/pgSQL users do not normally need to worry about that, since `FOR` loops automatically use a cursor internally to avoid memory problems.) A more interesting usage is to return a reference to a cursor that a function has created, allowing the caller to read the rows. This provides an efficient way to return large row sets from functions.

## 37.8.1. 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 CURSOR [ ( arguments ) ] FOR query ;

(`FOR` may be replaced by `IS` for Oracle compatibility.) `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. The actual values to substitute for these names will be specified later, when the cursor is opened.

Some examples:

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

All three of these variables have the data type `refcursor`, but the first may be used with any query, while the second has a fully specified query already bound to it, and the last has a parameterized query bound to it. (`key` will be replaced by an integer parameter value when the cursor is opened.) The variable `curs1` is said to be unbound since it is not bound to any particular query.

## 37.8.2. Opening Cursors

Before a cursor can be used to retrieve rows, it must be opened. (This is the equivalent action 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.

### 37.8.2.1. `OPEN FOR SELECT`

    OPEN unbound-cursor FOR SELECT ...;

The cursor variable is opened and given the specified query to execute. The cursor cannot be open already, and it must have been declared as an unbound cursor (that is, as a simple `refcursor` variable). The `SELECT` query is treated in the same way as other `SELECT` statements in PL/pgSQL: PL/pgSQL variable names are substituted, and the query plan is cached for possible reuse.

An example:

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

### 37.8.2.2. `OPEN FOR EXECUTE`

    OPEN unbound-cursor FOR EXECUTE query-string;

The cursor variable is opened and given the specified query to execute. The cursor cannot be open already, and it must have been declared as an unbound cursor (that is, as a simple `refcursor` variable). 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 can vary from one run to the next.

An example:

    OPEN curs1 FOR EXECUTE ''SELECT * FROM '' || quote_ident($1);

### 37.8.2.3. Opening a Bound Cursor

    OPEN bound-cursor [ ( argument_values ) ];

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.

Examples:

    OPEN curs2;
    OPEN curs3(42);

## 37.8.3. 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. (Internally, a `refcursor` value is simply the string name of a so-called portal containing the active query for the cursor. This name can be passed around, assigned to other `refcursor` variables, and so on, without disturbing the portal.)

All portals are implicitly closed at transaction end. Therefore a `refcursor` value is usable to reference an open cursor only until the end of the transaction.

### 37.8.3.1. `FETCH`

    FETCH cursor INTO target;

`FETCH` retrieves the next row from the cursor into a target, which may be a row variable, a record variable, or a comma-separated list of simple variables, just like `SELECT INTO`. As with `SELECT INTO`, the special variable `FOUND` may be checked to see whether a row was obtained or not.

An example:

    FETCH curs1 INTO rowvar;
    FETCH curs2 INTO foo, bar, baz;

### 37.8.3.2. `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;

### 37.8.3.3. 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;

    BEGIN;
    SELECT reffunc2();

          reffunc2
    --------------------
     <unnamed cursor 1>
    (1 row)

    FETCH ALL IN "<unnamed cursor 1>";
    COMMIT;

Prev | Home | Next
Control Structures | Up | Errors and Messages