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# Cursor types

Cursors are objects used to send commands to a PostgreSQL connection and to manage the results returned by it. They are normally created by the connection’s `cursor()` method.

Psycopg can manage different kinds of “cursors”, the objects used to send queries and retrieve results from the server. They differ from each other in aspects such as:

- Are the parameters bound on the client or on the server? Server-side binding can offer better performance (for instance allowing to use prepared statements) and reduced memory footprint, but may require stricter query definition and certain queries that work in `psycopg2` might need to be adapted.

- Is the query result stored on the client or on the server? Server-side cursors allow partial retrieval of large datasets, but they might offer less performance in everyday usage.

- Are queries manipulated by Python (to handle placeholders in `%s` and `%(name)s` Python-style) or sent as they are to the PostgreSQL server (which only supports `$1`, `$2` parameters)?

Psycopg exposes the following classes to implement the different strategies. All the classes are exposed by the main `psycopg` package. Every class has also an `Async`-prefixed counterparts, designed to be used in conjunction with `AsyncConnection` in `asyncio` programs.

Class | Binding | Storage | Placeholders | See also
--- | --- | --- | --- | ---
`Cursor` | server-side | client-side | `%s`, `%(name)s` | Client-side cursors
`ClientCursor` | client-side | client-side | `%s`, `%(name)s` | Client-side-binding cursors
`ServerCursor` | server-side | server-side | `%s`, `%(name)s` | Server-side cursors
`RawCursor` | server-side | client-side | `$1` | Raw query cursors
`RawServerCursor` | server-side | server-side | `$1` | Raw query cursors

If not specified by a `cursor_factory`, `cursor()` will usually produce `Cursor` objects.

## Client-side cursors

Client-side cursors are what Psycopg uses in its normal querying process. They are implemented by the `Cursor` and `AsyncCursor` classes. In such querying pattern, after a cursor sends a query to the server (usually calling `execute()`), the server replies transferring to the client the whole set of results requested, which is stored in the state of the same cursor and from where it can be read from Python code (using methods such as `fetchone()` and siblings).

This querying process is very scalable because, after a query result has been transmitted to the client, the server doesn’t keep any state. Because the results are already in the client memory, iterating its rows is very quick.

The downside of this querying method is that the entire result has to be transmitted completely to the client (with a time proportional to its size) and the client needs enough memory to hold it, so it is only suitable for reasonably small result sets.

## Client-side-binding cursors

Added in version 3.1.

The previously described client-side cursors send the query and the parameters separately to the server. This is the most efficient way to process parametrised queries and allows to build several features and optimizations. However, not all types of queries can be bound server-side; in particular no Data Definition Language query can. See Server-side binding for the description of these problems.

The `ClientCursor` (and its `AsyncClientCursor` async counterpart) merge the query on the client and send the query and the parameters merged together to the server. This allows to parametrize any type of PostgreSQL statement, not only queries (`SELECT`) and Data Manipulation statements (`INSERT`, `UPDATE`, `DELETE`).

Using `ClientCursor`, Psycopg 3 behaviour will be more similar to `psycopg2` (which only implements client-side binding) and could be useful to port Psycopg 2 programs more easily to Psycopg 3. The objects in the `sql` module allow for greater flexibility (for instance to parametrize a table name too, not only values); however, for simple cases, a `ClientCursor` could be the right object.

In order to obtain `ClientCursor` from a connection, you can set its `cursor_factory` (at init time or changing its attribute afterwards):

    from psycopg import connect, ClientCursor

    conn = psycopg.connect(DSN, cursor_factory=ClientCursor)
    cur = conn.cursor()
    # <psycopg.ClientCursor [no result] [IDLE] (database=piro) at 0x7fd977ae2880>

If you need to create a one-off client-side-binding cursor out of a normal connection, you can just use the `ClientCursor` class passing the connection as argument.

    conn = psycopg.connect(DSN)
    cur = psycopg.ClientCursor(conn)

Warning

Client-side cursors don’t support binary parameters and return values and don’t support prepared statements.

Tip

The best use for client-side binding cursors is probably to port large Psycopg 2 code to Psycopg 3, especially for programs making wide use of Data Definition Language statements.

The `psycopg.sql` module allows for more generic client-side query composition, to mix client- and server-side parameters binding, and allows to parametrize tables and fields names too, or entirely generic SQL snippets.

### Simple query protocol

Using the `ClientCursor` should ensure that psycopg will always use the simple query protocol for querying. In most cases, the choice of the fronted/backend protocol used is transparent on PostgreSQL. However, in some case using the simple query protocol is mandatory. This is the case querying the PgBouncer admin console for instance, which doesn’t support the extended query protocol.

    from psycopg import connect, ClientCursor

    conn = psycopg.connect(ADMIN_DSN, cursor_factory=ClientCursor)
    cur = conn.cursor()
    cur.execute("SHOW STATS")
    cur.fetchall()

Changed in version 3.1.20: While querying using the `ClientCursor` works well with PgBouncer, the connection’s COMMIT and ROLLBACK commands are only ensured to be executed using the simple query protocol starting from Psycopg 3.1.20.

In previous versions you should use an autocommit connection in order to query the PgBouncer admin console:

    from psycopg import connect, ClientCursor

    conn = psycopg.connect(ADMIN_DSN, cursor_factory=ClientCursor, autocommit=True)
    ...

## Server-side cursors

PostgreSQL has its own concept of cursor too (sometimes also called portal). When a database cursor is created, the query is not necessarily completely processed: the server might be able to produce results only as they are needed. Only the results requested are transmitted to the client: if the query result is very large but the client only needs the first few records it is possible to transmit only them.

The downside is that the server needs to keep track of the partially processed results, so it uses more memory and resources on the server.

Psycopg allows the use of server-side cursors using the classes `ServerCursor` and `AsyncServerCursor`. They are usually created by passing the `name` parameter to the `cursor()` method (reason for which, in `psycopg2`, they are usually called named cursors). The use of these classes is similar to their client-side counterparts: their interface is the same, but behind the scene they send commands to control the state of the cursor on the server (for instance when fetching new records or when moving using `scroll()`).

Using a server-side cursor it is possible to process datasets larger than what would fit in the client’s memory. However for small queries they are less efficient because it takes more commands to receive their result, so you should use them only if you need to process huge results or if only a partial result is needed.

See also

Server-side cursors are created and managed by `ServerCursor` using SQL commands such as DECLARE, FETCH, MOVE. The PostgreSQL documentation gives a good idea of what is possible to do with them.

### “Stealing” an existing cursor

A Psycopg `ServerCursor` can be also used to consume a cursor which was created in other ways than the `DECLARE` that `ServerCursor.execute()` runs behind the scene.

For instance if you have a PL/pgSQL function returning a cursor:

    CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS $$
    BEGIN
        OPEN $1 FOR SELECT col FROM test;
        RETURN $1;
    END;
    $$ LANGUAGE plpgsql;

you can run a one-off command in the same connection to call it (e.g. using `Connection.execute()`) in order to create the cursor on the server:

    conn.execute("SELECT reffunc('curname')")

after which you can create a server-side cursor declared by the same name, and directly call the fetch methods, skipping the `execute()` call:

    cur = conn.cursor('curname')
    # no cur.execute()
    for record in cur:  # or cur.fetchone(), cur.fetchmany()...
        # do something with record

## Raw query cursors

Added in version 3.2.

The `RawCursor` and `AsyncRawCursor` classes allow users to use PostgreSQL native placeholders (`$1`, `$2`, etc.) in their queries instead of the standard `%s` placeholder. This can be useful when it’s desirable to pass the query unmodified to PostgreSQL and rely on PostgreSQL’s placeholder functionality, such as when dealing with a very complex query containing `%s` inside strings, dollar-quoted strings or elsewhere.

One important note is that raw query cursors only accept positional arguments in the form of a list or tuple. This means you cannot use named arguments (i.e., dictionaries).

`RawCursor` behaves like `Cursor`, in returning the complete result from the server to the client. The `RawServerCursor` and `AsyncRawServerCursor` implement Server-side cursors with raw PostgreSQL placeholders.

There are two ways to use raw query cursors:

1. Using the cursor factory:

    from psycopg import connect, RawCursor

    with connect(dsn, cursor_factory=RawCursor) as conn:
        with conn.cursor() as cur:
            cur.execute("SELECT $1, $2", [1, "Hello"])
            assert cur.fetchone() == (1, "Hello")

2. Instantiating a cursor:

    from psycopg import connect, RawCursor

    with connect(dsn) as conn:
        with RawCursor(conn) as cur:
            cur.execute("SELECT $1, $2", [1, "Hello"])
            assert cur.fetchone() == (1, "Hello")