Cursor types

Psycopg can manage kinds of “cursors” which differ in where the state of a query being processed is stored: Client-side cursors and Server-side cursors.

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.

Server-side cursors

PostgreSQL also has its own concept of cursor (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