cursor.fetchall() vs list(cursor) in Python

Question

Both methods return a list of the returned items of the query, did I miss something here?
Or they have identical usages indeed?
Any differences performance-wise?

question from:https://stackoverflow.com/questions/17861152/cursor-fetchall-vs-listcursor-in-python

Answer 1

If you are using the default cursor, a MySQLdb.cursors.Cursor, the entire result set will be stored on the client side (i.e. in a Python list) by the time the cursor.execute() is completed.

Therefore, even if you use

for row in cursor:

you will not be getting any reduction in memory footprint. The entire result set has already been stored in a list (See self._rows in MySQLdb/cursors.py).

However, if you use an SSCursor or SSDictCursor:

import MySQLdb
import MySQLdb.cursors as cursors

conn = MySQLdb.connect(..., cursorclass=cursors.SSCursor)

then the result set is stored in the server, mysqld. Now you can write

cursor = conn.cursor()
cursor.execute('SELECT * FROM HUGETABLE')
for row in cursor:
    print(row)

and the rows will be fetched one-by-one from the server, thus not requiring Python to build a huge list of tuples first, and thus saving on memory.

Otherwise, as others have already stated, cursor.fetchall() and list(cursor) are essentially the same.