Question

MySQL ResultSets are by default retrieved completely from the server before any work can be done. In cases of huge result sets this becomes unusable. I would like instead to actually retrieve the rows one by one from the server.

In Java, following the instructions here (under ‘ResultSet’), I create a statement like this:

stmt = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY,               java.sql.ResultSet.CONCUR_READ_ONLY); stmt.setFetchSize(Integer.MIN_VALUE); 

This works nicely in Java. My question is: is there a way to do the same in python?

One thing I tried is to limit the query to a 1000 rows at a time, like this:

start_row = 0 while True:     cursor = conn.cursor()     cursor.execute('SELECT item FROM items LIMIT %d,1000' % start_row)     rows = cursor.fetchall()     if not rows:         break     start_row += 1000     # Do something with rows... 

However, this seems to get slower the higher start_row is.

And no, using fetchone() instead of fetchall() doesn’t change anything.

Clarification:

The naive code I use to reproduce this problem looks like this:

import MySQLdb  conn = MySQLdb.connect(user='user', passwd='password', db='mydb') cur = conn.cursor() print 'Executing query' cur.execute('SELECT * FROM bigtable');  print 'Starting loop' row = cur.fetchone() while row is not None:     print ', '.join([str(c) for c in row])     row = cur.fetchone()  cur.close() conn.close() 

On a ~700,000 rows table, this code runs quickly. But on a ~9,000,000 rows table it prints ‘Executing Query’ and then hangs for a long long time. That is why it makes no difference if I use fetchone() or fetchall().

Answer 1

I think you have to connect passing cursorclass = MySQLdb.cursors.SSCursor:

 MySQLdb.connect(user='user',                   passwd='password',                  db='mydb',                  cursorclass = MySQLdb.cursors.SSCursor                 ) 

The default cursor fetches all the data at once, even if you don’t use fetchall.

Edit: SSCursor or any other cursor class that supports server side resultsets – check the module docs on MySQLdb.cursors.