cursor.fetchall（）vs list（cursor） [英] cursor.fetchall() vs list(cursor) in Python

问题描述

这两种方法都返回查询返回项的列表，我在这里错过了什么吗？或者他们有相同的用法吗？任何差异性能明智？

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?

推荐答案

如果你使用默认游标，一个 MySQLdb.cursors.Cursor ， c> cursor.execute（）时，整个结果集将存储在客户端 code>已完成。

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.

因此，即使您使用

for row in cursor:

您不会获得任何减少的内存占用。整个结果集已经存储在一个列表中（参见MySQLdb / cursors.py中的 self._rows ）。

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).

但是，如果你使用SSCursor或SSDictCursor：

However, if you use an SSCursor or SSDictCursor:

import MySQLdb
import MySQLdb.cursors as cursors

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

然后结果集存储在服务器，mysqld。现在你可以写

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)

从服务器逐个获取，因此不需要Python首先构建一个庞大的元组列表，从而节省内存。

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.

否则，因为其他人已经说明， cursor.fetchall（）和列表（光标）本质上是相同的。

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

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