通过Python批量从MySQL检索数据 [英] Retrieving Data from MySQL in batches via Python

问题描述

由于数量多，我想分批进行此过程.

I would like to make this process in batches, because of the volume.

这是我的代码:

 getconn = conexiones()
 con = getconn.mysqlDWconnect()
 with con:
     cur = con.cursor(mdb.cursors.DictCursor)
     cur.execute("SELECT id, date, product_id, sales FROM sales")
     rows = cur.fetchall()

如何实现索引以批量获取数据?

How can I implement an index to fetch the data in batches?

推荐答案

第一点:python db-api.cursor是迭代器，因此除非您真的需要立即将整个批处理加载到内存中，您可以从使用此功能开始，即代替:

First point: a python db-api.cursor is an iterator, so unless you really need to load a whole batch in memory at once, you can just start with using this feature, ie instead of:

cursor.execute("SELECT * FROM mytable")
rows = cursor.fetchall()
for row in rows:
   do_something_with(row)

您可以:

cursor.execute("SELECT * FROM mytable")
for row in cursor:
   do_something_with(row)

然后，如果您的数据库连接器的实现仍未正确使用此功能，则是时候将LIMIT和OFFSET添加到混合中了:

Then if your db connector's implementation still doesn't make proper use of this feature, it will be time to add LIMIT and OFFSET to the mix:

# py2 / py3 compat
try:
    # xrange is defined in py2 only
    xrange
except NameError:
    # py3 range is actually p2 xrange
    xrange = range

cursor.execute("SELECT count(*) FROM mytable")
count = cursor.fetchone()[0]
batch_size = 42 # whatever

for offset in xrange(0, count, batch_size):
    cursor.execute(
        "SELECT * FROM mytable LIMIT %s OFFSET %s", 
        (batch_size, offset))
   for row in cursor:
       do_something_with(row)

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