Python db-api: fetchone vs fetchmany vs fetchall

我今天和一些同事讨论了 Python 的 db-api fetchone vs fetchmany vs fetchall。

我确信每种方法的用例都取决于我正在使用的 db-api 的实现，但是一般来说，fetchone 和 fetchmany 和 fetchall 的用例是什么呢？

换句话说，下面是等价的吗？还是有一个比其他的更受欢迎？如果是，在什么情况下？

cursor.execute("SELECT id, name FROM `table`")
for i in xrange(cursor.rowcount):
id, name = cursor.fetchone()
print id, name




cursor.execute("SELECT id, name FROM `table`")
result = cursor.fetchmany()
while result:
for id, name in result:
print id, name
result = cursor.fetchmany()




cursor.execute("SELECT id, name FROM `table`")
for id, name in cursor.fetchall():
print id, name
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最佳答案

I think it indeed depends on the implementation, but you can get an idea of the differences by looking into MySQLdb sources. Depending on the options, mysqldb fetch* keep the current set of rows in memory or server side, so fetchmany vs fetchone has some flexibility here to know what to keep in (python's) memory and what to keep db server side.

PEP 249 does not give much detail, so I guess this is to optimize things depending on the database while exact semantics are implementation-defined.

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These are implementation specific.

  • fetchall

Will get all the results from the table. This will work better when size of the table is small. If the table size is bigger, fetchall will fail in those cases.

Will use most of the memory.

Will cause some issues will can occur if the queries is done on network.

  • fetchmany

fetchmany will get only required number of results. You can yield the results and process. Simple Snippet of implementation of fetchmany.

   while True:
results = cursor.fetchmany(arraysize)
if not results:
break
for result in results:
yield result
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As per official psycopg2 documentation

fetchone()

Fetch the next row of a query result set, returning a single tuple, or None when no more data is available:

>>> cur.execute("SELECT * FROM test WHERE id = %s", (3,))
>>> cur.fetchone()


(3, 42, 'bar')

A ProgrammingError is raised if the previous call to execute*() did not produce any result set or no call was issued yet.

fetchmany([size=cursor.arraysize])

Fetch the next set of rows of a query result, returning a list of tuples. An empty list is returned when no more rows are available.

The number of rows to fetch per call is specified by the parameter. If it is not given, the cursor’s arraysize determines the number of rows to be fetched. The method should try to fetch as many rows as indicated by the size parameter. If this is not possible due to the specified number of rows not being available, fewer rows may be returned:

>>> cur.execute("SELECT * FROM test;")
>>> cur.fetchmany(2)
[(1, 100, "abc'def"), (2, None, 'dada')]
>>> cur.fetchmany(2)
[(3, 42, 'bar')]
>>> cur.fetchmany(2)
[]

A ProgrammingError is raised if the previous call to execute*() did not produce any result set or no call was issued yet.

Note there are performance considerations involved with the size parameter. For optimal performance, it is usually best to use the arraysize attribute. If the size parameter is used, then it is best for it to retain the same value from one fetchmany() call to the next.

List item

fetchall()

Fetch all (remaining) rows of a query result, returning them as a list of tuples. An empty list is returned if there is no more record to fetch.

>>> cur.execute("SELECT * FROM test;")
>>> cur.fetchall()
[(1, 100, "abc'def"), (2, None, 'dada'), (3, 42, 'bar')]

A ProgrammingError is raised if the previous call to execute*() did not produce any result set or no call was issued yet.


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