这里是COPY的二进制等效FROM用于Python 3:
from io import BytesIO
from struct import pack
import psycopg2
# Two rows of data; "id" is not in the upstream data source
# Columns: node, ts, val1, val2
data = [(23253, 342, -15.336734, 2494627.949375),
(23256, 348, 43.23524, 2494827.949375)]
conn = psycopg2.connect("dbname=mydb user=postgres")
curs = conn.cursor()
# Determine starting value for sequence
curs.execute("SELECT nextval('num_data_id_seq')")
id_seq = curs.fetchone()[0]
# Make a binary file object for COPY FROM
cpy = BytesIO()
# 11-byte signature, no flags, no header extension
cpy.write(pack('!11sii', b'PGCOPY\n\377\r\n\0', 0, 0))
# Columns: id, node, ts, val1, val2
# Zip: (column position, format, size)
row_format = list(zip(range(-1, 4),
('i', 'i', 'h', 'f', 'd'),
(4, 4, 2, 4, 8)))
for row in data:
# Number of columns/fields (always 5)
cpy.write(pack('!h', 5))
for col, fmt, size in row_format:
value = (id_seq if col == -1 else row[col])
cpy.write(pack('!i' + fmt, size, value))
id_seq += 1 # manually increment sequence outside of database
# File trailer
cpy.write(pack('!h', -1))
# Copy data to database
cpy.seek(0)
curs.copy_expert("COPY num_data FROM STDIN WITH BINARY", cpy)
# Update sequence on database
curs.execute("SELECT setval('num_data_id_seq', %s, false)", (id_seq,))
conn.commit()
更新
我重写了上面的方法来为COPY写入文件。我在Python中的数据是NumPy数组,所以使用它们是有意义的。下面是用1M的行,列7一些示例data
:
import psycopg2
import numpy as np
from struct import pack
from io import BytesIO
from datetime import datetime
conn = psycopg2.connect("dbname=mydb user=postgres")
curs = conn.cursor()
# NumPy record array
shape = (7, 2000, 500)
print('Generating data with %i rows, %i columns' % (shape[1]*shape[2], shape[0]))
dtype = ([('id', 'i4'), ('node', 'i4'), ('ts', 'i2')] +
[('s' + str(x), 'f4') for x in range(shape[0])])
data = np.empty(shape[1]*shape[2], dtype)
data['id'] = np.arange(shape[1]*shape[2]) + 1
data['node'] = np.tile(np.arange(shape[1]) + 1, shape[2])
data['ts'] = np.repeat(np.arange(shape[2]) + 1, shape[1])
data['s0'] = np.random.rand(shape[1]*shape[2]) * 100
prv = 's0'
for nxt in data.dtype.names[4:]:
data[nxt] = data[prv] + np.random.rand(shape[1]*shape[2]) * 10
prv = nxt
在我的数据库,我有一个看起来像两个表:
CREATE TABLE num_data_binary
(
id integer PRIMARY KEY,
node integer NOT NULL,
ts smallint NOT NULL,
s0 real,
s1 real,
s2 real,
s3 real,
s4 real,
s5 real,
s6 real
) WITH (OIDS=FALSE);
,并命名为num_data_text
另一个类似的表。
这里有一些简单的辅助功能由NumPy的记录阵列中使用这些信息来准备数据COPY(文本和二进制格式):
def prepare_text(dat):
cpy = BytesIO()
for row in dat:
cpy.write('\t'.join([repr(x) for x in row]) + '\n')
return(cpy)
def prepare_binary(dat):
pgcopy_dtype = [('num_fields','>i2')]
for field, dtype in dat.dtype.descr:
pgcopy_dtype += [(field + '_length', '>i4'),
(field, dtype.replace('<', '>'))]
pgcopy = np.empty(dat.shape, pgcopy_dtype)
pgcopy['num_fields'] = len(dat.dtype)
for i in range(len(dat.dtype)):
field = dat.dtype.names[i]
pgcopy[field + '_length'] = dat.dtype[i].alignment
pgcopy[field] = dat[field]
cpy = BytesIO()
cpy.write(pack('!11sii', b'PGCOPY\n\377\r\n\0', 0, 0))
cpy.write(pgcopy.tostring()) # all rows
cpy.write(pack('!h', -1)) # file trailer
return(cpy)
这我如何使用辅助函数基准的两个拷贝格式的方法:
def time_pgcopy(dat, table, binary):
print('Processing copy object for ' + table)
tstart = datetime.now()
if binary:
cpy = prepare_binary(dat)
else: # text
cpy = prepare_text(dat)
tendw = datetime.now()
print('Copy object prepared in ' + str(tendw - tstart) + '; ' +
str(cpy.tell()) + ' bytes; transfering to database')
cpy.seek(0)
if binary:
curs.copy_expert('COPY ' + table + ' FROM STDIN WITH BINARY', cpy)
else: # text
curs.copy_from(cpy, table)
conn.commit()
tend = datetime.now()
print('Database copy time: ' + str(tend - tendw))
print(' Total time: ' + str(tend - tstart))
return
time_pgcopy(data, 'num_data_text', binary=False)
time_pgcopy(data, 'num_data_binary', binary=True)
下面是最后两个time_pgcopy
命令的输出:
Processing copy object for num_data_text
Copy object prepared in 0:01:15.288695; 84355016 bytes; transfering to database
Database copy time: 0:00:37.929166
Total time: 0:01:53.217861
Processing copy object for num_data_binary
Copy object prepared in 0:00:01.296143; 80000021 bytes; transfering to database
Database copy time: 0:00:23.325952
Total time: 0:00:24.622095
因此,使用二进制方法NumPy→文件和文件→数据库步骤都快得多。明显的区别是Python如何准备COPY文件,这对文本来说确实很慢。一般来说,二进制格式以2/3的时间作为该模式的文本格式加载到数据库中。
最后,我比较了数据库中两个表中的值,看看数字是否不同。对于列s0
,大约1.46%的行具有不同的值,并且s6
(可能与我使用的随机方法有关)的这一部分增加到6.17%。所有70M 32位浮点值之间的非零绝对差值介于9.3132257e-010和7.6293945e-006之间。文本和二进制加载方法之间的这些细微差异是由于文本格式方法所需的float→text→float转换的精度损失所致。
那么,你可以[导入二进制文件COPY](http://www.postgresql.org/docs/9.1/interactive/sql-copy.html),但为此,整个文件必须在一个特定的二进制格式,而不仅仅是一个值。 –
@Erwin,是的,我读了关于COPY的二进制模式,但我不确定它是否被psycopg2支持,或者我是否应该使用不同的方法。 –
我使用的二进制文件格式的唯一应用是导入从* PostgreSQL导出*的文件。我不知道任何其他可以编写特定格式的程序。尽管如此,这并不意味着它不可能出现在那里。如果是用于重复操作,则可以以文本形式复制到Postgres一次,下一次写入二进制文件和'COPY FROM .. FORMAT BINARY'。 –