Question

我有以下Postgres查询，其中我要从具有约2500万行的table1中获取数据，并希望将以下查询的输出写入多个文件中。

query = """ WITH sequence AS (
                SELECT 
                        a,
                        b,
                        c
                FROM table1 )                    

select * from sequence;"""

下面是获取完整数据集的python脚本。如何修改脚本以将其提取到多个文件（例如，每个文件有10000行）

#IMPORT LIBRARIES ########################
import psycopg2
from pandas import DataFrame

#CREATE DATABASE CONNECTION ########################
connect_str = "dbname='x' user='x' host='x' " "password='x' port = x"
conn = psycopg2.connect(connect_str)
cur = conn.cursor()
conn.autocommit = True

cur.execute(query)
df = DataFrame(cur.fetchall())

谢谢

Answer 1

以下3种方法可能会有所帮助

使用 psycopg2命名为cursor cursor.itersize = 2000

代码段

 with conn.cursor(name='fetch_large_result') as cursor:

    cursor.itersize = 20000

    query = "SELECT * FROM ..."
    cursor.execute(query)

    for row in cursor:
....

使用 psycopg2命名为光标fetchmany（size = 2000）

代码段

conn = psycopg2.connect(conn_url)
cursor = conn.cursor(name='fetch_large_result')
cursor.execute('SELECT * FROM <large_table>')

while True:
    # consume result over a series of iterations
    # with each iteration fetching 2000 records
    records = cursor.fetchmany(size=2000)

    if not records:
        break

    for r in records:
        ....

cursor.close() #  cleanup
conn.close()

最后，您可以定义一个滚动光标

定义 SCROLL CURSOR

代码段

BEGIN MY_WORK;
-- Set up a cursor:
DECLARE scroll_cursor_bd SCROLL CURSOR FOR SELECT * FROM My_Table;

-- Fetch the first 5 rows in the cursor scroll_cursor_bd:

FETCH FORWARD 5 FROM scroll_cursor_bd;
CLOSE scroll_cursor_bd;
COMMIT MY_WORK;

请注意：未在psycopg2中命名光标将导致光标位于客户端而不是服务器端。

从Postgres数据库中批量获取数据（Python）

1 个答案: