Question

我们需要对Postgres DB中的许多行进行批量更新，并希望使用下面的SQL语法。我们如何使用psycopg2做到这一点？

UPDATE table_to_be_updated
SET msg = update_payload.msg
FROM (VALUES %(update_payload)s) AS update_payload(id, msg)
WHERE table_to_be_updated.id = update_payload.id
RETURNING *

尝试1-传递值

我们需要将嵌套的可迭代格式传递给psycopg2查询。对于update_payload，我尝试传递一个列表列表，元组列表和元组元组。全部失败，并显示各种错误。

尝试2-使用__conform __

编写自定义类

我尝试编写一个可用于这些操作的自定义类，该类将返回

(VALUES (row1_col1, row1_col2), (row2_col1, row2_col2), (...))

我已经按照here的说明进行了编码，但是很明显我做错了。例如，在这种方法中，我将不得不处理表中所有值的引用，这会很麻烦并且容易出错。

class ValuesTable(list):
    def __init__(self, *args, **kwargs):
        super(ValuesTable, self).__init__(*args, **kwargs)

    def __repr__(self):
        data_in_sql = ""
        for row in self:
            str_values = ", ".join([str(value) for value in row])
            data_in_sql += "({})".format(str_values)
        return "(VALUES {})".format(data_in_sql)

    def __conform__(self, proto):
        return self.__repr__()

    def getquoted(self):
        return self.__repr__()

    def __str__(self):
        return self.__repr__()

Answer 1

要求：

Postgres表，由字段ID和msg（以及其他可能的字段）组成
包含msg新值的Python数据
Postgres表应通过psycopg2更新

示例表

CREATE TABLE einstein(
   id CHAR(5) PRIMARY KEY,
   msg VARCHAR(1024) NOT NULL
);

测试数据

INSERT INTO einstein VALUES ('a', 'empty');
INSERT INTO einstein VALUES ('b', 'empty');
INSERT INTO einstein VALUES ('c', 'empty');

Python程序

假想的，独立的示例程序，引用了著名物理学家的话。

import sys
import psycopg2
from psycopg2.extras import execute_values


def print_table(con):
    cur = con.cursor()
    cur.execute("SELECT * FROM einstein")
    rows = cur.fetchall()
    for row in rows:
        print(f"{row[0]} {row[1]}")


def update(con, einstein_quotes):
    cur = con.cursor()
    execute_values(cur, """UPDATE einstein 
                           SET msg = update_payload.msg 
                           FROM (VALUES %s) AS update_payload (id, msg) 
                           WHERE einstein.id = update_payload.id""", einstein_quotes)
    con.commit()


def main():
    con = None
    einstein_quotes = [("a", "Few are those who see with their own eyes and feel with their own hearts."),
                       ("b", "I have no special talent. I am only passionately curious."),
                       ("c", "Life is like riding a bicycle. To keep your balance you must keep moving.")]

    try:
        con = psycopg2.connect("dbname='stephan' user='stephan' host='localhost' password=''")
        print_table(con)
        update(con, einstein_quotes)
        print("rows updated:")
        print_table(con)

    except psycopg2.DatabaseError as e:

        print(f'Error {e}')
        sys.exit(1)

    finally:

        if con:
            con.close()


if __name__ == '__main__':
    main()

准备报表替代

import sys
import psycopg2
from psycopg2.extras import execute_batch


def print_table(con):
    cur = con.cursor()
    cur.execute("SELECT * FROM einstein")
    rows = cur.fetchall()
    for row in rows:
        print(f"{row[0]} {row[1]}")


def update(con, einstein_quotes, page_size):
    cur = con.cursor()
    cur.execute("PREPARE updateStmt AS UPDATE einstein SET msg=$1 WHERE id=$2")
    execute_batch(cur, "EXECUTE updateStmt (%(msg)s, %(id)s)", einstein_quotes, page_size=page_size)
    cur.execute("DEALLOCATE updateStmt")
    con.commit()


def main():
    con = None
    einstein_quotes = ({"id": "a", "msg": "Few are those who see with their own eyes and feel with their own hearts."},
                       {"id": "b", "msg": "I have no special talent. I am only passionately curious."},
                       {"id": "c", "msg": "Life is like riding a bicycle. To keep your balance you must keep moving."})

    try:
        con = psycopg2.connect("dbname='stephan' user='stephan' host='localhost' password=''")
        print_table(con)
        update(con, einstein_quotes, 100)  #choose some meaningful page_size here
        print("rows updated:")
        print_table(con)

    except psycopg2.DatabaseError as e:

        print(f'Error {e}')
        sys.exit(1)

    finally:

        if con:
            con.close()


if __name__ == '__main__':
    main()

输出

上面的程序会将以下内容输出到调试控制台：

a     empty
b     empty
c     empty
rows updated:
a     Few are those who see with their own eyes and feel with their own hearts.
b     I have no special talent. I am only passionately curious.
c     Life is like riding a bicycle. To keep your balance you must keep moving.

使用psycopg2

1 个答案: