python postgres 在读取结果时更改行的值

Question

我正在使用 python3、postgress 10 和 Psycopg2 来查询多条记录

import psycopg2
conn = psycopg2.connect(<my connection string>)
with conn:
    with conn.cursor() as cur:        
        cur.execute('select id,field1 from table1')
        for id, field1 from cur.fetchall():
           print(id,field1)
           #todo: how up update field1 to be f(field1) where f is an arbitrary python function

我的问题是:我如何更新我正在读取的行的值并将 field1 的值设置为一些基于 python 的任意计算

编辑:目的是更新表中的行

Answer 1

您需要另一个光标，例如:

with conn:
    with conn.cursor() as cur:        
        cur.execute('select id,field1 from table1')
        for id, field1 in cur.fetchall():
            print(id,field1)
            with conn.cursor() as cur_update:
                cur_update.execute('update table1 set field1 = %s where id = %s', (f(field1), id))

但是请注意，这涉及与选定行一样多的更新，这显然效率不高。可以使用 psycopg2.extras.execute_values(): 在单个查询中完成更新

from psycopg2.extras import execute_values
    
with conn:
    with conn.cursor() as cur:        
        cur.execute('select id,field1 from table1')
        rows = cur.fetchall()
        for id, field1 in rows:
            print(id,field1)
                
    # convert rows to new values of field1
    values = [(id, f(field1)) for id, field1 in rows]
    sql = '''
        with upd (id, field1) as (values %s)
        update table1 t
        set field1 = upd.field1
        from upd
        where upd.id = t.id
        '''
    with conn.cursor() as cur:
        execute_values(cur, sql, values)