使用 psycopg2 缓存中间体 table

Question

以涉及两个 SELECTs 的 psycopg2 调用为例：

import psycopg2

with psycopg2.connect("dbname=test user=postgres") as conn:
    with conn.cursor() as cur:
        cur.execute("SELECT a, b, c FROM table WHERE a > 5 and d < 10;")
        r1 = cur.fetchall()
        cur.execute("SELECT a, b, c FROM table WHERE a > 5 and d > 20;")
        r2 = cur.fetchall()

这有点低效；潜在的 O(N) 检查 WHERE a > 5 执行了两次，而它似乎只能执行一次，并对该中间结果执行子查询。

通过 psycopg2 API 执行此操作的规范方法是什么？

类似于：

with psycopg2.connect("dbname=test user=postgres") as conn:
    with conn.cursor() as cur:
        cur.execute("SELECT a, b, c FROM table WHERE a > 5")
        # ...
        cur.execute("SELECT a, b, c FROM temp_table WHERE d < 10;")
        r1 = cur.fetchall()
        cur.execute("SELECT a, b, c FROM temp_table WHERE d > 20;")
        r2 = cur.fetchall()

使用文字是最好的解决方案吗"CREATE TEMP TABLE..."？

我是从 Django ORM 的角度来看这个问题的，其中 QuerySet 的后续评估会重用缓存的结果。 psycopg2 API 是否提供类似的东西？

Answer 1

您可以执行单个查询并将结果分成两个列表：

with psycopg2.connect("dbname=test user=postgres") as conn:
    with conn.cursor() as cur:
        cur.execute("SELECT a, b, c, d FROM my_table WHERE a > 5 and (d < 10 or d > 20);")
        rows = cur.fetchall()

r1 = [(i[0], i[1], i[2]) for i in rows if i[3] < 10]
r2 = [(i[0], i[1], i[2]) for i in rows if i[3] > 20]

在结果集不是很大的情况下，上述解决方案应该是最有效的。或者，您可以创建一个临时 table:

with psycopg2.connect("dbname=test user=postgres") as conn:
    with conn.cursor() as cur:
        cur.execute("""
            CREATE TEMP TABLE t AS
            SELECT a, b, c, d 
            FROM my_table 
            WHERE a > 5 and (d < 10 or d > 20);""")
        cur.execute("SELECT a, b, c FROM t WHERE d < 10;")
        r1 = cur.fetchall()
        cur.execute("SELECT a, b, c FROM t WHERE d > 20;")
        r2 = cur.fetchall()

临时文件table会在连接关闭时自动删除。

如果结果集太大而无法在客户端实际处理，请使用 server-side cursor. When you fetch single rows in a loop the rows are actually retrieved from the server in buckets. You can define the size of the buckets by setting itersize.。

r1 = []
r2 = []
with psycopg2.connect("dbname=test user=postgres") as conn:
    with conn.cursor('my_cursor') as cur:
        cur.itersize = 1000
        cur.execute("SELECT a, b, c, d FROM my_table WHERE a < 5 and (d < 10 or d > 20);")
        for row in cur:
            if row[3] < 10:
                r1.append((row[0], row[1], row[2]))
            else:
                r2.append((row[0], row[1], row[2]))

使用 psycopg2 缓存中间体 table

Caching intermediate table with psycopg2

python

postgresql

psycopg2

python-3.x