Python Postgres是否正在缓存我们的查询？我们如何绕过它？

我正在尝试运行以下python3代码：

import os
import psycopg2
import logging

# Set max attempts before giving up
MAX_ATTEMPTS = 5

# Set basic logging config to debug (i.e. log everything).
# By default, this will log te stdout (i.e. it will behave the same as print)
logging.basicConfig(level=logging.DEBUG)

# Grab DB url from env variable
database_url = os.environ.get('DATABASE_URL')

assert database_url is not None, 'DATABASE_URL env variable must be set to a postgres connection string.'

# Initiate psycopg2 and instantiate a cursor object
conn = psycopg2.connect(database_url)
cursor = conn.cursor()


# Define function to delete old records
def delete_old_records(cur):
    # execute a query to delete old records. We're going to refer to this as the "delete" command
    query = 'DELETE FROM my_table WHERE id NOT IN ( SELECT id FROM ( SELECT id FROM my_table ORDER BY id DESC LIMIT 1850 ) foo);'
    cur.execute(query)


# Set variables to keep track of loop
successful = False
attempts = 0

# While not successful and max attempts not reached
while not successful and attempts < MAX_ATTEMPTS:
    try:
        # Attempt to delete old records
        delete_old_records(cursor)
        # Set successful to True if no errors were encountered in the previous line
        successful = True
        # Log a message
        logging.info('Successfully truncated old records!')
    # If some psycopg2 error happens
    except psycopg2.Error as e:
        # Log the error
        logging.exception('Got exception when executing query')
        # Rollback the cursor and get ready to try again
        conn.rollback()
        # Increment attempts by 1
        attempts += 1

# If not able to perform operation after max attempts, log message to indicate failure
if not successful:
    logging.warning(f'Was not successfully able to truncate logs after {MAX_ATTEMPTS} retries. '
                    f'Check logs for traceback (console output by default).')

我们得到1860而不是1850（即，行未被删除）

在psql或postico中手动运行delete命令时，我们分别在psql或postico中运行COUNT命令时得到正确的结果。但是，在ipython中运行命令时，我们会得到不同的结果

当我打开与计算机A上ipython上的db的连接，并运行delete命令，然后打开另一个与计算机B上ipython上的db的连接器并运行count命令时，我发现db行计数没有改变，即仍然是1860，没有切到1850

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我怀疑缓存/备忘录化，但我不确定我的命令是否真的有效。psycopg2、Postco或postgres本身是否有可能导致这种情况？我们该如何应对呢？我们在Postco或psycopg2/postgres上看不到任何清除缓存

不涉及缓存。PostgreSQL不缓存查询结果

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您只是忘记了提交删除事务，因此它的效果在任何并发事务中都不可见。

我尝试了执行connection.COMMIT（）和connection as conn:with conn.cursor（）as cur:cur.execute（query），但Postco仍然将count查询的结果显示为1860@劳伦斯：没关系，我和往常一样是个白痴。错误的数据库连接字符串。谢谢！你能详细说明一下你的答案吗？什么数据库引擎有或没有缓存，以及缓存是如何工作的？我只知道MySQL有一个查询缓存，但我认为这是一个不合适的功能，在99%的情况下最好关闭它。我不知道它是怎么工作的。

SELECT count(*) from my_table;