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我正在尝试运行以下一段 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).')

这是问题所在：

  1. 代码执行成功且没有错误。但是，当我们在 postico（Mac 的 Postgres GUI）上运行以下命令（以下称为“count”命令）时：

    SELECT count(*) from my_table;

    我们得到 1860 而不是 1850（即行没有被删除）。

  2. 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。

  3. 当我在计算机 A 上与 ipython 上的 db 建立开放连接时，我运行 delete 命令，并在计算机 B 上打开另一个连接到 ipython 上的 db 并运行 count 命令，我看到 db 行计数没有改变了即仍然是1860，而不是削减到1850。

我怀疑缓存/记忆，但我不确定我的命令是否真的有效。psycopg2、postico 或 postgres 本身是否有可能导致此问题？我们如何绕过它？我们在 postico 或 psycopg2/postgres 上看不到任何清晰的缓存。

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1 回答 1

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不涉及缓存。PostgreSQL 不缓存查询结果。

您只是忘记COMMIT了删除事务，因此它的影响在任何并发事务中都不可见。

于 2018-09-28T08:29:58.223 回答