Question

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).')

问题出在这里

代码成功执行且没有错误。但是，当我们在postico（适用于Mac的Postgres GUI）上运行以下命令（以下称为“ count”命令）时：
```
SELECT count(*) from my_table;
```
我们得到的是1860而不是1850（即，行没有删除）。
在psql或postico中手动运行delete命令时，分别在psql或postico中运行COUNT命令时，我们将获得正确的结果。但是，在ipython中运行命令时会得到不同的结果。
当我与计算机A上的ipython上的数据库建立开放连接，并且运行delete命令，并且在计算机B上的ipython上打开与数据库的另一个连接器并运行count命令时，我看到db行数没有改变，即仍然是1860，没有减少到1850。

我怀疑缓存/存储，但是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现象的原因？以及我们如何解决呢？我们在postico或psycopg2 / postgres上看不到任何清除的缓存。

Answer 1

不涉及缓存。 PostgreSQL不缓存查询结果。

您只是忘记了COMMIT删除的事务，因此其影响在任何并发事务中都不可见。

Postgres是否在缓存我们的查询，以及如何解决它？

1 个答案: