Postgres 是否缓存了我们的查询，我们如何解决它?

Question

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).')

问题是这样的：

1. 代码成功执行且没有错误。但是，当我们在postico (Postgres GUI for Mac(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(。

在 psql 或 postico 中手动运行 delete 命令时

2. ，我们分别在 psql 或 postico 中运行 COUNT 命令时会得到正确的结果。但是，在 ipython 中运行命令时，我们会得到不同的结果。

当我在计算机 A 上的 ipython 上打开与数据库的连接时，我运行删除命令，

3. 并在计算机 B 上的 ipython 上打开另一个连接器并运行 count 命令，我看到数据库行计数没有改变，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本身中是否有可能导致这种情况的东西？我们如何绕过它？我们在postico或psycopg2/postgres上没有看到任何清除缓存。

Answer 1

不涉及缓存。PostgreSQL 不缓存查询结果。

您只是忘记COMMIT删除事务，因此其影响在任何并发事务中都不可见。