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xiazhibin/数据库小笔记.md

Last active January 18, 2018 08:19
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数据库小笔记
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数据库小笔记.md

select with update 问题

  • 同一个进程对表的同一行select with update是没有问题的
  • 不同进程对表的同一行select with update是会造成lock transaction。要等其中一方commit。
  • idle in transaction，如果开启了一个空闲的事务，会有一个超时时间，postgres会断开这个transaction。解决方法是try catch掉，重新commit

数人数，里面可能有重复的

z_uid     c_uid
1 a
1 b
2 m
  • select DISTINCT z_uid from t
  • select count(DISTINCT z_uid) from t
  • select DISTINCT T.z_uid from (select z.uid as z_uid,c.uid as c_uid from z,c) as T
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