为什么PG会提示增加max_locks_per_transaction的值

这篇文章主要介绍“为什么PG会提示增加max_locks_per_transaction的值”，在日常操作中，相信很多人在为什么PG会提示增加max_locks_per_transaction的值问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”为什么PG会提示增加max_locks_per_transaction的值”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！

有时候我们可能会在PG的日志发现如下信息：

2020-01-09 16:29:19.062 CST,"pg12","testdb",6193,"[local]",5e16dccd.1831,1,"CREATE TABLE",2020-01-09 15:57:01 CST,2/34,1512004206,ERROR,53200,"out of shared memory",,"You might need to increase max_locks_per_transaction.",,,,"CREATE TABLE a13030 (id int);",,,"psql" 2020-01-09 16:29:19.379 CST,"pg12","testdb",6193,"[local]",5e16dccd.1831,2,"CREATE TABLE",2020-01-09 15:57:01 CST,2/0,1512004206,ERROR,25P02,"current transaction is aborted, commands ignored until end of transaction block",,,,,,"CREATE TABLE a13031 (id int);",,,"psql"

直观上来看，OOM似乎与max_locks_per_transaction扯不上什么关系，为什么PG会提示增加max_locks_per_transaction的值呢？在一个事务中，shared lock table最大可以跟踪max_locks_per_transaction * (max_connections + max_prepared_transactions) 个对象（如数据表），超过的会报OOM错误。注意：锁粒度是object(如relation等)，跟行数无关。

OOM场景模拟
下面是一个模拟场景，在同一个事务中创建1w张表：

\pset footer off \o /tmp/drop.sql SELECT 'drop table if exists tbl' || id || ' ;' as "--"        FROM generate_series(1, 20000) AS id; \i /tmp/drop.sql \pset footer off \pset tuples_only \o /tmp/create.sql SELECT 'CREATE TABLE tbl' || id || ' (id int);' as "--"        FROM generate_series(1, 20000) AS id; \o /tmp/ret.txt begin; \i /tmp/create.sql

使用watch监控输出

watch -n1 "psql -c \"select locktype,mode,count(*) from pg_locks group by locktype,mode;\"" Every 1.0s: psql -c "select locktype,mode,count(*) from pg_locks group by locktype,mode;"  Fri Jan 10 14:41:26 2020 Expanded display is used automatically.    locktype    |        mode         | count ---------------+---------------------+-------  object        | AccessShareLock     |     1  relation      | AccessShareLock     |     1  virtualxid    | ExclusiveLock       |     2  relation      | AccessExclusiveLock |  3776  transactionid | ExclusiveLock       |     1 (5 rows) ... Every 1.0s: psql -c "select locktype,mode,count(*) from pg_locks group by locktype,mode;"  Fri Jan 10 14:41:50 2020 Expanded display is used automatically.    locktype    |        mode         | count ---------------+---------------------+-------  object        | AccessShareLock     |     1  relation      | AccessShareLock     |     1  virtualxid    | ExclusiveLock       |     2  relation      | AccessExclusiveLock | 10000  transactionid | ExclusiveLock       |     1 (5 rows) ...

在执行到tbl13034时报错

2020-01-10 14:44:18.855 CST,"pg12","testdb",32120,"[local]",5e181bea.7d78,3,"CREATE TABLE",2020-01-10 14:38:34 CST,2/106085,1512036258,ERROR,53200,"out of shared memory",,"You might need to increase max_locks_per_transaction.",,,,"CREATE TABLE tbl13034 (id int);",,,"psql" 2020-01-10 14:44:19.202 CST,"pg12","testdb",32120,"[local]",5e181bea.7d78,4,"CREATE TABLE",2020-01-10 14:38:34 CST,2/0,1512036258,ERROR,25P02,"current transaction is aborted, commands ignored until end of transaction block",,,,,,"CREATE TABLE tbl13035 (id int);",,,"psql"

相关源码
搜索You might need to increase max_locks_per_transaction.该错误信息出现在lock.c中

 /*   * LockAcquireExtended - allows us to specify additional options   *   * reportMemoryError specifies whether a lock request that fills the lock   * table should generate an ERROR or not.  Passing "false" allows the caller   * to attempt to recover from lock-table-full situations, perhaps by forcibly   * cancelling other lock holders and then retrying.  Note, however, that the   * return code for that is LOCKACQUIRE_NOT_AVAIL, so that it's unsafe to use   * in combination with dontWait = true, as the cause of failure couldn't be   * distinguished.   *   * If locallockp isn't NULL, *locallockp receives a pointer to the LOCALLOCK   * table entry if a lock is successfully acquired, or NULL if not.   */  LockAcquireResult  LockAcquireExtended(const LOCKTAG *locktag,                      LOCKMODE lockmode,                      bool sessionLock,                      bool dontWait,                      bool reportMemoryError,                      LOCALLOCK **locallockp)  {      ...      /*       * If this lock could potentially have been taken via the fast-path by       * some other backend, we must (temporarily) disable further use of the       * fast-path for this lock tag, and migrate any locks already taken via       * this method to the main lock table.       */      if (ConflictsWithRelationFastPath(locktag, lockmode))      {          uint32      fasthashcode = FastPathStrongLockHashPartition(hashcode);          BeginStrongLockAcquire(locallock, fasthashcode);          if (!FastPathTransferRelationLocks(lockMethodTable, locktag,                                             hashcode))          {              AbortStrongLockAcquire();              if (locallock->nLocks == 0)                  RemoveLocalLock(locallock);              if (locallockp)                  *locallockp = NULL;              if (reportMemoryError)                  ereport(ERROR,                          (errcode(ERRCODE_OUT_OF_MEMORY),                           errmsg("out of shared memory"),                           errhint("You might need to increase max_locks_per_transaction.")));              else                  return LOCKACQUIRE_NOT_AVAIL;          }      }      ...

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