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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！
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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;
}
}
...到此，关于“为什么PG会提示增加max_locks_per_transaction的值”的学习就结束了，希望能够解决大家的疑惑。理论与实践的搭配能更好的帮助大家学习，快去试试吧！若想继续学习更多相关知识，请继续关注创新互联网站，小编会继续努力为大家带来更多实用的文章！
