PostgreSQL ERROR: out of shared memory处理

使用pg_dump命令导出一个库的时候，报

pg_dump: error: query failed: ERROR: out of shared memory

HINT: You might need to increase "max_locks_per_transaction".

从错误字面上看是超出内存大小了，建议增加max_locks_per_transaction参数

本环境中：

cpp 复制代码

[postgres174@geoscene ~]$ psql
psql (17.4)
Type "help" for help.

postgres=# show max_locks_per_transaction ;
 max_locks_per_transaction
---------------------------
 64
(1 row)

原因：

pg_dump -d dbname的时候会对库中所有的表执行lock table in shared mode 操作。

该操作会将表锁记录到表锁所在的共享结构中。

cpp 复制代码

lock.c: SetupLockInTable

lock = (LOCK *) hash_search_with_hash_value(LockMethodLockHash,
												locktag,
												hashcode,
												HASH_ENTER_NULL,
												&found);

LockMethodLockHash 结构在InitLocks初始化

cpp 复制代码

lock.c
InitLocks(void)
{
        HASHCTL         info;
        long            init_table_size,
                                max_table_size;
        bool            found;

        /*
         * Compute init/max size to request for lock hashtables.  Note these
         * calculations must agree with LockShmemSize!
         */

        //最大表大小
        max_table_size = NLOCKENTS();
        init_table_size = max_table_size / 2;

        /*
         * Allocate hash table for LOCK structs.  This stores per-locked-object
         * information.
         */
        info.keysize = sizeof(LOCKTAG);
        info.entrysize = sizeof(LOCK);
        info.num_partitions = NUM_LOCK_PARTITIONS;

        LockMethodLockHash = ShmemInitHash("LOCK hash",
                                                                           init_table_size,
                                                                           max_table_size,
                                                                           &info,
                                                                           HASH_ELEM | HASH_BLOBS | HASH_PARTITION);
...

查看

cpp 复制代码

#define NLOCKENTS() \
	mul_size(max_locks_per_xact, add_size(MaxBackends, max_prepared_xacts))


Size
mul_size(Size s1, Size s2)
{
	Size		result;

	if (s1 == 0 || s2 == 0)
		return 0;
	result = s1 * s2;
	/* We are assuming Size is an unsigned type here... */
	if (result / s2 != s1)
		ereport(ERROR,
				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
				 errmsg("requested shared memory size overflows size_t")));
	return result;
}

可以看出来，跟最大连接数和max_prepared_xacts相关

所以解决这个问题除了增加max_locks_per_xact参数的数量，增加最大连接数实际也可以。