how many objects can be locked per transaction

在PostgreSQL中，通过pg_locks这个视图可以查看到当前被跟踪的锁状态。
或者直接通过如下SQL查询:
SELECT l.locktype, l.database, l.relation, l.page, l.tuple, l.virtualxid, l.transactionid, l.classid, l.objid, l.objsubid, l.virtualtransaction, l.pid, l.mode, l.granted FROM pg_lock_status() l(locktype, database, relation, page, tuple, virtualxid, transactionid,classid, objid, objsubid, virtualtransaction, pid, mode, granted);
这些锁的信息被存储在System V的共享内存中，理论上只要共享内存足够，就可以继续获得锁。
但是文档却是这么描述的（实际上不是精确的描述，但是这个参数是会影响System V的分配的，所以间接的影响了可以持有锁的数量）：
max_locks_per_transaction (integer) -- 注意这是一个平均数，并不是用于限制单个事务的锁对象数量。

    The shared lock table tracks locks on max_locks_per_transaction * (max_connections + max_prepared_transactions) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have clients that touch many different tables in a single transaction. This parameter can only be set at server start.

    Increasing this parameter might cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See Section 17.4.1 for information on how to adjust those parameters, if necessary.

    When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server.

例:
假设 max_locks_per_transaction = 10
max_prepared_transactions = 0
max_connections = 40
按照文档的说明，整个数据库集群在同一时刻可以被锁的对象数应该等于400.但是实际上不只这个数量，（实际受到System V共享内存的限制）
创建两个测试函数，一个用于批量创建表，一个用于批量锁表。
CREATE OR REPLACE FUNCTION digoal.f_create_table(i_min integer, i_max integer)
 RETURNS void
 LANGUAGE plpgsql
AS $function$
declare
begin
for i in i_min..i_max loop
execute 'create table tbl_user_'||i||' (like tbl_user);';
end loop;
return;
end;
$function$

CREATE OR REPLACE FUNCTION digoal.f_lock_table(i_min integer, i_max integer)
 RETURNS void
 LANGUAGE plpgsql
AS $function$
declare
begin
for i in i_min..i_max loop
execute 'lock table tbl_user_'||i||' in exclusive mode;';
end loop;
return;
end;
$function$

digoal=> begin;
BEGIN
digoal=> select * from f_create_table(756,1857);
 f_create_table
----------------
 
(1 row)
digoal=> select count(distinct (database,relation)),count(*) from pg_locks;
 count | count
-------+-------
  1106 |  1107
(1 row)
此时数据库集群中被锁对象已经达到1106个,锁插槽使用1107个.已经超出400的限制，
继续申请锁看看会发生什么？
digoal=> select * from f_lock_table(1,4);
WARNING:  out of shared memory
CONTEXT:  SQL statement "lock table tbl_user_4 in exclusive mode;"
PL/pgSQL function "f_lock_table" line 4 at EXECUTE statement
ERROR:  out of shared memory
HINT:  You might need to increase max_locks_per_transaction.
CONTEXT:  SQL statement "lock table tbl_user_4 in exclusive mode;"
PL/pgSQL function "f_lock_table" line 4 at EXECUTE statement
再开连接的话也会有 out of shared memory 的报错.
postgres@db-172-16-3-33-> psql -h 127.0.0.1 postgres postgres
WARNING:  out of shared memory
psql: FATAL:  out of shared memory
HINT:  You might need to increase max_locks_per_transaction.

通 过增加max_locks_per_transaction或max_connections都可以达到加大System V共享内存的效果，但是具体能锁多少个对象不能使用max_locks_per_transaction * (max_connections + max_prepared_transactions)来精确计算，只是一个大概值。
当然，由于每一个连接都会消耗部分System V共享内存，因此当连接达到max_connections的时候，文档给出的公式会更加接近精确值。