Can rename table but can not truncate table

一个表无法truncate可是能够rename，这个乍听起来认为好奇怪，以下模拟该过程。
3个session：
session1运行truncate和rename操作。
session2运行lock表操作；
session3进行监控。
 
session1：

[gpadmin@wx60 contrib]$ psql gtlions
psql (8.2.15)
Type "help" for help.
 
gtlions=# d test
             Table "public.test"
 Column |          Type          | Modifiers 
--------+------------------------+-----------
 id     | integer                | 
 name   | character varying(200) | 
Indexes:
    "idxtestid" btree (id)
    "idxtestname" btree (name)
Distributed by: (id)
 
gtlions=# select pg_backend_pid();
 pg_backend_pid 
----------------
           1473
(1 row)
 
gtlions=# truncate table test;
Cancel request sent
ERROR:  relation "test" does not exist
gtlions=# alter table test rename to test1;
ALTER TABLE

 
session2：

[gpadmin@wx60 ~]$ psql gtlions
psql (8.2.15)
Type "help" for help.
 
gtlions=# select pg_backend_pid();
 pg_backend_pid 
----------------
           1555
(1 row)
 
gtlions=# begin;
BEGIN
gtlions=# select * from test limit 10;
  id   |    name    
-------+------------
 19672 | 19672-asfd
 19674 | 19674-asfd
 19676 | 19676-asfd
 19678 | 19678-asfd
 19680 | 19680-asfd
 19682 | 19682-asfd
 19684 | 19684-asfd
 19686 | 19686-asfd
 19688 | 19688-asfd
 19690 | 19690-asfd
(10 rows)
 
gtlions=# end;
COMMIT

 
session3：

[gpadmin@wx60 ~]$ psql gtlions
psql (8.2.15)
Type "help" for help.
 
gtlions=# select locktype,relation::regclass,pid,mode,granted,gp_segment_id from pg_locks where pid in (1473,1555) order by pid;
 locktype | relation | pid | mode | granted | gp_segment_id 
----------+----------+-----+------+---------+---------------
(0 rows)
 
gtlions=# select locktype,relation::regclass,pid,mode,granted,gp_segment_id from pg_locks where pid in (1473,1555) order by pid;
   locktype    |  relation   | pid  |      mode       | granted | gp_segment_id 
---------------+-------------+------+-----------------+---------+---------------
 relation      | test        | 1555 | AccessShareLock | t       |            -1
 relation      | idxtestname | 1555 | AccessShareLock | t       |            -1
 transactionid |             | 1555 | ExclusiveLock   | t       |            -1
 relation      | idxtestid   | 1555 | AccessShareLock | t       |            -1
(4 rows)
 
gtlions=# select locktype,relation::regclass,pid,mode,granted,gp_segment_id from pg_locks where pid in (1473,1555) order by pid;
   locktype    |  relation   | pid  |        mode         | granted | gp_segment_id 
---------------+-------------+------+---------------------+---------+---------------
 transactionid |             | 1473 | ExclusiveLock       | t       |            -1
 relation      | test        | 1473 | AccessExclusiveLock | f       |            -1
 transactionid |             | 1555 | ExclusiveLock       | t       |            -1
 relation      | idxtestid   | 1555 | AccessShareLock     | t       |            -1
 relation      | idxtestname | 1555 | AccessShareLock     | t       |            -1
 relation      | test        | 1555 | AccessShareLock     | t       |            -1
(6 rows)

 
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