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The following bug has been logged on the website:

Bug reference:      17077
Logged by:          yanliang lei
Email address:      leiyanliang@highgo.com
PostgreSQL version: 13.3
Operating system:   CentOS7.6
Description:        

Three parameters are
'transaction_deferrable','transaction_isolation','transaction_read_only'。

in the following sql statement,there are  three
parameters（'transaction_deferrable','transaction_isolation','transaction_read_only'）

[pg133@VM-0-8-centos ~]$ psql -d postgres
psql (13.3)
Type "help" for help.

postgres=# select * from pg_settings where name in
('transaction_deferrable','transaction_isolation','transaction_read_only');
          name          |    setting     | unit |                   
category                     |                                              
        short_desc                                                        |
extra_desc | c
ontext | vartype |  source  | min_val | max_val |                           
   enumvals                               |    boot_val    |   reset_val   
| sourcefile | sourceline | pending_restart 
------------------------+----------------+------+-------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------+------------+--
-------+---------+----------+---------+---------+----------------------------------------------------------------------+----------------+----------------+------------+------------+-----------------
 transaction_deferrable | off            |      | Client Connection Defaults
/ Statement Behavior | Whether to defer a read-only serializable transaction
until it can be executed with no possible serialization failures. |         
  | u
ser    | bool    | override |         |         |                           
                                          | off            | off           
|            |            | f
 transaction_isolation  | read committed |      | Client Connection Defaults
/ Statement Behavior | Sets the current transaction's isolation level.      
                                                                  |         
  | u
ser    | enum    | override |         |         | {serializable,"repeatable
read","read committed","read uncommitted"} | read committed | read committed
|            |            | f
 transaction_read_only  | off            |      | Client Connection Defaults
/ Statement Behavior | Sets the current transaction's read-only status.     
                                                                  |         
  | u
ser    | bool    | override |         |         |                           
                                          | off            | off           
|            |            | f
(3 rows)

postgres=# 

in the
https://www.postgresql.org/docs/current/runtime-config-client.html#RUNTIME-CONFIG-CLIENT-STATEMENT

there is no description about these three parameters。