wal_level (enum)
wal_level决定有多少信息被写入到WAL中。 默认值是最小的，其中写入唯一从崩溃或立即关机中恢复的所需信息。 archive补充WAL归档需要的日志记录，
以及 hot_standby进一步增加在备用服务器上运行只读查询所需的信息 。这个参数只能在服务器启动时设置。
在最小级别中，安全地忽略一些批量操作的WAL-日志， 这可以使那些操作快很多（参阅第 14.4.7 节）。 这种优化可以应用到：
CREATE TABLE AS
CREATE INDEX
CLUSTER
COPY into tables that were created or truncated in the same transaction
但是最小的WAL不包含从基本的备份和WAL日志中重建数据的足够信息， 所以archive或者hot_standby级别必须用来启动 WAL归档(archive_mode)和流复制。
在hot_standby级别，相同的信息被记录为archive， 加上需要重建从WAL运行的事务状态信息。 为了在备用服务器上启用只读查询，主库上wal_level必须设置为hot_standby，
必须启动备库上的hot_standby。 使用hot_standby和archive级别之间的性能几乎没有可测量的差异 ，如果任何生产的影响是明显的，则是值得欢迎的。


[postgres@zhaopin-200-43 /data/postgresql/rdrc_94/data/pg_log]$ more postgresql-2015-11-25_02.log
[    2015-11-25 02:37:35.583 UTC 115397 56551eef.1c2c5 1 0]LOG:  database system was shut down at 2015-11-25 02:37:34 UTC
[    2015-11-25 02:37:35.594 UTC 115397 56551eef.1c2c5 2 0]LOG:  MultiXact member wraparound protections are now enabled
[    2015-11-25 02:37:35.597 UTC 115395 56551eef.1c2c3 3 0]LOG:  database system is ready to accept connections
[    2015-11-25 02:37:35.597 UTC 115401 56551eef.1c2c9 1 0]LOG:  autovacuum launcher started


postgres=# select * from pg_
postgres-# ^C
postgres=# insert into test_logical(id) values (2),(3),(4);
INSERT 0 3
postgres=# select * from test_logical;
 id
----
  1
  2
  3
  4
(4 rows)


postgres=# select * from pg_logical_slot_get_changes('log_slot1',null,null);
  location   | xid  |                       data
-------------+------+--------------------------------------------------
 71/99000028 | 2000 | BEGIN 2000
 71/99000028 | 2000 | table public.test_logical: INSERT: id[integer]:2
 71/99000068 | 2000 | table public.test_logical: INSERT: id[integer]:3
 71/990000A8 | 2000 | table public.test_logical: INSERT: id[integer]:4
 71/99000128 | 2000 | COMMIT 2000
(5 rows)


postgres=# insert into test_logical (id) values (7);
INSERT 0 1
postgres=# select * from pg_logical_slot_peek_changes('log_slot1',null,null);
  location   | xid  |                       data
-------------+------+--------------------------------------------------
 71/A5000028 | 2001 | BEGIN 2001
 71/A5000028 | 2001 | table public.test_logical: INSERT: id[integer]:7
 71/A50000A8 | 2001 | COMMIT 2001
(3 rows)


postgres=# select * from pg_logical_slot_peek_changes('log_slot1',null,null);
  location   | xid  |                       data
-------------+------+--------------------------------------------------
 71/A5000028 | 2001 | BEGIN 2001
 71/A5000028 | 2001 | table public.test_logical: INSERT: id[integer]:7
 71/A50000A8 | 2001 | COMMIT 2001
(3 rows)


postgres=# select * from pg_logical_slot_peek_changes('log_slot1',null,null);
  location   | xid  |                       data
-------------+------+--------------------------------------------------
 71/A5000028 | 2001 | BEGIN 2001
 71/A5000028 | 2001 | table public.test_logical: INSERT: id[integer]:7
 71/A50000A8 | 2001 | COMMIT 2001
(3 rows)


postgres=# select * from pg_logical_slot_peek_changes('log_slot1',null,null);
  location   | xid  |                       data
-------------+------+--------------------------------------------------
 71/A5000028 | 2001 | BEGIN 2001
 71/A5000028 | 2001 | table public.test_logical: INSERT: id[integer]:7
 71/A50000A8 | 2001 | COMMIT 2001
(3 rows)


postgres=# create table test_1(id int4);
CREATE TABLE
postgres=# insert into test_1 values (1);
INSERT 0 1
postgres=# insert into test_1 values (2);
INSERT 0 1
postgres=# delete from test_1 where id=1;






COMMIT 2003
BEGIN 2004
table public.test_1: INSERT: id[integer]:2
COMMIT 2004
BEGIN 2005
table public.test_1: DELETE: (no-tuple-data)
COMMIT 2005
pg_recvlogical: could not receive data from WAL stream: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
pg_recvlogical: disconnected; waiting 5 seconds to try again
BEGIN 2001
table public.test_logical: INSERT: id[integer]:7
COMMIT 2001
BEGIN 2002
COMMIT 2002
BEGIN 2003
table public.test_1: INSERT: id[integer]:1
COMMIT 2003
BEGIN 2004
table public.test_1: INSERT: id[integer]:2
COMMIT 2004
BEGIN 2005
table public.test_1: DELETE: (no-tuple-data)
COMMIT 2005
pg_recvlogical: could not receive data from WAL stream: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
pg_recvlogical: disconnected; waiting 5 seconds to try again
BEGIN 2001
table public.test_logical: INSERT: id[integer]:7
COMMIT 2001
BEGIN 2002
COMMIT 2002
BEGIN 2003
table public.test_1: INSERT: id[integer]:1
COMMIT 2003
BEGIN 2004
table public.test_1: INSERT: id[integer]:2
COMMIT 2004
BEGIN 2005
table public.test_1: DELETE: (no-tuple-data)
COMMIT 2005
pg_recvlogical: could not receive data from WAL stream: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
pg_recvlogical: disconnected; waiting 5 seconds to try again
BEGIN 2001
table public.test_logical: INSERT: id[integer]:7
COMMIT 2001
BEGIN 2002
COMMIT 2002
BEGIN 2003
table public.test_1: INSERT: id[integer]:1
COMMIT 2003
BEGIN 2004
table public.test_1: INSERT: id[integer]:2
COMMIT 2004
BEGIN 2005
table public.test_1: DELETE: (no-tuple-data)
COMMIT 2005
pg_recvlogical: could not receive data from WAL stream: server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.