PostgreSQL 9.4.4 中文手册
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# 46.1. 逻辑解码示例

下面的示例演示了使用SQL接口控制逻辑解码。

在你可以使用逻辑解码之前，必须将wal_level设置为 `logical`，将max_replication_slots 设置为至少为1。然后，以超级用户的身份连接到目标数据库 （在下面的示例中，是`postgres`）。

    postgres=# -- Create a slot named 'regression_slot' using the output plugin 'test_decoding'
    postgres=# SELECT * FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
        slot_name    | xlog_position
    -----------------+---------------
     regression_slot | 0/16B1970
    (1 row)

    postgres=# SELECT * FROM pg_replication_slots;
        slot_name    |    plugin     | slot_type | datoid | database | active |  xmin  | catalog_xmin | restart_lsn
    -----------------+---------------+-----------+--------+----------+--------+--------+--------------+-------------
     regression_slot | test_decoding | logical   |  12052 | postgres | f      |        |          684 | 0/16A4408
    (1 row)

    postgres=# -- There are no changes to see yet
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     location | xid | data
    ----------+-----+------
    (0 rows)

    postgres=# CREATE TABLE data(id serial primary key, data text);
    CREATE TABLE

    postgres=# -- DDL isn't replicated, so all you'll see is the transaction
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     location  | xid |    data
    -----------+-----+------------
     0/16D5D48 | 688 | BEGIN 688
     0/16E0380 | 688 | COMMIT 688
    (2 rows)

    postgres=# -- Once changes are read, they're consumed and not emitted
    postgres=# -- in a subsequent call:
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     location | xid | data
    ----------+-----+------
    (0 rows)

    postgres=# BEGIN;
    postgres=# INSERT INTO data(data) VALUES('1');
    postgres=# INSERT INTO data(data) VALUES('2');
    postgres=# COMMIT;

    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     location  | xid |                     data
    -----------+-----+-----------------------------------------------
     0/16E0478 | 689 | BEGIN 689
     0/16E0478 | 689 | table public.data: INSERT: id[integer]:1 data[text]:'1'
     0/16E0580 | 689 | table public.data: INSERT: id[integer]:2 data[text]:'2'
     0/16E0650 | 689 | COMMIT 689
    (4 rows)

    postgres=# INSERT INTO data(data) VALUES('3');

    postgres=# -- You can also peek ahead in the change stream without consuming changes
    postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
     location  | xid |                     data
    -----------+-----+-----------------------------------------------
     0/16E09C0 | 690 | BEGIN 690
     0/16E09C0 | 690 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/16E0B90 | 690 | COMMIT 690
    (3 rows)

    postgres=# -- The next call to pg_logical_slot_peek_changes() returns the same changes again
    postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
     location  | xid |                     data
    -----------+-----+-----------------------------------------------
     0/16E09C0 | 690 | BEGIN 690
     0/16E09C0 | 690 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/16E0B90 | 690 | COMMIT 690
    (3 rows)

    postgres=# -- options can be passed to output plugin, to influence the formatting
    postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-timestamp', 'on');
     location  | xid |                     data
    -----------+-----+-----------------------------------------------
     0/16E09C0 | 690 | BEGIN 690
     0/16E09C0 | 690 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/16E0B90 | 690 | COMMIT 690 (at 2014-02-27 16:41:51.863092+01)
    (3 rows)

    postgres=# -- Remember to destroy a slot you no longer need to stop it consuming
    postgres=# -- server resources:
    postgres=# SELECT pg_drop_replication_slot('regression_slot');
     pg_drop_replication_slot
    -----------------------

    (1 row)

下面的示例显示了流复制协议是如何控制逻辑解码的，使用PostgreSQL发布中包含的程序 pg_recvlogical。这要求客户端认证设置为允许复制连接 （参阅第 25.2.5.1 节）， 并且`max_wal_senders`的设置对于额外连接来说足够大。

    $ pg_recvlogical -d postgres --slot test --create-slot
    $ pg_recvlogical -d postgres --slot test --start -f -
    Control+Z
    $ psql -d postgres -c "INSERT INTO data(data) VALUES('4');"
    $ fg
    BEGIN 693
    table public.data: INSERT: id[integer]:4 data[text]:'4'
    COMMIT 693
    Control+C
    $ pg_recvlogical -d postgres --slot test --drop-slot

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