51.1. Logical Decoding Examples
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## 51.1. Logical Decoding Examples

The following example demonstrates controlling logical decoding using the SQL interface.

Before you can use logical decoding, you must set wal_level to `logical` and max_replication_slots to at least 1. Then, you should connect to the target database (in the example below, `postgres`) as a superuser.

    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', false, true);
        slot_name    |    lsn
    -----------------+-----------
     regression_slot | 0/16B1970
    (1 row)

    postgres=# SELECT slot_name, plugin, slot_type, database, active, restart_lsn, confirmed_flush_lsn FROM pg_replication_slots;
        slot_name    |    plugin     | slot_type | database | active | restart_lsn | confirmed_flush_lsn
    -----------------+---------------+-----------+----------+--------+-------------+-----------------
     regression_slot | test_decoding | logical   | postgres | f      | 0/16A4408   | 0/16A4440
    (1 row)

    postgres=# -- There are no changes to see yet
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     lsn | 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);
        lsn    |  xid  |     data
    -----------+-------+--------------
     0/BA2DA58 | 10297 | BEGIN 10297
     0/BA5A5A0 | 10297 | COMMIT 10297
    (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);
     lsn | 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);
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A688 | 10298 | BEGIN 10298
     0/BA5A6F0 | 10298 | table public.data: INSERT: id[integer]:1 data[text]:'1'
     0/BA5A7F8 | 10298 | table public.data: INSERT: id[integer]:2 data[text]:'2'
     0/BA5A8A8 | 10298 | COMMIT 10298
    (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);
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A8E0 | 10299 | BEGIN 10299
     0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/BA5A990 | 10299 | COMMIT 10299
    (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);
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A8E0 | 10299 | BEGIN 10299
     0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/BA5A990 | 10299 | COMMIT 10299
    (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');
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A8E0 | 10299 | BEGIN 10299
     0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/BA5A990 | 10299 | COMMIT 10299 (at 2017-05-10 12:07:21.272494-04)
    (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)

The following examples shows how logical decoding is controlled over the streaming replication protocol, using the program pg_recvlogical included in the PostgreSQL distribution. This requires that client authentication is set up to allow replication connections (see Section 27.2.5.1) and that `max_wal_senders` is set sufficiently high to allow an additional connection. The second example shows how to stream two-phase transactions. Before you use two-phase commands, you must set max_prepared_transactions to at least 1.

    Example 1:
    $ 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

    Example 2:
    $ pg_recvlogical -d postgres --slot=test --create-slot --two-phase
    $ pg_recvlogical -d postgres --slot=test --start -f -
    Control+Z
    $ psql -d postgres -c "BEGIN;INSERT INTO data(data) VALUES('5');PREPARE TRANSACTION 'test';"
    $ fg
    BEGIN 694
    table public.data: INSERT: id[integer]:5 data[text]:'5'
    PREPARE TRANSACTION 'test', txid 694
    Control+Z
    $ psql -d postgres -c "COMMIT PREPARED 'test';"
    $ fg
    COMMIT PREPARED 'test', txid 694
    Control+C
    $ pg_recvlogical -d postgres --slot=test --drop-slot

The following example shows SQL interface that can be used to decode prepared transactions. Before you use two-phase commit commands, you must set `max_prepared_transactions` to at least 1. You must also have set the two-phase parameter as 'true' while creating the slot using `pg_create_logical_replication_slot` Note that we will stream the entire transaction after the commit if it is not already decoded.

    postgres=# BEGIN;
    postgres=*# INSERT INTO data(data) VALUES('5');
    postgres=*# PREPARE TRANSACTION 'test_prepared1';

    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                          data
    -----------+-----+---------------------------------------------------------
     0/1689DC0 | 529 | BEGIN 529
     0/1689DC0 | 529 | table public.data: INSERT: id[integer]:3 data[text]:'5'
     0/1689FC0 | 529 | PREPARE TRANSACTION 'test_prepared1', txid 529
    (3 rows)

    postgres=# COMMIT PREPARED 'test_prepared1';
    postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                    data
    -----------+-----+--------------------------------------------
     0/168A060 | 529 | COMMIT PREPARED 'test_prepared1', txid 529
    (4 row)

    postgres=#-- you can also rollback a prepared transaction
    postgres=# BEGIN;
    postgres=*# INSERT INTO data(data) VALUES('6');
    postgres=*# PREPARE TRANSACTION 'test_prepared2';
    postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                          data
    -----------+-----+---------------------------------------------------------
     0/168A180 | 530 | BEGIN 530
     0/168A1E8 | 530 | table public.data: INSERT: id[integer]:4 data[text]:'6'
     0/168A430 | 530 | PREPARE TRANSACTION 'test_prepared2', txid 530
    (3 rows)

    postgres=# ROLLBACK PREPARED 'test_prepared2';
    postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                     data
    -----------+-----+----------------------------------------------
     0/168A4B8 | 530 | ROLLBACK PREPARED 'test_prepared2', txid 530
    (1 row)

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