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');
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 example 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 26.2.5.1) and that max_wal_senders is set sufficiently high to allow an additional connection.
