49.1. 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)