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 show 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. 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)