Logical replication is a method of replicating data objects and their changes, based upon their replication identity (usually a primary key). We use the term logical in contrast to physical replication, which uses exact block addresses and byte-by-byte replication. PostgreSQL supports both mechanisms concurrently, see . Logical replication allows fine-grained control over both data replication and security. Logical replication uses a publish and subscribe model with one or more subscribers subscribing to one or more publications on a publisher node. Subscribers pull data from the publications they subscribe to and may subsequently re-publish data to allow cascading replication or more complex configurations. Logical replication of a table typically starts with taking a snapshot of the data on the publisher database and copying that to the subscriber. Once that is done, the changes on the publisher are sent to the subscriber as they occur in real-time. The subscriber applies the data in the same order as the publisher so that transactional consistency is guaranteed for publications within a single subscription. This method of data replication is sometimes referred to as transactional replication. The typical use-cases for logical replication are: Sending incremental changes in a single database or a subset of a database to subscribers as they occur. Firing triggers for individual changes as they arrive on the subscriber. Consolidating multiple databases into a single one (for example for analytical purposes). Replicating between different major versions of PostgreSQL. Replicating between PostgreSQL instances on different platforms (for example Linux to Windows) Giving access to replicated data to different groups of users. Sharing a subset of the database between multiple databases. The subscriber database behaves in the same way as any other PostgreSQL instance and can be used as a publisher for other databases by defining its own publications. When the subscriber is treated as read-only by application, there will be no conflicts from a single subscription. On the other hand, if there are other writes done either by an application or by other subscribers to the same set of tables, conflicts can arise. A publication can be defined on any physical replication primary. The node where a publication is defined is referred to as publisher. A publication is a set of changes generated from a table or a group of tables, and might also be described as a change set or replication set. Each publication exists in only one database. Publications are different from schemas and do not affect how the table is accessed. Each table can be added to multiple publications if needed. Publications may currently only contain tables and all tables in schema. Objects must be added explicitly, except when a publication is created for ALL TABLES. Publications can choose to limit the changes they produce to any combination of INSERT, UPDATE, DELETE, and TRUNCATE, similar to how triggers are fired by particular event types. By default, all operation types are replicated. These publication specifications apply only for DML operations; they do not affect the initial data synchronization copy. (Row filters have no effect for TRUNCATE. See ). A published table must have a replica identity configured in order to be able to replicate UPDATE and DELETE operations, so that appropriate rows to update or delete can be identified on the subscriber side. By default, this is the primary key, if there is one. Another unique index (with certain additional requirements) can also be set to be the replica identity. If the table does not have any suitable key, then it can be set to replica identity full, which means the entire row becomes the key. This, however, is very inefficient and should only be used as a fallback if no other solution is possible. If a replica identity other than full is set on the publisher side, a replica identity comprising the same or fewer columns must also be set on the subscriber side. See for details on how to set the replica identity. If a table without a replica identity is added to a publication that replicates UPDATE or DELETE operations then subsequent UPDATE or DELETE operations will cause an error on the publisher. INSERT operations can proceed regardless of any replica identity. Every publication can have multiple subscribers. A publication is created using the CREATE PUBLICATION command and may later be altered or dropped using corresponding commands. The individual tables can be added and removed dynamically using ALTER PUBLICATION. Both the ADD TABLE and DROP TABLE operations are transactional; so the table will start or stop replicating at the correct snapshot once the transaction has committed. A subscription is the downstream side of logical replication. The node where a subscription is defined is referred to as the subscriber. A subscription defines the connection to another database and set of publications (one or more) to which it wants to subscribe. The subscriber database behaves in the same way as any other PostgreSQL instance and can be used as a publisher for other databases by defining its own publications. A subscriber node may have multiple subscriptions if desired. It is possible to define multiple subscriptions between a single publisher-subscriber pair, in which case care must be taken to ensure that the subscribed publication objects don't overlap. Each subscription will receive changes via one replication slot (see ). Additional replication slots may be required for the initial data synchronization of pre-existing table data and those will be dropped at the end of data synchronization. A logical replication subscription can be a standby for synchronous replication (see ). The standby name is by default the subscription name. An alternative name can be specified as application_name in the connection information of the subscription. Subscriptions are dumped by pg_dump if the current user is a superuser. Otherwise a warning is written and subscriptions are skipped, because non-superusers cannot read all subscription information from the pg_subscription catalog. The subscription is added using CREATE SUBSCRIPTION and can be stopped/resumed at any time using the ALTER SUBSCRIPTION command and removed using DROP SUBSCRIPTION. When a subscription is dropped and recreated, the synchronization information is lost. This means that the data has to be resynchronized afterwards. The schema definitions are not replicated, and the published tables must exist on the subscriber. Only regular tables may be the target of replication. For example, you can't replicate to a view. The tables are matched between the publisher and the subscriber using the fully qualified table name. Replication to differently-named tables on the subscriber is not supported. Columns of a table are also matched by name. The order of columns in the subscriber table does not need to match that of the publisher. The data types of the columns do not need to match, as long as the text representation of the data can be converted to the target type. For example, you can replicate from a column of type integer to a column of type bigint. The target table can also have additional columns not provided by the published table. Any such columns will be filled with the default value as specified in the definition of the target table. As mentioned earlier, each (active) subscription receives changes from a replication slot on the remote (publishing) side. Additional table synchronization slots are normally transient, created internally to perform initial table synchronization and dropped automatically when they are no longer needed. These table synchronization slots have generated names: pg_%u_sync_%u_%llu (parameters: Subscription oid, Table relid, system identifier sysid) Normally, the remote replication slot is created automatically when the subscription is created using CREATE SUBSCRIPTION and it is dropped automatically when the subscription is dropped using DROP SUBSCRIPTION. In some situations, however, it can be useful or necessary to manipulate the subscription and the underlying replication slot separately. Here are some scenarios: When creating a subscription, the replication slot already exists. In that case, the subscription can be created using the create_slot = false option to associate with the existing slot. When creating a subscription, the remote host is not reachable or in an unclear state. In that case, the subscription can be created using the connect = false option. The remote host will then not be contacted at all. This is what pg_dump uses. The remote replication slot will then have to be created manually before the subscription can be activated. When dropping a subscription, the replication slot should be kept. This could be useful when the subscriber database is being moved to a different host and will be activated from there. In that case, disassociate the slot from the subscription using ALTER SUBSCRIPTION before attempting to drop the subscription. When dropping a subscription, the remote host is not reachable. In that case, disassociate the slot from the subscription using ALTER SUBSCRIPTION before attempting to drop the subscription. If the remote database instance no longer exists, no further action is then necessary. If, however, the remote database instance is just unreachable, the replication slot (and any still remaining table synchronization slots) should then be dropped manually; otherwise it/they would continue to reserve WAL and might eventually cause the disk to fill up. Such cases should be carefully investigated. Create some test tables on the publisher. test_pub=# CREATE TABLE t1(a int, b text, PRIMARY KEY(a)); CREATE TABLE test_pub=# CREATE TABLE t2(c int, d text, PRIMARY KEY(c)); CREATE TABLE test_pub=# CREATE TABLE t3(e int, f text, PRIMARY KEY(e)); CREATE TABLE Create the same tables on the subscriber. test_sub=# CREATE TABLE t1(a int, b text, PRIMARY KEY(a)); CREATE TABLE test_sub=# CREATE TABLE t2(c int, d text, PRIMARY KEY(c)); CREATE TABLE test_sub=# CREATE TABLE t3(e int, f text, PRIMARY KEY(e)); CREATE TABLE Insert data to the tables at the publisher side. test_pub=# INSERT INTO t1 VALUES (1, 'one'), (2, 'two'), (3, 'three'); INSERT 0 3 test_pub=# INSERT INTO t2 VALUES (1, 'A'), (2, 'B'), (3, 'C'); INSERT 0 3 test_pub=# INSERT INTO t3 VALUES (1, 'i'), (2, 'ii'), (3, 'iii'); INSERT 0 3 Create publications for the tables. The publications pub2 and pub3a disallow some publish operations. The publication pub3b has a row filter (see ). test_pub=# CREATE PUBLICATION pub1 FOR TABLE t1; CREATE PUBLICATION test_pub=# CREATE PUBLICATION pub2 FOR TABLE t2 WITH (publish = 'truncate'); CREATE PUBLICATION test_pub=# CREATE PUBLICATION pub3a FOR TABLE t3 WITH (publish = 'truncate'); CREATE PUBLICATION test_pub=# CREATE PUBLICATION pub3b FOR TABLE t3 WHERE (e > 5); CREATE PUBLICATION Create subscriptions for the publications. The subscription sub3 subscribes to both pub3a and pub3b. All subscriptions will copy initial data by default. test_sub=# CREATE SUBSCRIPTION sub1 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub1' test_sub-# PUBLICATION pub1; CREATE SUBSCRIPTION test_sub=# CREATE SUBSCRIPTION sub2 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub2' test_sub-# PUBLICATION pub2; CREATE SUBSCRIPTION test_sub=# CREATE SUBSCRIPTION sub3 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub3' test_sub-# PUBLICATION pub3a, pub3b; CREATE SUBSCRIPTION Observe that initial table data is copied, regardless of the publish operation of the publication. test_sub=# SELECT * FROM t1; a | b ---+------- 1 | one 2 | two 3 | three (3 rows) test_sub=# SELECT * FROM t2; c | d ---+--- 1 | A 2 | B 3 | C (3 rows) Furthermore, because the initial data copy ignores the publish operation, and because publication pub3a has no row filter, it means the copied table t3 contains all rows even when they do not match the row filter of publication pub3b. test_sub=# SELECT * FROM t3; e | f ---+----- 1 | i 2 | ii 3 | iii (3 rows) Insert more data to the tables at the publisher side. test_pub=# INSERT INTO t1 VALUES (4, 'four'), (5, 'five'), (6, 'six'); INSERT 0 3 test_pub=# INSERT INTO t2 VALUES (4, 'D'), (5, 'E'), (6, 'F'); INSERT 0 3 test_pub=# INSERT INTO t3 VALUES (4, 'iv'), (5, 'v'), (6, 'vi'); INSERT 0 3 Now the publisher side data looks like: test_pub=# SELECT * FROM t1; a | b ---+------- 1 | one 2 | two 3 | three 4 | four 5 | five 6 | six (6 rows) test_pub=# SELECT * FROM t2; c | d ---+--- 1 | A 2 | B 3 | C 4 | D 5 | E 6 | F (6 rows) test_pub=# SELECT * FROM t3; e | f ---+----- 1 | i 2 | ii 3 | iii 4 | iv 5 | v 6 | vi (6 rows) Observe that during normal replication the appropriate publish operations are used. This means publications pub2 and pub3a will not replicate the INSERT. Also, publication pub3b will only replicate data that matches the row filter of pub3b. Now the subscriber side data looks like: test_sub=# SELECT * FROM t1; a | b ---+------- 1 | one 2 | two 3 | three 4 | four 5 | five 6 | six (6 rows) test_sub=# SELECT * FROM t2; c | d ---+--- 1 | A 2 | B 3 | C (3 rows) test_sub=# SELECT * FROM t3; e | f ---+----- 1 | i 2 | ii 3 | iii 6 | vi (4 rows) By default, all data from all published tables will be replicated to the appropriate subscribers. The replicated data can be reduced by using a row filter. A user might choose to use row filters for behavioral, security or performance reasons. If a published table sets a row filter, a row is replicated only if its data satisfies the row filter expression. This allows a set of tables to be partially replicated. The row filter is defined per table. Use a WHERE clause after the table name for each published table that requires data to be filtered out. The WHERE clause must be enclosed by parentheses. See for details. Row filters are applied before publishing the changes. If the row filter evaluates to false or NULL then the row is not replicated. The WHERE clause expression is evaluated with the same role used for the replication connection (i.e. the role specified in the CONNECTION clause of the ). Row filters have no effect for TRUNCATE command. The WHERE clause allows only simple expressions. It cannot contain user-defined functions, operators, types, and collations, system column references or non-immutable built-in functions. If a publication publishes UPDATE or DELETE operations, the row filter WHERE clause must contain only columns that are covered by the replica identity (see ). If a publication publishes only INSERT operations, the row filter WHERE clause can use any column. Whenever an UPDATE is processed, the row filter expression is evaluated for both the old and new row (i.e. using the data before and after the update). If both evaluations are true, it replicates the UPDATE change. If both evaluations are false, it doesn't replicate the change. If only one of the old/new rows matches the row filter expression, the UPDATE is transformed to INSERT or DELETE, to avoid any data inconsistency. The row on the subscriber should reflect what is defined by the row filter expression on the publisher. If the old row satisfies the row filter expression (it was sent to the subscriber) but the new row doesn't, then, from a data consistency perspective the old row should be removed from the subscriber. So the UPDATE is transformed into a DELETE. If the old row doesn't satisfy the row filter expression (it wasn't sent to the subscriber) but the new row does, then, from a data consistency perspective the new row should be added to the subscriber. So the UPDATE is transformed into an INSERT. summarizes the applied transformations. Old rowNew rowTransformation no matchno matchdon't replicate no matchmatchINSERT matchno matchDELETE matchmatchUPDATE If the publication contains a partitioned table, the publication parameter publish_via_partition_root determines which row filter is used. If publish_via_partition_root is true, the root partitioned table's row filter is used. Otherwise, if publish_via_partition_root is false (default), each partition's row filter is used. If the subscription requires copying pre-existing table data and a publication contains WHERE clauses, only data that satisfies the row filter expressions is copied to the subscriber. If the subscription has several publications in which a table has been published with different WHERE clauses, rows that satisfy any of the expressions will be copied. See for details. Because initial data synchronization does not take into account the publish parameter when copying existing table data, some rows may be copied that would not be replicated using DML. Refer to , and see for examples. If the subscriber is in a release prior to 15, copy pre-existing data doesn't use row filters even if they are defined in the publication. This is because old releases can only copy the entire table data. If the subscription has several publications in which the same table has been published with different row filters (for the same publish operation), those expressions get ORed together, so that rows satisfying any of the expressions will be replicated. This means all the other row filters for the same table become redundant if: One of the publications has no row filter. One of the publications was created using FOR ALL TABLES. This clause does not allow row filters. One of the publications was created using FOR TABLES IN SCHEMA and the table belongs to the referred schema. This clause does not allow row filters. Create some tables to be used in the following examples. test_pub=# CREATE TABLE t1(a int, b int, c text, PRIMARY KEY(a,c)); CREATE TABLE test_pub=# CREATE TABLE t2(d int, e int, f int, PRIMARY KEY(d)); CREATE TABLE test_pub=# CREATE TABLE t3(g int, h int, i int, PRIMARY KEY(g)); CREATE TABLE Create some publications. Publication p1 has one table (t1) and that table has a row filter. Publication p2 has two tables. Table t1 has no row filter, and table t2 has a row filter. Publication p3 has two tables, and both of them have a row filter. test_pub=# CREATE PUBLICATION p1 FOR TABLE t1 WHERE (a > 5 AND c = 'NSW'); CREATE PUBLICATION test_pub=# CREATE PUBLICATION p2 FOR TABLE t1, t2 WHERE (e = 99); CREATE PUBLICATION test_pub=# CREATE PUBLICATION p3 FOR TABLE t2 WHERE (d = 10), t3 WHERE (g = 10); CREATE PUBLICATION psql can be used to show the row filter expressions (if defined) for each publication. test_pub=# \dRp+ Publication p1 Owner | All tables | Inserts | Updates | Deletes | Truncates | Via root ----------+------------+---------+---------+---------+-----------+---------- postgres | f | t | t | t | t | f Tables: "public.t1" WHERE ((a > 5) AND (c = 'NSW'::text)) Publication p2 Owner | All tables | Inserts | Updates | Deletes | Truncates | Via root ----------+------------+---------+---------+---------+-----------+---------- postgres | f | t | t | t | t | f Tables: "public.t1" "public.t2" WHERE (e = 99) Publication p3 Owner | All tables | Inserts | Updates | Deletes | Truncates | Via root ----------+------------+---------+---------+---------+-----------+---------- postgres | f | t | t | t | t | f Tables: "public.t2" WHERE (d = 10) "public.t3" WHERE (g = 10) psql can be used to show the row filter expressions (if defined) for each table. See that table t1 is a member of two publications, but has a row filter only in p1. See that table t2 is a member of two publications, and has a different row filter in each of them. test_pub=# \d t1 Table "public.t1" Column | Type | Collation | Nullable | Default --------+---------+-----------+----------+--------- a | integer | | not null | b | integer | | | c | text | | not null | Indexes: "t1_pkey" PRIMARY KEY, btree (a, c) Publications: "p1" WHERE ((a > 5) AND (c = 'NSW'::text)) "p2" test_pub=# \d t2 Table "public.t2" Column | Type | Collation | Nullable | Default --------+---------+-----------+----------+--------- d | integer | | not null | e | integer | | | f | integer | | | Indexes: "t2_pkey" PRIMARY KEY, btree (d) Publications: "p2" WHERE (e = 99) "p3" WHERE (d = 10) test_pub=# \d t3 Table "public.t3" Column | Type | Collation | Nullable | Default --------+---------+-----------+----------+--------- g | integer | | not null | h | integer | | | i | integer | | | Indexes: "t3_pkey" PRIMARY KEY, btree (g) Publications: "p3" WHERE (g = 10) On the subscriber node, create a table t1 with the same definition as the one on the publisher, and also create the subscription s1 that subscribes to the publication p1. test_sub=# CREATE TABLE t1(a int, b int, c text, PRIMARY KEY(a,c)); CREATE TABLE test_sub=# CREATE SUBSCRIPTION s1 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=s1' test_sub-# PUBLICATION p1; CREATE SUBSCRIPTION Insert some rows. Only the rows satisfying the t1 WHERE clause of publication p1 are replicated. test_pub=# INSERT INTO t1 VALUES (2, 102, 'NSW'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (3, 103, 'QLD'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (4, 104, 'VIC'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (5, 105, 'ACT'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (6, 106, 'NSW'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (7, 107, 'NT'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (8, 108, 'QLD'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES (9, 109, 'NSW'); INSERT 0 1 test_pub=# SELECT * FROM t1; a | b | c ---+-----+----- 2 | 102 | NSW 3 | 103 | QLD 4 | 104 | VIC 5 | 105 | ACT 6 | 106 | NSW 7 | 107 | NT 8 | 108 | QLD 9 | 109 | NSW (8 rows) test_sub=# SELECT * FROM t1; a | b | c ---+-----+----- 6 | 106 | NSW 9 | 109 | NSW (2 rows) Update some data, where the old and new row values both satisfy the t1 WHERE clause of publication p1. The UPDATE replicates the change as normal. test_pub=# UPDATE t1 SET b = 999 WHERE a = 6; UPDATE 1 test_pub=# SELECT * FROM t1; a | b | c ---+-----+----- 2 | 102 | NSW 3 | 103 | QLD 4 | 104 | VIC 5 | 105 | ACT 7 | 107 | NT 8 | 108 | QLD 9 | 109 | NSW 6 | 999 | NSW (8 rows) test_sub=# SELECT * FROM t1; a | b | c ---+-----+----- 9 | 109 | NSW 6 | 999 | NSW (2 rows) Update some data, where the old row values did not satisfy the t1 WHERE clause of publication p1, but the new row values do satisfy it. The UPDATE is transformed into an INSERT and the change is replicated. See the new row on the subscriber. test_pub=# UPDATE t1 SET a = 555 WHERE a = 2; UPDATE 1 test_pub=# SELECT * FROM t1; a | b | c -----+-----+----- 3 | 103 | QLD 4 | 104 | VIC 5 | 105 | ACT 7 | 107 | NT 8 | 108 | QLD 9 | 109 | NSW 6 | 999 | NSW 555 | 102 | NSW (8 rows) test_sub=# SELECT * FROM t1; a | b | c -----+-----+----- 9 | 109 | NSW 6 | 999 | NSW 555 | 102 | NSW (3 rows) Update some data, where the old row values satisfied the t1 WHERE clause of publication p1, but the new row values do not satisfy it. The UPDATE is transformed into a DELETE and the change is replicated. See that the row is removed from the subscriber. test_pub=# UPDATE t1 SET c = 'VIC' WHERE a = 9; UPDATE 1 test_pub=# SELECT * FROM t1; a | b | c -----+-----+----- 3 | 103 | QLD 4 | 104 | VIC 5 | 105 | ACT 7 | 107 | NT 8 | 108 | QLD 6 | 999 | NSW 555 | 102 | NSW 9 | 109 | VIC (8 rows) test_sub=# SELECT * FROM t1; a | b | c -----+-----+----- 6 | 999 | NSW 555 | 102 | NSW (2 rows) The following examples show how the publication parameter publish_via_partition_root determines whether the row filter of the parent or child table will be used in the case of partitioned tables. Create a partitioned table on the publisher. test_pub=# CREATE TABLE parent(a int PRIMARY KEY) PARTITION BY RANGE(a); CREATE TABLE test_pub=# CREATE TABLE child PARTITION OF parent DEFAULT; CREATE TABLE Create the same tables on the subscriber. test_sub=# CREATE TABLE parent(a int PRIMARY KEY) PARTITION BY RANGE(a); CREATE TABLE test_sub=# CREATE TABLE child PARTITION OF parent DEFAULT; CREATE TABLE Create a publication p4, and then subscribe to it. The publication parameter publish_via_partition_root is set as true. There are row filters defined on both the partitioned table (parent), and on the partition (child). test_pub=# CREATE PUBLICATION p4 FOR TABLE parent WHERE (a < 5), child WHERE (a >= 5) test_pub-# WITH (publish_via_partition_root=true); CREATE PUBLICATION test_sub=# CREATE SUBSCRIPTION s4 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=s4' test_sub-# PUBLICATION p4; CREATE SUBSCRIPTION Insert some values directly into the parent and child tables. They replicate using the row filter of parent (because publish_via_partition_root is true). test_pub=# INSERT INTO parent VALUES (2), (4), (6); INSERT 0 3 test_pub=# INSERT INTO child VALUES (3), (5), (7); INSERT 0 3 test_pub=# SELECT * FROM parent ORDER BY a; a --- 2 3 4 5 6 7 (6 rows) test_sub=# SELECT * FROM parent ORDER BY a; a --- 2 3 4 (3 rows) Repeat the same test, but with a different value for publish_via_partition_root. The publication parameter publish_via_partition_root is set as false. A row filter is defined on the partition (child). test_pub=# DROP PUBLICATION p4; DROP PUBLICATION test_pub=# CREATE PUBLICATION p4 FOR TABLE parent, child WHERE (a >= 5) test_pub-# WITH (publish_via_partition_root=false); CREATE PUBLICATION test_sub=# ALTER SUBSCRIPTION s4 REFRESH PUBLICATION; ALTER SUBSCRIPTION Do the inserts on the publisher same as before. They replicate using the row filter of child (because publish_via_partition_root is false). test_pub=# TRUNCATE parent; TRUNCATE TABLE test_pub=# INSERT INTO parent VALUES (2), (4), (6); INSERT 0 3 test_pub=# INSERT INTO child VALUES (3), (5), (7); INSERT 0 3 test_pub=# SELECT * FROM parent ORDER BY a; a --- 2 3 4 5 6 7 (6 rows) test_sub=# SELECT * FROM child ORDER BY a; a --- 5 6 7 (3 rows) Each publication can optionally specify which columns of each table are replicated to subscribers. The table on the subscriber side must have at least all the columns that are published. If no column list is specified, then all columns on the publisher are replicated. See for details on the syntax. The choice of columns can be based on behavioral or performance reasons. However, do not rely on this feature for security: a malicious subscriber is able to obtain data from columns that are not specifically published. If security is a consideration, protections can be applied at the publisher side. If no column list is specified, any columns added later are automatically replicated. This means that having a column list which names all columns is not the same as having no column list at all. A column list can contain only simple column references. The order of columns in the list is not preserved. Specifying a column list when the publication also publishes FOR TABLES IN SCHEMA is not supported. For partitioned tables, the publication parameter publish_via_partition_root determines which column list is used. If publish_via_partition_root is true, the root partitioned table's column list is used. Otherwise, if publish_via_partition_root is false (the default), each partition's column list is used. If a publication publishes UPDATE or DELETE operations, any column list must include the table's replica identity columns (see ). If a publication publishes only INSERT operations, then the column list may omit replica identity columns. Column lists have no effect for the TRUNCATE command. During initial data synchronization, only the published columns are copied. However, if the subscriber is from a release prior to 15, then all the columns in the table are copied during initial data synchronization, ignoring any column lists. There's currently no support for subscriptions comprising several publications where the same table has been published with different column lists. disallows creating such subscriptions, but it is still possible to get into that situation by adding or altering column lists on the publication side after a subscription has been created. This means changing the column lists of tables on publications that are already subscribed could lead to errors being thrown on the subscriber side. If a subscription is affected by this problem, the only way to resume replication is to adjust one of the column lists on the publication side so that they all match; and then either recreate the subscription, or use ALTER SUBSCRIPTION ... DROP PUBLICATION to remove one of the offending publications and add it again. Create a table t1 to be used in the following example. test_pub=# CREATE TABLE t1(id int, a text, b text, c text, d text, e text, PRIMARY KEY(id)); CREATE TABLE Create a publication p1. A column list is defined for table t1 to reduce the number of columns that will be replicated. Notice that the order of column names in the column list does not matter. test_pub=# CREATE PUBLICATION p1 FOR TABLE t1 (id, b, a, d); CREATE PUBLICATION psql can be used to show the column lists (if defined) for each publication. test_pub=# \dRp+ Publication p1 Owner | All tables | Inserts | Updates | Deletes | Truncates | Via root ----------+------------+---------+---------+---------+-----------+---------- postgres | f | t | t | t | t | f Tables: "public.t1" (id, a, b, d) psql can be used to show the column lists (if defined) for each table. test_pub=# \d t1 Table "public.t1" Column | Type | Collation | Nullable | Default --------+---------+-----------+----------+--------- id | integer | | not null | a | text | | | b | text | | | c | text | | | d | text | | | e | text | | | Indexes: "t1_pkey" PRIMARY KEY, btree (id) Publications: "p1" (id, a, b, d) On the subscriber node, create a table t1 which now only needs a subset of the columns that were on the publisher table t1, and also create the subscription s1 that subscribes to the publication p1. test_sub=# CREATE TABLE t1(id int, b text, a text, d text, PRIMARY KEY(id)); CREATE TABLE test_sub=# CREATE SUBSCRIPTION s1 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=s1' test_sub-# PUBLICATION p1; CREATE SUBSCRIPTION On the publisher node, insert some rows to table t1. test_pub=# INSERT INTO t1 VALUES(1, 'a-1', 'b-1', 'c-1', 'd-1', 'e-1'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES(2, 'a-2', 'b-2', 'c-2', 'd-2', 'e-2'); INSERT 0 1 test_pub=# INSERT INTO t1 VALUES(3, 'a-3', 'b-3', 'c-3', 'd-3', 'e-3'); INSERT 0 1 test_pub=# SELECT * FROM t1 ORDER BY id; id | a | b | c | d | e ----+-----+-----+-----+-----+----- 1 | a-1 | b-1 | c-1 | d-1 | e-1 2 | a-2 | b-2 | c-2 | d-2 | e-2 3 | a-3 | b-3 | c-3 | d-3 | e-3 (3 rows) Only data from the column list of publication p1 is replicated. test_sub=# SELECT * FROM t1 ORDER BY id; id | b | a | d ----+-----+-----+----- 1 | b-1 | a-1 | d-1 2 | b-2 | a-2 | d-2 3 | b-3 | a-3 | d-3 (3 rows) Logical replication behaves similarly to normal DML operations in that the data will be updated even if it was changed locally on the subscriber node. If incoming data violates any constraints the replication will stop. This is referred to as a conflict. When replicating UPDATE or DELETE operations, missing data will not produce a conflict and such operations will simply be skipped. Logical replication operations are performed with the privileges of the role which owns the subscription. Permissions failures on target tables will cause replication conflicts, as will enabled row-level security on target tables that the subscription owner is subject to, without regard to whether any policy would ordinarily reject the INSERT, UPDATE, DELETE or TRUNCATE which is being replicated. This restriction on row-level security may be lifted in a future version of PostgreSQL. A conflict will produce an error and will stop the replication; it must be resolved manually by the user. Details about the conflict can be found in the subscriber's server log. The resolution can be done either by changing data or permissions on the subscriber so that it does not conflict with the incoming change or by skipping the transaction that conflicts with the existing data. When a conflict produces an error, the replication won't proceed, and the logical replication worker will emit the following kind of message to the subscriber's server log: ERROR: duplicate key value violates unique constraint "test_pkey" DETAIL: Key (c)=(1) already exists. CONTEXT: processing remote data for replication origin "pg_16395" during "INSERT" for replication target relation "public.test" in transaction 725 finished at 0/14C0378 The LSN of the transaction that contains the change violating the constraint and the replication origin name can be found from the server log (LSN 0/14C0378 and replication origin pg_16395 in the above case). The transaction that produced the conflict can be skipped by using ALTER SUBSCRIPTION ... SKIP with the finish LSN (i.e., LSN 0/14C0378). The finish LSN could be an LSN at which the transaction is committed or prepared on the publisher. Alternatively, the transaction can also be skipped by calling the pg_replication_origin_advance() function. Before using this function, the subscription needs to be disabled temporarily either by ALTER SUBSCRIPTION ... DISABLE or, the subscription can be used with the disable_on_error option. Then, you can use pg_replication_origin_advance() function with the node_name (i.e., pg_16395) and the next LSN of the finish LSN (i.e., 0/14C0379). The current position of origins can be seen in the pg_replication_origin_status system view. Please note that skipping the whole transaction includes skipping changes that might not violate any constraint. This can easily make the subscriber inconsistent. Logical replication currently has the following restrictions or missing functionality. These might be addressed in future releases. The database schema and DDL commands are not replicated. The initial schema can be copied by hand using pg_dump --schema-only. Subsequent schema changes would need to be kept in sync manually. (Note, however, that there is no need for the schemas to be absolutely the same on both sides.) Logical replication is robust when schema definitions change in a live database: When the schema is changed on the publisher and replicated data starts arriving at the subscriber but does not fit into the table schema, replication will error until the schema is updated. In many cases, intermittent errors can be avoided by applying additive schema changes to the subscriber first. Sequence data is not replicated. The data in serial or identity columns backed by sequences will of course be replicated as part of the table, but the sequence itself would still show the start value on the subscriber. If the subscriber is used as a read-only database, then this should typically not be a problem. If, however, some kind of switchover or failover to the subscriber database is intended, then the sequences would need to be updated to the latest values, either by copying the current data from the publisher (perhaps using pg_dump) or by determining a sufficiently high value from the tables themselves. Replication of TRUNCATE commands is supported, but some care must be taken when truncating groups of tables connected by foreign keys. When replicating a truncate action, the subscriber will truncate the same group of tables that was truncated on the publisher, either explicitly specified or implicitly collected via CASCADE, minus tables that are not part of the subscription. This will work correctly if all affected tables are part of the same subscription. But if some tables to be truncated on the subscriber have foreign-key links to tables that are not part of the same (or any) subscription, then the application of the truncate action on the subscriber will fail. Large objects (see ) are not replicated. There is no workaround for that, other than storing data in normal tables. Replication is only supported by tables, including partitioned tables. Attempts to replicate other types of relations, such as views, materialized views, or foreign tables, will result in an error. When replicating between partitioned tables, the actual replication originates, by default, from the leaf partitions on the publisher, so partitions on the publisher must also exist on the subscriber as valid target tables. (They could either be leaf partitions themselves, or they could be further subpartitioned, or they could even be independent tables.) Publications can also specify that changes are to be replicated using the identity and schema of the partitioned root table instead of that of the individual leaf partitions in which the changes actually originate (see CREATE PUBLICATION). Logical replication starts by copying a snapshot of the data on the publisher database. Once that is done, changes on the publisher are sent to the subscriber as they occur in real time. The subscriber applies data in the order in which commits were made on the publisher so that transactional consistency is guaranteed for the publications within any single subscription. Logical replication is built with an architecture similar to physical streaming replication (see ). It is implemented by walsender and apply processes. The walsender process starts logical decoding (described in ) of the WAL and loads the standard logical decoding output plugin (pgoutput). The plugin transforms the changes read from WAL to the logical replication protocol (see ) and filters the data according to the publication specification. The data is then continuously transferred using the streaming replication protocol to the apply worker, which maps the data to local tables and applies the individual changes as they are received, in correct transactional order. The apply process on the subscriber database always runs with session_replication_role set to replica. This means that, by default, triggers and rules will not fire on a subscriber. Users can optionally choose to enable triggers and rules on a table using the ALTER TABLE command and the ENABLE TRIGGER and ENABLE RULE clauses. The logical replication apply process currently only fires row triggers, not statement triggers. The initial table synchronization, however, is implemented like a COPY command and thus fires both row and statement triggers for INSERT. The initial data in existing subscribed tables are snapshotted and copied in a parallel instance of a special kind of apply process. This process will create its own replication slot and copy the existing data. As soon as the copy is finished the table contents will become visible to other backends. Once existing data is copied, the worker enters synchronization mode, which ensures that the table is brought up to a synchronized state with the main apply process by streaming any changes that happened during the initial data copy using standard logical replication. During this synchronization phase, the changes are applied and committed in the same order as they happened on the publisher. Once synchronization is done, control of the replication of the table is given back to the main apply process where replication continues as normal. The publication publish parameter only affects what DML operations will be replicated. The initial data synchronization does not take this parameter into account when copying the existing table data. Because logical replication is based on a similar architecture as physical streaming replication, the monitoring on a publication node is similar to monitoring of a physical replication primary (see ). The monitoring information about subscription is visible in pg_stat_subscription. This view contains one row for every subscription worker. A subscription can have zero or more active subscription workers depending on its state. Normally, there is a single apply process running for an enabled subscription. A disabled subscription or a crashed subscription will have zero rows in this view. If the initial data synchronization of any table is in progress, there will be additional workers for the tables being synchronized. A user able to modify the schema of subscriber-side tables can execute arbitrary code as the role which owns any subscription which modifies those tables. Limit ownership and TRIGGER privilege on such tables to trusted roles. Moreover, if untrusted users can create tables, use only publications that list tables explicitly. That is to say, create a subscription FOR ALL TABLES or FOR TABLES IN SCHEMA only when superusers trust every user permitted to create a non-temp table on the publisher or the subscriber. The role used for the replication connection must have the REPLICATION attribute (or be a superuser). If the role lacks SUPERUSER and BYPASSRLS, publisher row security policies can execute. If the role does not trust all table owners, include options=-crow_security=off in the connection string; if a table owner then adds a row security policy, that setting will cause replication to halt rather than execute the policy. Access for the role must be configured in pg_hba.conf and it must have the LOGIN attribute. In order to be able to copy the initial table data, the role used for the replication connection must have the SELECT privilege on a published table (or be a superuser). To create a publication, the user must have the CREATE privilege in the database. To add tables to a publication, the user must have ownership rights on the table. To add all tables in schema to a publication, the user must be a superuser. To create a publication that publishes all tables or all tables in schema automatically, the user must be a superuser. To create a subscription, the user must be a superuser. The subscription apply process will run in the local database with the privileges of the subscription owner. On the publisher, privileges are only checked once at the start of a replication connection and are not re-checked as each change record is read. On the subscriber, the subscription owner's privileges are re-checked for each transaction when applied. If a worker is in the process of applying a transaction when the ownership of the subscription is changed by a concurrent transaction, the application of the current transaction will continue under the old owner's privileges. Logical replication requires several configuration options to be set. On the publisher side, wal_level must be set to logical, and max_replication_slots must be set to at least the number of subscriptions expected to connect, plus some reserve for table synchronization. And max_wal_senders should be set to at least the same as max_replication_slots plus the number of physical replicas that are connected at the same time. max_replication_slots must also be set on the subscriber. It should be set to at least the number of subscriptions that will be added to the subscriber, plus some reserve for table synchronization. max_logical_replication_workers must be set to at least the number of subscriptions, again plus some reserve for the table synchronization. Additionally the max_worker_processes may need to be adjusted to accommodate for replication workers, at least (max_logical_replication_workers + 1). Note that some extensions and parallel queries also take worker slots from max_worker_processes. First set the configuration options in postgresql.conf: wal_level = logical The other required settings have default values that are sufficient for a basic setup. pg_hba.conf needs to be adjusted to allow replication (the values here depend on your actual network configuration and user you want to use for connecting): host all repuser 0.0.0.0/0 md5 Then on the publisher database: CREATE PUBLICATION mypub FOR TABLE users, departments; And on the subscriber database: CREATE SUBSCRIPTION mysub CONNECTION 'dbname=foo host=bar user=repuser' PUBLICATION mypub; The above will start the replication process, which synchronizes the initial table contents of the tables users and departments and then starts replicating incremental changes to those tables.