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‎202403/20240309_01.md

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## PostgreSQL 17 preview - Add support for `DEFAULT` in `ALTER TABLE .. SET ACCESS METHOD`
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### 作者
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digoal
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### 日期
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2024-03-09
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### 标签
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PostgreSQL , PolarDB , DuckDB , table access method , default_table_access_method
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----
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## 背景
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https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=d61a6cad6418f643a5773352038d0dfe5d3535b8
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```
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Add support for DEFAULT in ALTER TABLE .. SET ACCESS METHOD
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author Michael Paquier <michael@paquier.xyz>
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Fri, 8 Mar 2024 00:31:52 +0000 (09:31 +0900)
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committer Michael Paquier <michael@paquier.xyz>
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Fri, 8 Mar 2024 00:31:52 +0000 (09:31 +0900)
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commit d61a6cad6418f643a5773352038d0dfe5d3535b8
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tree 10791cac8cc32610b5784768d966b2eaa0109f67 tree
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parent 4f8c1e7aaf11c42fa658eeab9baef0a035e76fe2 commit | diff
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Add support for DEFAULT in ALTER TABLE .. SET ACCESS METHOD
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This option can be used to switch a relation to use the access method
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set by default_table_access_method when running the command.
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This has come up when discussing the possibility to support setting
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pg_class.relam for partitioned tables (left out here as future work),
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while being useful on its own for relations with physical storage as
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these must have an access method set.
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Per suggestion from Justin Pryzby.
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Author: Michael Paquier
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Reviewed-by: Justin Pryzby
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Discussion: https://postgr.es/m/ZeCZ89xAVFeOmrQC@pryzbyj2023
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```
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例子
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```
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+-- DEFAULT access method
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+BEGIN;
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+SET LOCAL default_table_access_method TO heap2;
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+ALTER TABLE heaptable SET ACCESS METHOD DEFAULT;
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+SELECT amname FROM pg_class c, pg_am am
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+ WHERE c.relam = am.oid AND c.oid = 'heaptable'::regclass;
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+ amname
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+--------
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+ heap2
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+(1 row)
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+
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+SET LOCAL default_table_access_method TO heap;
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+ALTER TABLE heaptable SET ACCESS METHOD DEFAULT;
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+SELECT amname FROM pg_class c, pg_am am
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+ WHERE c.relam = am.oid AND c.oid = 'heaptable'::regclass;
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+ amname
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+--------
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+ heap
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+(1 row)
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+
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+ROLLBACK;
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```
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#### [期望 PostgreSQL|开源PolarDB 增加什么功能?](https://github.com/digoal/blog/issues/76 "269ac3d1c492e938c0191101c7238216")
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#### [PolarDB 开源数据库](https://openpolardb.com/home "57258f76c37864c6e6d23383d05714ea")
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#### [PolarDB 学习图谱](https://www.aliyun.com/database/openpolardb/activity "8642f60e04ed0c814bf9cb9677976bd4")
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#### [购买PolarDB云服务折扣活动进行中, 55元起](https://www.aliyun.com/activity/new/polardb-yunparter?userCode=bsb3t4al "e0495c413bedacabb75ff1e880be465a")
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#### [PostgreSQL 解决方案集合](../201706/20170601_02.md "40cff096e9ed7122c512b35d8561d9c8")
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#### [德哥 / digoal's Github - 公益是一辈子的事.](https://github.com/digoal/blog/blob/master/README.md "22709685feb7cab07d30f30387f0a9ae")
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#### [About 德哥](https://github.com/digoal/blog/blob/master/me/readme.md "a37735981e7704886ffd590565582dd0")
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![digoal's wechat](../pic/digoal_weixin.jpg "f7ad92eeba24523fd47a6e1a0e691b59")
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‎202403/20240309_02.md

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## PostgreSQL 17 preview - 增加GUC standby_slot_names , 保证这些standby已接收并flush所有逻辑slot向下游发送逻辑数据对应的WAL
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### 作者
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digoal
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### 日期
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2024-03-09
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### 标签
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PostgreSQL , PolarDB , DuckDB , 逻辑复制 , 切换 , 堵塞wal sender
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----
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## 背景
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PostgreSQL 自从16版本开始支持standby开启逻辑复制, 17版本对这个功能进行了进一步的更新, 例如primary节点支持将逻辑复制slot的LSN状态同步给standby, 以便在数据库出现primary-standby HA 切换后新的primary下游逻辑subscriber可以不多不少的继续从slot订阅数据.
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[《PostgreSQL 17 preview - Add a new slot sync worker to synchronize logical slots》](../202402/20240223_01.md)
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[《PostgreSQL 17 preview - sync logical replication slot LSN, Failover & Switchover》](../202402/20240214_02.md)
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[《PostgreSQL 17 preview - 支持逻辑复制槽failover to 流复制standby节点. `pg_create_logical_replication_slot(... failover = true|false ...)`》](../202401/20240126_01.md)
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[《PostgreSQL 16 preview - standby 支持 logical decoding》](../202304/20230410_07.md)
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本文提到的patch Introduce a new GUC 'standby_slot_names' 就是要让数据库主从正常切换可以保证切换后新的primary下游逻辑subscriber可以不多不少的继续从slot订阅数据.
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原理:
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- wal sender在向下游发送逻辑日志信息前, 必须保证在standby_slot_names中的standby已经接收并flush了逻辑日志对应的WAL.
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- 所以不要随意配置standby_slot_names, 最好是性能好、网络好、稳定的standby配置在里面, 否则逻辑复制容易出现堵塞或较大delay.
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https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=bf279ddd1c28ce0251446ee90043a4cb96e5db0f
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```
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Introduce a new GUC 'standby_slot_names'.
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author Amit Kapila <akapila@postgresql.org>
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Fri, 8 Mar 2024 02:40:45 +0000 (08:10 +0530)
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committer Amit Kapila <akapila@postgresql.org>
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Fri, 8 Mar 2024 02:40:45 +0000 (08:10 +0530)
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commit bf279ddd1c28ce0251446ee90043a4cb96e5db0f
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tree 642eb01435995795793fc61cedcc66949c824f1d tree
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parent 453c46873774219243501b8efc16b2b5a5e9d194 commit | diff
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Introduce a new GUC 'standby_slot_names'.
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This patch provides a way to ensure that physical standbys that are
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potential failover candidates have received and flushed changes before
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the primary server making them visible to subscribers. Doing so guarantees
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that the promoted standby server is not lagging behind the subscribers
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when a failover is necessary.
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The logical walsender now guarantees that all local changes are sent and
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flushed to the standby servers corresponding to the replication slots
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specified in 'standby_slot_names' before sending those changes to the
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subscriber.
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Additionally, the SQL functions pg_logical_slot_get_changes,
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pg_logical_slot_peek_changes and pg_replication_slot_advance are modified
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to ensure that they process changes for failover slots only after physical
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slots specified in 'standby_slot_names' have confirmed WAL receipt for those.
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Author: Hou Zhijie and Shveta Malik
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Reviewed-by: Masahiko Sawada, Peter Smith, Bertrand Drouvot, Ajin Cherian, Nisha Moond, Amit Kapila
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Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
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```
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```
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+#standby_slot_names = '' # streaming replication standby server slot names that
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+ # logical walsender processes will wait for
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```
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```
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+ <varlistentry id="guc-standby-slot-names" xreflabel="standby_slot_names">
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+ <term><varname>standby_slot_names</varname> (<type>string</type>)
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+ <indexterm>
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+ <primary><varname>standby_slot_names</varname> configuration parameter</primary>
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+ </indexterm>
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+ </term>
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+ <listitem>
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+ <para>
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+ A comma-separated list of streaming replication standby server slot names
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+ that logical WAL sender processes will wait for. Logical WAL sender processes
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+ will send decoded changes to plugins only after the specified replication
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+ slots confirm receiving WAL. This guarantees that logical replication
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+ failover slots do not consume changes until those changes are received
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+ and flushed to corresponding physical standbys. If a
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+ logical replication connection is meant to switch to a physical standby
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+ after the standby is promoted, the physical replication slot for the
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+ standby should be listed here. Note that logical replication will not
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+ proceed if the slots specified in the
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+ <varname>standby_slot_names</varname> do not exist or are invalidated.
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+ Additionally, the replication management functions
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+ <link linkend="pg-replication-slot-advance">
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+ <function>pg_replication_slot_advance</function></link>,
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+ <link linkend="pg-logical-slot-get-changes">
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+ <function>pg_logical_slot_get_changes</function></link>, and
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+ <link linkend="pg-logical-slot-peek-changes">
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+ <function>pg_logical_slot_peek_changes</function></link>,
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+ when used with logical failover slots, will block until all
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+ physical slots specified in <varname>standby_slot_names</varname> have
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+ confirmed WAL receipt.
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+ </para>
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+ <para>
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+ The standbys corresponding to the physical replication slots in
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+ <varname>standby_slot_names</varname> must configure
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+ <literal>sync_replication_slots = true</literal> so they can receive
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+ logical failover slot changes from the primary.
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+ </para>
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+ </listitem>
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+ </varlistentry>
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```
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```
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+ It's highly recommended that the said physical replication slot is named in
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+ <link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>
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+ list on the primary, to prevent the subscriber from consuming changes
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+ faster than the hot standby. Even when correctly configured, some latency
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+ is expected when sending changes to logical subscribers due to the waiting
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+ on slots named in
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+ <link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>.
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+ When <varname>standby_slot_names</varname> is utilized, the
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+ primary server will not completely shut down until the corresponding
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+ standbys, associated with the physical replication slots specified
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+ in <varname>standby_slot_names</varname>, have confirmed
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+ receiving the WAL up to the latest flushed position on the primary server.
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```
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#### [期望 PostgreSQL|开源PolarDB 增加什么功能?](https://github.com/digoal/blog/issues/76 "269ac3d1c492e938c0191101c7238216")
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#### [PolarDB 开源数据库](https://openpolardb.com/home "57258f76c37864c6e6d23383d05714ea")
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#### [PolarDB 学习图谱](https://www.aliyun.com/database/openpolardb/activity "8642f60e04ed0c814bf9cb9677976bd4")
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#### [购买PolarDB云服务折扣活动进行中, 55元起](https://www.aliyun.com/activity/new/polardb-yunparter?userCode=bsb3t4al "e0495c413bedacabb75ff1e880be465a")
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#### [PostgreSQL 解决方案集合](../201706/20170601_02.md "40cff096e9ed7122c512b35d8561d9c8")
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#### [德哥 / digoal's Github - 公益是一辈子的事.](https://github.com/digoal/blog/blob/master/README.md "22709685feb7cab07d30f30387f0a9ae")
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#### [About 德哥](https://github.com/digoal/blog/blob/master/me/readme.md "a37735981e7704886ffd590565582dd0")
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![digoal's wechat](../pic/digoal_weixin.jpg "f7ad92eeba24523fd47a6e1a0e691b59")
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‎202403/readme.md

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### 文章列表
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----
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##### 20240309_02.md [《PostgreSQL 17 preview - 增加GUC standby_slot_names , 保证这些standby已接收并flush所有逻辑slot向下游发送逻辑数据对应的WAL》](20240309_02.md)
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##### 20240309_01.md [《PostgreSQL 17 preview - Add support for `DEFAULT` in `ALTER TABLE .. SET ACCESS METHOD`》](20240309_01.md)
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##### 20240308_01.md [《老白说PG有隐藏的安全隐患, 真的吗?》](20240308_01.md)
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##### 20240306_02.md [《和总监们一起聊聊数据库三害》](20240306_02.md)
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##### 20240306_01.md [《PostgreSQL 17 preview - Add `--copy-file-range` option to `pg_upgrade`》](20240306_01.md)

‎README.md

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### 所有文档如下
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----
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##### 202403/20240309_02.md [《PostgreSQL 17 preview - 增加GUC standby_slot_names , 保证这些standby已接收并flush所有逻辑slot向下游发送逻辑数据对应的WAL》](202403/20240309_02.md)
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##### 202403/20240309_01.md [《PostgreSQL 17 preview - Add support for `DEFAULT` in `ALTER TABLE .. SET ACCESS METHOD`》](202403/20240309_01.md)
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##### 202403/20240308_01.md [《老白说PG有隐藏的安全隐患, 真的吗?》](202403/20240308_01.md)
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##### 202403/20240306_02.md [《和总监们一起聊聊数据库三害》](202403/20240306_02.md)
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##### 202403/20240306_01.md [《PostgreSQL 17 preview - Add `--copy-file-range` option to `pg_upgrade`》](202403/20240306_01.md)

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