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Postgres FD Implementation
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<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.131 2009/12/19 01:32:30 sriggs Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.132 2009/12/19 17:49:50 momjian Exp $ -->
<chapter id="backup"> <chapter id="backup">
<title>Backup and Restore</title> <title>Backup and Restore</title>
...@@ -1430,10 +1430,10 @@ archive_command = 'local_backup_script.sh' ...@@ -1430,10 +1430,10 @@ archive_command = 'local_backup_script.sh'
<para> <para>
Operations on hash indexes are not presently WAL-logged, so Operations on hash indexes are not presently WAL-logged, so
replay will not update these indexes. This will mean that any new inserts replay will not update these indexes. This will mean that any new inserts
will be ignored by the index, updated rows will apparently disappear and will be ignored by the index, updated rows will apparently disappear and
deleted rows will still retain pointers. In other words, if you modify a deleted rows will still retain pointers. In other words, if you modify a
table with a hash index on it then you will get incorrect query results table with a hash index on it then you will get incorrect query results
on a standby server. When recovery completes it is recommended that you on a standby server. When recovery completes it is recommended that you
manually <xref linkend="sql-reindex" endterm="sql-reindex-title"> manually <xref linkend="sql-reindex" endterm="sql-reindex-title">
each such index after completing a recovery operation. each such index after completing a recovery operation.
</para> </para>
...@@ -1895,117 +1895,117 @@ if (!triggered) ...@@ -1895,117 +1895,117 @@ if (!triggered)
</indexterm> </indexterm>
<para> <para>
Hot Standby is the term used to describe the ability to connect to Hot Standby is the term used to describe the ability to connect to
the server and run queries while the server is in archive recovery. This the server and run queries while the server is in archive recovery. This
is useful for both log shipping replication and for restoring a backup is useful for both log shipping replication and for restoring a backup
to an exact state with great precision. to an exact state with great precision.
The term Hot Standby also refers to the ability of the server to move The term Hot Standby also refers to the ability of the server to move
from recovery through to normal running while users continue running from recovery through to normal running while users continue running
queries and/or continue their connections. queries and/or continue their connections.
</para> </para>
<para> <para>
Running queries in recovery is in many ways the same as normal running Running queries in recovery is in many ways the same as normal running
though there are a large number of usage and administrative points though there are a large number of usage and administrative points
to note. to note.
</para> </para>
<sect2 id="hot-standby-users"> <sect2 id="hot-standby-users">
<title>User's Overview</title> <title>User's Overview</title>
<para> <para>
Users can connect to the database while the server is in recovery Users can connect to the database while the server is in recovery
and perform read-only queries. Read-only access to catalogs and views and perform read-only queries. Read-only access to catalogs and views
will also occur as normal. will also occur as normal.
</para> </para>
<para> <para>
The data on the standby takes some time to arrive from the primary server The data on the standby takes some time to arrive from the primary server
so there will be a measurable delay between primary and standby. Running the so there will be a measurable delay between primary and standby. Running the
same query nearly simultaneously on both primary and standby might therefore same query nearly simultaneously on both primary and standby might therefore
return differing results. We say that data on the standby is eventually return differing results. We say that data on the standby is eventually
consistent with the primary. consistent with the primary.
Queries executed on the standby will be correct with regard to the transactions Queries executed on the standby will be correct with regard to the transactions
that had been recovered at the start of the query, or start of first statement, that had been recovered at the start of the query, or start of first statement,
in the case of serializable transactions. In comparison with the primary, in the case of serializable transactions. In comparison with the primary,
the standby returns query results that could have been obtained on the primary the standby returns query results that could have been obtained on the primary
at some exact moment in the past. at some exact moment in the past.
</para> </para>
<para> <para>
When a transaction is started in recovery, the parameter When a transaction is started in recovery, the parameter
<varname>transaction_read_only</> will be forced to be true, regardless of the <varname>transaction_read_only</> will be forced to be true, regardless of the
<varname>default_transaction_read_only</> setting in <filename>postgresql.conf</>. <varname>default_transaction_read_only</> setting in <filename>postgresql.conf</>.
It can't be manually set to false either. As a result, all transactions It can't be manually set to false either. As a result, all transactions
started during recovery will be limited to read-only actions only. In all started during recovery will be limited to read-only actions only. In all
other ways, connected sessions will appear identical to sessions other ways, connected sessions will appear identical to sessions
initiated during normal processing mode. There are no special commands initiated during normal processing mode. There are no special commands
required to initiate a connection at this time, so all interfaces required to initiate a connection at this time, so all interfaces
work normally without change. After recovery finishes, the session work normally without change. After recovery finishes, the session
will allow normal read-write transactions at the start of the next will allow normal read-write transactions at the start of the next
transaction, if these are requested. transaction, if these are requested.
</para> </para>
<para> <para>
Read-only here means "no writes to the permanent database tables". Read-only here means "no writes to the permanent database tables".
There are no problems with queries that make use of transient sort and There are no problems with queries that make use of transient sort and
work files. work files.
</para> </para>
<para> <para>
The following actions are allowed The following actions are allowed
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
Query access - SELECT, COPY TO including views and SELECT RULEs Query access - SELECT, COPY TO including views and SELECT RULEs
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Cursor commands - DECLARE, FETCH, CLOSE, Cursor commands - DECLARE, FETCH, CLOSE,
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Parameters - SHOW, SET, RESET Parameters - SHOW, SET, RESET
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Transaction management commands Transaction management commands
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
BEGIN, END, ABORT, START TRANSACTION BEGIN, END, ABORT, START TRANSACTION
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
SAVEPOINT, RELEASE, ROLLBACK TO SAVEPOINT SAVEPOINT, RELEASE, ROLLBACK TO SAVEPOINT
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
EXCEPTION blocks and other internal subtransactions EXCEPTION blocks and other internal subtransactions
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
LOCK TABLE, though only when explicitly in one of these modes: LOCK TABLE, though only when explicitly in one of these modes:
ACCESS SHARE, ROW SHARE or ROW EXCLUSIVE. ACCESS SHARE, ROW SHARE or ROW EXCLUSIVE.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Plans and resources - PREPARE, EXECUTE, DEALLOCATE, DISCARD Plans and resources - PREPARE, EXECUTE, DEALLOCATE, DISCARD
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Plugins and extensions - LOAD Plugins and extensions - LOAD
</para> </para>
</listitem> </listitem>
...@@ -2013,125 +2013,125 @@ if (!triggered) ...@@ -2013,125 +2013,125 @@ if (!triggered)
</para> </para>
<para> <para>
These actions produce error messages These actions produce error messages
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
Data Definition Language (DML) - INSERT, UPDATE, DELETE, COPY FROM, TRUNCATE. Data Definition Language (DML) - INSERT, UPDATE, DELETE, COPY FROM, TRUNCATE.
Note that there are no allowed actions that result in a trigger Note that there are no allowed actions that result in a trigger
being executed during recovery. being executed during recovery.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Data Definition Language (DDL) - CREATE, DROP, ALTER, COMMENT. Data Definition Language (DDL) - CREATE, DROP, ALTER, COMMENT.
This also applies to temporary tables currently because currently their This also applies to temporary tables currently because currently their
definition causes writes to catalog tables. definition causes writes to catalog tables.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
SELECT ... FOR SHARE | UPDATE which cause row locks to be written SELECT ... FOR SHARE | UPDATE which cause row locks to be written
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
RULEs on SELECT statements that generate DML commands. RULEs on SELECT statements that generate DML commands.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
LOCK TABLE, in short default form, since it requests ACCESS EXCLUSIVE MODE. LOCK TABLE, in short default form, since it requests ACCESS EXCLUSIVE MODE.
LOCK TABLE that explicitly requests a mode higher than ROW EXCLUSIVE MODE. LOCK TABLE that explicitly requests a mode higher than ROW EXCLUSIVE MODE.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Transaction management commands that explicitly set non-read only state Transaction management commands that explicitly set non-read only state
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
BEGIN READ WRITE, BEGIN READ WRITE,
START TRANSACTION READ WRITE START TRANSACTION READ WRITE
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
SET TRANSACTION READ WRITE, SET TRANSACTION READ WRITE,
SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
SET transaction_read_only = off SET transaction_read_only = off
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Two-phase commit commands - PREPARE TRANSACTION, COMMIT PREPARED, Two-phase commit commands - PREPARE TRANSACTION, COMMIT PREPARED,
ROLLBACK PREPARED because even read-only transactions need to write ROLLBACK PREPARED because even read-only transactions need to write
WAL in the prepare phase (the first phase of two phase commit). WAL in the prepare phase (the first phase of two phase commit).
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
sequence update - nextval() sequence update - nextval()
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
LISTEN, UNLISTEN, NOTIFY since they currently write to system tables LISTEN, UNLISTEN, NOTIFY since they currently write to system tables
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
<para> <para>
Note that current behaviour of read only transactions when not in Note that current behaviour of read only transactions when not in
recovery is to allow the last two actions, so there are small and recovery is to allow the last two actions, so there are small and
subtle differences in behaviour between read-only transactions subtle differences in behaviour between read-only transactions
run on standby and during normal running. run on standby and during normal running.
It is possible that the restrictions on LISTEN, UNLISTEN, NOTIFY and It is possible that the restrictions on LISTEN, UNLISTEN, NOTIFY and
temporary tables may be lifted in a future release, if their internal temporary tables may be lifted in a future release, if their internal
implementation is altered to make this possible. implementation is altered to make this possible.
</para> </para>
<para> <para>
If failover or switchover occurs the database will switch to normal If failover or switchover occurs the database will switch to normal
processing mode. Sessions will remain connected while the server processing mode. Sessions will remain connected while the server
changes mode. Current transactions will continue, though will remain changes mode. Current transactions will continue, though will remain
read-only. After recovery is complete, it will be possible to initiate read-only. After recovery is complete, it will be possible to initiate
read-write transactions. read-write transactions.
</para> </para>
<para> <para>
Users will be able to tell whether their session is read-only by Users will be able to tell whether their session is read-only by
issuing SHOW transaction_read_only. In addition a set of issuing SHOW transaction_read_only. In addition a set of
functions <xref linkend="functions-recovery-info-table"> allow users to functions <xref linkend="functions-recovery-info-table"> allow users to
access information about Hot Standby. These allow you to write access information about Hot Standby. These allow you to write
functions that are aware of the current state of the database. These functions that are aware of the current state of the database. These
can be used to monitor the progress of recovery, or to allow you to can be used to monitor the progress of recovery, or to allow you to
write complex programs that restore the database to particular states. write complex programs that restore the database to particular states.
</para> </para>
<para> <para>
In recovery, transactions will not be permitted to take any table lock In recovery, transactions will not be permitted to take any table lock
higher than RowExclusiveLock. In addition, transactions may never assign higher than RowExclusiveLock. In addition, transactions may never assign
a TransactionId and may never write WAL. a TransactionId and may never write WAL.
Any <command>LOCK TABLE</> command that runs on the standby and requests Any <command>LOCK TABLE</> command that runs on the standby and requests
a specific lock mode higher than ROW EXCLUSIVE MODE will be rejected. a specific lock mode higher than ROW EXCLUSIVE MODE will be rejected.
</para> </para>
<para> <para>
In general queries will not experience lock conflicts with the database In general queries will not experience lock conflicts with the database
changes made by recovery. This is becase recovery follows normal changes made by recovery. This is becase recovery follows normal
concurrency control mechanisms, known as <acronym>MVCC</>. There are concurrency control mechanisms, known as <acronym>MVCC</>. There are
some types of change that will cause conflicts, covered in the following some types of change that will cause conflicts, covered in the following
section. section.
</para> </para>
</sect2> </sect2>
...@@ -2139,188 +2139,188 @@ if (!triggered) ...@@ -2139,188 +2139,188 @@ if (!triggered)
<title>Handling query conflicts</title> <title>Handling query conflicts</title>
<para> <para>
The primary and standby nodes are in many ways loosely connected. Actions The primary and standby nodes are in many ways loosely connected. Actions
on the primary will have an effect on the standby. As a result, there is on the primary will have an effect on the standby. As a result, there is
potential for negative interactions or conflicts between them. The easiest potential for negative interactions or conflicts between them. The easiest
conflict to understand is performance: if a huge data load is taking place conflict to understand is performance: if a huge data load is taking place
on the primary then this will generate a similar stream of WAL records on the on the primary then this will generate a similar stream of WAL records on the
standby, so standby queries may contend for system resources, such as I/O. standby, so standby queries may contend for system resources, such as I/O.
</para> </para>
<para> <para>
There are also additional types of conflict that can occur with Hot Standby. There are also additional types of conflict that can occur with Hot Standby.
These conflicts are <emphasis>hard conflicts</> in the sense that we may These conflicts are <emphasis>hard conflicts</> in the sense that we may
need to cancel queries and in some cases disconnect sessions to resolve them. need to cancel queries and in some cases disconnect sessions to resolve them.
The user is provided with a number of optional ways to handle these The user is provided with a number of optional ways to handle these
conflicts, though we must first understand the possible reasons behind a conflict. conflicts, though we must first understand the possible reasons behind a conflict.
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
Access Exclusive Locks from primary node, including both explicit Access Exclusive Locks from primary node, including both explicit
LOCK commands and various kinds of DDL action LOCK commands and various kinds of DDL action
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Dropping tablespaces on the primary while standby queries are using Dropping tablespaces on the primary while standby queries are using
those tablespace for temporary work files (work_mem overflow) those tablespace for temporary work files (work_mem overflow)
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Dropping databases on the primary while that role is connected on standby. Dropping databases on the primary while that role is connected on standby.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Waiting to acquire buffer cleanup locks (for which there is no time out) Waiting to acquire buffer cleanup locks (for which there is no time out)
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Early cleanup of data still visible to the current query's snapshot Early cleanup of data still visible to the current query's snapshot
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
<para> <para>
Some WAL redo actions will be for DDL actions. These DDL actions are Some WAL redo actions will be for DDL actions. These DDL actions are
repeating actions that have already committed on the primary node, so repeating actions that have already committed on the primary node, so
they must not fail on the standby node. These DDL locks take priority they must not fail on the standby node. These DDL locks take priority
and will automatically *cancel* any read-only transactions that get in and will automatically *cancel* any read-only transactions that get in
their way, after a grace period. This is similar to the possibility of their way, after a grace period. This is similar to the possibility of
being canceled by the deadlock detector, but in this case the standby being canceled by the deadlock detector, but in this case the standby
process always wins, since the replayed actions must not fail. This process always wins, since the replayed actions must not fail. This
also ensures that replication doesn't fall behind while we wait for a also ensures that replication doesn't fall behind while we wait for a
query to complete. Again, we assume that the standby is there for high query to complete. Again, we assume that the standby is there for high
availability purposes primarily. availability purposes primarily.
</para> </para>
<para> <para>
An example of the above would be an Administrator on Primary server An example of the above would be an Administrator on Primary server
runs a <command>DROP TABLE</> on a table that's currently being queried runs a <command>DROP TABLE</> on a table that's currently being queried
in the standby server. in the standby server.
Clearly the query cannot continue if we let the <command>DROP TABLE</> Clearly the query cannot continue if we let the <command>DROP TABLE</>
proceed. If this situation occurred on the primary, the <command>DROP TABLE</> proceed. If this situation occurred on the primary, the <command>DROP TABLE</>
would wait until the query has finished. When the query is on the standby would wait until the query has finished. When the query is on the standby
and the <command>DROP TABLE</> is on the primary, the primary doesn't have and the <command>DROP TABLE</> is on the primary, the primary doesn't have
information about which queries are running on the standby and so the query information about which queries are running on the standby and so the query
does not wait on the primary. The WAL change records come through to the does not wait on the primary. The WAL change records come through to the
standby while the standby query is still running, causing a conflict. standby while the standby query is still running, causing a conflict.
</para> </para>
<para> <para>
The most common reason for conflict between standby queries and WAL redo is The most common reason for conflict between standby queries and WAL redo is
"early cleanup". Normally, <productname>PostgreSQL</> allows cleanup of old "early cleanup". Normally, <productname>PostgreSQL</> allows cleanup of old
row versions when there are no users who may need to see them to ensure correct row versions when there are no users who may need to see them to ensure correct
visibility of data (the heart of MVCC). If there is a standby query that has visibility of data (the heart of MVCC). If there is a standby query that has
been running for longer than any query on the primary then it is possible been running for longer than any query on the primary then it is possible
for old row versions to be removed by either a vacuum or HOT. This will for old row versions to be removed by either a vacuum or HOT. This will
then generate WAL records that, if applied, would remove data on the then generate WAL records that, if applied, would remove data on the
standby that might *potentially* be required by the standby query. standby that might *potentially* be required by the standby query.
In more technical language, the primary's xmin horizon is later than In more technical language, the primary's xmin horizon is later than
the standby's xmin horizon, allowing dead rows to be removed. the standby's xmin horizon, allowing dead rows to be removed.
</para> </para>
<para> <para>
Experienced users should note that both row version cleanup and row version Experienced users should note that both row version cleanup and row version
freezing will potentially conflict with recovery queries. Running a freezing will potentially conflict with recovery queries. Running a
manual <command>VACUUM FREEZE</> is likely to cause conflicts even on tables manual <command>VACUUM FREEZE</> is likely to cause conflicts even on tables
with no updated or deleted rows. with no updated or deleted rows.
</para> </para>
<para> <para>
We have a number of choices for resolving query conflicts. The default We have a number of choices for resolving query conflicts. The default
is that we wait and hope the query completes. The server will wait is that we wait and hope the query completes. The server will wait
automatically until the lag between primary and standby is at most automatically until the lag between primary and standby is at most
<varname>max_standby_delay</> seconds. Once that grace period expires, <varname>max_standby_delay</> seconds. Once that grace period expires,
we take one of the following actions: we take one of the following actions:
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
If the conflict is caused by a lock, we cancel the conflicting standby If the conflict is caused by a lock, we cancel the conflicting standby
transaction immediately. If the transaction is idle-in-transaction transaction immediately. If the transaction is idle-in-transaction
then currently we abort the session instead, though this may change then currently we abort the session instead, though this may change
in the future. in the future.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
If the conflict is caused by cleanup records we tell the standby query If the conflict is caused by cleanup records we tell the standby query
that a conflict has occurred and that it must cancel itself to avoid the that a conflict has occurred and that it must cancel itself to avoid the
risk that it silently fails to read relevant data because risk that it silently fails to read relevant data because
that data has been removed. (This is regrettably very similar to the that data has been removed. (This is regrettably very similar to the
much feared and iconic error message "snapshot too old"). Some cleanup much feared and iconic error message "snapshot too old"). Some cleanup
records only cause conflict with older queries, though some types of records only cause conflict with older queries, though some types of
cleanup record affect all queries. cleanup record affect all queries.
</para> </para>
<para> <para>
If cancellation does occur, the query and/or transaction can always If cancellation does occur, the query and/or transaction can always
be re-executed. The error is dynamic and will not necessarily occur be re-executed. The error is dynamic and will not necessarily occur
the same way if the query is executed again. the same way if the query is executed again.
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
<para> <para>
<varname>max_standby_delay</> is set in <filename>postgresql.conf</>. <varname>max_standby_delay</> is set in <filename>postgresql.conf</>.
The parameter applies to the server as a whole so if the delay is all used The parameter applies to the server as a whole so if the delay is all used
up by a single query then there may be little or no waiting for queries that up by a single query then there may be little or no waiting for queries that
follow immediately, though they will have benefited equally from the initial follow immediately, though they will have benefited equally from the initial
waiting period. The server may take time to catch up again before the grace waiting period. The server may take time to catch up again before the grace
period is available again, though if there is a heavy and constant stream period is available again, though if there is a heavy and constant stream
of conflicts it may seldom catch up fully. of conflicts it may seldom catch up fully.
</para> </para>
<para> <para>
Users should be clear that tables that are regularly and heavily updated on Users should be clear that tables that are regularly and heavily updated on
primary server will quickly cause cancellation of longer running queries on primary server will quickly cause cancellation of longer running queries on
the standby. In those cases <varname>max_standby_delay</> can be the standby. In those cases <varname>max_standby_delay</> can be
considered somewhat but not exactly the same as setting considered somewhat but not exactly the same as setting
<varname>statement_timeout</>. <varname>statement_timeout</>.
</para> </para>
<para> <para>
Other remedial actions exist if the number of cancellations is unacceptable. Other remedial actions exist if the number of cancellations is unacceptable.
The first option is to connect to primary server and keep a query active The first option is to connect to primary server and keep a query active
for as long as we need to run queries on the standby. This guarantees that for as long as we need to run queries on the standby. This guarantees that
a WAL cleanup record is never generated and we don't ever get query a WAL cleanup record is never generated and we don't ever get query
conflicts as described above. This could be done using contrib/dblink conflicts as described above. This could be done using contrib/dblink
and pg_sleep(), or via other mechanisms. If you do this, you should note and pg_sleep(), or via other mechanisms. If you do this, you should note
that this will delay cleanup of dead rows by vacuum or HOT and many that this will delay cleanup of dead rows by vacuum or HOT and many
people may find this undesirable. However, we should remember that people may find this undesirable. However, we should remember that
primary and standby nodes are linked via the WAL, so this situation is no primary and standby nodes are linked via the WAL, so this situation is no
different to the case where we ran the query on the primary node itself different to the case where we ran the query on the primary node itself
except we have the benefit of off-loading the execution onto the standby. except we have the benefit of off-loading the execution onto the standby.
</para> </para>
<para> <para>
It is also possible to set <varname>vacuum_defer_cleanup_age</> on the primary It is also possible to set <varname>vacuum_defer_cleanup_age</> on the primary
to defer the cleanup of records by autovacuum, vacuum and HOT. This may allow to defer the cleanup of records by autovacuum, vacuum and HOT. This may allow
more time for queries to execute before they are cancelled on the standby, more time for queries to execute before they are cancelled on the standby,
without the need for setting a high <varname>max_standby_delay</>. without the need for setting a high <varname>max_standby_delay</>.
</para> </para>
<para> <para>
Three-way deadlocks are possible between AccessExclusiveLocks arriving from Three-way deadlocks are possible between AccessExclusiveLocks arriving from
the primary, cleanup WAL records that require buffer cleanup locks and the primary, cleanup WAL records that require buffer cleanup locks and
user requests that are waiting behind replayed AccessExclusiveLocks. Deadlocks user requests that are waiting behind replayed AccessExclusiveLocks. Deadlocks
are currently resolved by the cancellation of user processes that would are currently resolved by the cancellation of user processes that would
need to wait on a lock. This is heavy-handed and generates more query need to wait on a lock. This is heavy-handed and generates more query
cancellations than we need to, though does remove the possibility of deadlock. cancellations than we need to, though does remove the possibility of deadlock.
This behaviour is expected to improve substantially for the main release This behaviour is expected to improve substantially for the main release
version of 8.5. version of 8.5.
</para> </para>
<para> <para>
Dropping tablespaces or databases is discussed in the administrator's Dropping tablespaces or databases is discussed in the administrator's
section since they are not typical user situations. section since they are not typical user situations.
</para> </para>
</sect2> </sect2>
...@@ -2328,13 +2328,13 @@ if (!triggered) ...@@ -2328,13 +2328,13 @@ if (!triggered)
<title>Administrator's Overview</title> <title>Administrator's Overview</title>
<para> <para>
If there is a <filename>recovery.conf</> file present the server will start If there is a <filename>recovery.conf</> file present the server will start
in Hot Standby mode by default, though <varname>recovery_connections</> can in Hot Standby mode by default, though <varname>recovery_connections</> can
be disabled via <filename>postgresql.conf</>, if required. The server may take be disabled via <filename>postgresql.conf</>, if required. The server may take
some time to enable recovery connections since the server must first complete some time to enable recovery connections since the server must first complete
sufficient recovery to provide a consistent state against which queries sufficient recovery to provide a consistent state against which queries
can run before enabling read only connections. Look for these messages can run before enabling read only connections. Look for these messages
in the server logs in the server logs
<programlisting> <programlisting>
LOG: initializing recovery connections LOG: initializing recovery connections
...@@ -2345,233 +2345,233 @@ LOG: consistent recovery state reached ...@@ -2345,233 +2345,233 @@ LOG: consistent recovery state reached
LOG: database system is ready to accept read only connections LOG: database system is ready to accept read only connections
</programlisting> </programlisting>
Consistency information is recorded once per checkpoint on the primary, as long Consistency information is recorded once per checkpoint on the primary, as long
as <varname>recovery_connections</> is enabled (on the primary). If this parameter as <varname>recovery_connections</> is enabled (on the primary). If this parameter
is disabled, it will not be possible to enable recovery connections on the standby. is disabled, it will not be possible to enable recovery connections on the standby.
The consistent state can also be delayed in the presence of both of these conditions The consistent state can also be delayed in the presence of both of these conditions
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
a write transaction has more than 64 subtransactions a write transaction has more than 64 subtransactions
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
very long-lived write transactions very long-lived write transactions
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
If you are running file-based log shipping ("warm standby"), you may need If you are running file-based log shipping ("warm standby"), you may need
to wait until the next WAL file arrives, which could be as long as the to wait until the next WAL file arrives, which could be as long as the
<varname>archive_timeout</> setting on the primary. <varname>archive_timeout</> setting on the primary.
</para> </para>
<para> <para>
The setting of some parameters on the standby will need reconfiguration The setting of some parameters on the standby will need reconfiguration
if they have been changed on the primary. The value on the standby must if they have been changed on the primary. The value on the standby must
be equal to or greater than the value on the primary. If these parameters be equal to or greater than the value on the primary. If these parameters
are not set high enough then the standby will not be able to track work are not set high enough then the standby will not be able to track work
correctly from recovering transactions. If these values are set too low the correctly from recovering transactions. If these values are set too low the
the server will halt. Higher values can then be supplied and the server the server will halt. Higher values can then be supplied and the server
restarted to begin recovery again. restarted to begin recovery again.
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
<varname>max_connections</> <varname>max_connections</>
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
<varname>max_prepared_transactions</> <varname>max_prepared_transactions</>
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
<varname>max_locks_per_transaction</> <varname>max_locks_per_transaction</>
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
<para> <para>
It is important that the administrator consider the appropriate setting It is important that the administrator consider the appropriate setting
of <varname>max_standby_delay</>, set in <filename>postgresql.conf</>. of <varname>max_standby_delay</>, set in <filename>postgresql.conf</>.
There is no optimal setting and should be set according to business There is no optimal setting and should be set according to business
priorities. For example if the server is primarily tasked as a High priorities. For example if the server is primarily tasked as a High
Availability server, then you may wish to lower Availability server, then you may wish to lower
<varname>max_standby_delay</> or even set it to zero, though that is a <varname>max_standby_delay</> or even set it to zero, though that is a
very aggressive setting. If the standby server is tasked as an additional very aggressive setting. If the standby server is tasked as an additional
server for decision support queries then it may be acceptable to set this server for decision support queries then it may be acceptable to set this
to a value of many hours (in seconds). It is also possible to set to a value of many hours (in seconds). It is also possible to set
<varname>max_standby_delay</> to -1 which means wait forever for queries <varname>max_standby_delay</> to -1 which means wait forever for queries
to complete, if there are conflicts; this will be useful when performing to complete, if there are conflicts; this will be useful when performing
an archive recovery from a backup. an archive recovery from a backup.
</para> </para>
<para> <para>
Transaction status "hint bits" written on primary are not WAL-logged, Transaction status "hint bits" written on primary are not WAL-logged,
so data on standby will likely re-write the hints again on the standby. so data on standby will likely re-write the hints again on the standby.
Thus the main database blocks will produce write I/Os even though Thus the main database blocks will produce write I/Os even though
all users are read-only; no changes have occurred to the data values all users are read-only; no changes have occurred to the data values
themselves. Users will be able to write large sort temp files and themselves. Users will be able to write large sort temp files and
re-generate relcache info files, so there is no part of the database re-generate relcache info files, so there is no part of the database
that is truly read-only during hot standby mode. There is no restriction that is truly read-only during hot standby mode. There is no restriction
on the use of set returning functions, or other users of tuplestore/tuplesort on the use of set returning functions, or other users of tuplestore/tuplesort
code. Note also that writes to remote databases will still be possible, code. Note also that writes to remote databases will still be possible,
even though the transaction is read-only locally. even though the transaction is read-only locally.
</para> </para>
<para> <para>
The following types of administrator command are not accepted The following types of administrator command are not accepted
during recovery mode during recovery mode
<itemizedlist> <itemizedlist>
<listitem> <listitem>
<para> <para>
Data Definition Language (DDL) - e.g. CREATE INDEX Data Definition Language (DDL) - e.g. CREATE INDEX
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Privilege and Ownership - GRANT, REVOKE, REASSIGN Privilege and Ownership - GRANT, REVOKE, REASSIGN
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Maintenance commands - ANALYZE, VACUUM, CLUSTER, REINDEX Maintenance commands - ANALYZE, VACUUM, CLUSTER, REINDEX
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
</para> </para>
<para> <para>
Note again that some of these commands are actually allowed during Note again that some of these commands are actually allowed during
"read only" mode transactions on the primary. "read only" mode transactions on the primary.
</para> </para>
<para> <para>
As a result, you cannot create additional indexes that exist solely As a result, you cannot create additional indexes that exist solely
on the standby, nor can statistics that exist solely on the standby. on the standby, nor can statistics that exist solely on the standby.
If these administrator commands are needed they should be executed If these administrator commands are needed they should be executed
on the primary so that the changes will propagate through to the on the primary so that the changes will propagate through to the
standby. standby.
</para> </para>
<para> <para>
<function>pg_cancel_backend()</> will work on user backends, but not the <function>pg_cancel_backend()</> will work on user backends, but not the
Startup process, which performs recovery. pg_stat_activity does not Startup process, which performs recovery. pg_stat_activity does not
show an entry for the Startup process, nor do recovering transactions show an entry for the Startup process, nor do recovering transactions
show as active. As a result, pg_prepared_xacts is always empty during show as active. As a result, pg_prepared_xacts is always empty during
recovery. If you wish to resolve in-doubt prepared transactions recovery. If you wish to resolve in-doubt prepared transactions
then look at pg_prepared_xacts on the primary and issue commands to then look at pg_prepared_xacts on the primary and issue commands to
resolve those transactions there. resolve those transactions there.
</para> </para>
<para> <para>
pg_locks will show locks held by backends as normal. pg_locks also shows pg_locks will show locks held by backends as normal. pg_locks also shows
a virtual transaction managed by the Startup process that owns all a virtual transaction managed by the Startup process that owns all
AccessExclusiveLocks held by transactions being replayed by recovery. AccessExclusiveLocks held by transactions being replayed by recovery.
Note that Startup process does not acquire locks to Note that Startup process does not acquire locks to
make database changes and thus locks other than AccessExclusiveLocks make database changes and thus locks other than AccessExclusiveLocks
do not show in pg_locks for the Startup process, they are just presumed do not show in pg_locks for the Startup process, they are just presumed
to exist. to exist.
</para> </para>
<para> <para>
<productname>check_pgsql</> will work, but it is very simple. <productname>check_pgsql</> will work, but it is very simple.
<productname>check_postgres</> will also work, though many some actions <productname>check_postgres</> will also work, though many some actions
could give different or confusing results. could give different or confusing results.
e.g. last vacuum time will not be maintained for example, since no e.g. last vacuum time will not be maintained for example, since no
vacuum occurs on the standby (though vacuums running on the primary do vacuum occurs on the standby (though vacuums running on the primary do
send their changes to the standby). send their changes to the standby).
</para> </para>
<para> <para>
WAL file control commands will not work during recovery WAL file control commands will not work during recovery
e.g. <function>pg_start_backup</>, <function>pg_switch_xlog</> etc.. e.g. <function>pg_start_backup</>, <function>pg_switch_xlog</> etc..
</para> </para>
<para> <para>
Dynamically loadable modules work, including pg_stat_statements. Dynamically loadable modules work, including pg_stat_statements.
</para> </para>
<para> <para>
Advisory locks work normally in recovery, including deadlock detection. Advisory locks work normally in recovery, including deadlock detection.
Note that advisory locks are never WAL logged, so it is not possible for Note that advisory locks are never WAL logged, so it is not possible for
an advisory lock on either the primary or the standby to conflict with WAL an advisory lock on either the primary or the standby to conflict with WAL
replay. Nor is it possible to acquire an advisory lock on the primary replay. Nor is it possible to acquire an advisory lock on the primary
and have it initiate a similar advisory lock on the standby. Advisory and have it initiate a similar advisory lock on the standby. Advisory
locks relate only to a single server on which they are acquired. locks relate only to a single server on which they are acquired.
</para> </para>
<para> <para>
Trigger-based replication systems such as <productname>Slony</>, Trigger-based replication systems such as <productname>Slony</>,
<productname>Londiste</> and <productname>Bucardo</> won't run on the <productname>Londiste</> and <productname>Bucardo</> won't run on the
standby at all, though they will run happily on the primary server as standby at all, though they will run happily on the primary server as
long as the changes are not sent to standby servers to be applied. long as the changes are not sent to standby servers to be applied.
WAL replay is not trigger-based so you cannot relay from the WAL replay is not trigger-based so you cannot relay from the
standby to any system that requires additional database writes or standby to any system that requires additional database writes or
relies on the use of triggers. relies on the use of triggers.
</para> </para>
<para> <para>
New oids cannot be assigned, though some <acronym>UUID</> generators may still New oids cannot be assigned, though some <acronym>UUID</> generators may still
work as long as they do not rely on writing new status to the database. work as long as they do not rely on writing new status to the database.
</para> </para>
<para> <para>
Currently, temp table creation is not allowed during read only Currently, temp table creation is not allowed during read only
transactions, so in some cases existing scripts will not run correctly. transactions, so in some cases existing scripts will not run correctly.
It is possible we may relax that restriction in a later release. This is It is possible we may relax that restriction in a later release. This is
both a SQL Standard compliance issue and a technical issue. both a SQL Standard compliance issue and a technical issue.
</para> </para>
<para> <para>
<command>DROP TABLESPACE</> can only succeed if the tablespace is empty. <command>DROP TABLESPACE</> can only succeed if the tablespace is empty.
Some standby users may be actively using the tablespace via their Some standby users may be actively using the tablespace via their
<varname>temp_tablespaces</> parameter. If there are temp files in the <varname>temp_tablespaces</> parameter. If there are temp files in the
tablespace we currently cancel all active queries to ensure that temp tablespace we currently cancel all active queries to ensure that temp
files are removed, so that we can remove the tablespace and continue with files are removed, so that we can remove the tablespace and continue with
WAL replay. WAL replay.
</para> </para>
<para> <para>
Running <command>DROP DATABASE</>, <command>ALTER DATABASE ... SET TABLESPACE</>, Running <command>DROP DATABASE</>, <command>ALTER DATABASE ... SET TABLESPACE</>,
or <command>ALTER DATABASE ... RENAME</> on primary will generate a log message or <command>ALTER DATABASE ... RENAME</> on primary will generate a log message
that will cause all users connected to that database on the standby to be that will cause all users connected to that database on the standby to be
forcibly disconnected, once <varname>max_standby_delay</> has been reached. forcibly disconnected, once <varname>max_standby_delay</> has been reached.
</para> </para>
<para> <para>
In normal running, if you issue <command>DROP USER</> or <command>DROP ROLE</> In normal running, if you issue <command>DROP USER</> or <command>DROP ROLE</>
for a role with login capability while that user is still connected then for a role with login capability while that user is still connected then
nothing happens to the connected user - they remain connected. The user cannot nothing happens to the connected user - they remain connected. The user cannot
reconnect however. This behaviour applies in recovery also, so a reconnect however. This behaviour applies in recovery also, so a
<command>DROP USER</> on the primary does not disconnect that user on the standby. <command>DROP USER</> on the primary does not disconnect that user on the standby.
</para> </para>
<para> <para>
Stats collector is active during recovery. All scans, reads, blocks, Stats collector is active during recovery. All scans, reads, blocks,
index usage etc will all be recorded normally on the standby. Replayed index usage etc will all be recorded normally on the standby. Replayed
actions will not duplicate their effects on primary, so replaying an actions will not duplicate their effects on primary, so replaying an
insert will not increment the Inserts column of pg_stat_user_tables. insert will not increment the Inserts column of pg_stat_user_tables.
The stats file is deleted at start of recovery, so stats from primary The stats file is deleted at start of recovery, so stats from primary
and standby will differ; this is considered a feature not a bug. and standby will differ; this is considered a feature not a bug.
</para> </para>
<para> <para>
Autovacuum is not active during recovery, though will start normally Autovacuum is not active during recovery, though will start normally
at the end of recovery. at the end of recovery.
</para> </para>
<para> <para>
Background writer is active during recovery and will perform Background writer is active during recovery and will perform
restartpoints (similar to checkpoints on primary) and normal block restartpoints (similar to checkpoints on primary) and normal block
cleaning activities. The <command>CHECKPOINT</> command is accepted during recovery, cleaning activities. The <command>CHECKPOINT</> command is accepted during recovery,
though performs a restartpoint rather than a new checkpoint. though performs a restartpoint rather than a new checkpoint.
</para> </para>
</sect2> </sect2>
...@@ -2579,22 +2579,22 @@ LOG: database system is ready to accept read only connections ...@@ -2579,22 +2579,22 @@ LOG: database system is ready to accept read only connections
<title>Hot Standby Parameter Reference</title> <title>Hot Standby Parameter Reference</title>
<para> <para>
Various parameters have been mentioned above in the <xref linkend="hot-standby-admin"> Various parameters have been mentioned above in the <xref linkend="hot-standby-admin">
and <xref linkend="hot-standby-conflict"> sections. and <xref linkend="hot-standby-conflict"> sections.
</para> </para>
<para> <para>
On the primary, parameters <varname>recovery_connections</> and On the primary, parameters <varname>recovery_connections</> and
<varname>vacuum_defer_cleanup_age</> can be used to enable and control the <varname>vacuum_defer_cleanup_age</> can be used to enable and control the
primary server to assist the successful configuration of Hot Standby servers. primary server to assist the successful configuration of Hot Standby servers.
<varname>max_standby_delay</> has no effect if set on the primary. <varname>max_standby_delay</> has no effect if set on the primary.
</para> </para>
<para> <para>
On the standby, parameters <varname>recovery_connections</> and On the standby, parameters <varname>recovery_connections</> and
<varname>max_standby_delay</> can be used to enable and control Hot Standby. <varname>max_standby_delay</> can be used to enable and control Hot Standby.
standby server to assist the successful configuration of Hot Standby servers. standby server to assist the successful configuration of Hot Standby servers.
<varname>vacuum_defer_cleanup_age</> has no effect during recovery. <varname>vacuum_defer_cleanup_age</> has no effect during recovery.
</para> </para>
</sect2> </sect2>
...@@ -2610,40 +2610,40 @@ LOG: database system is ready to accept read only connections ...@@ -2610,40 +2610,40 @@ LOG: database system is ready to accept read only connections
<para> <para>
Operations on hash indexes are not presently WAL-logged, so Operations on hash indexes are not presently WAL-logged, so
replay will not update these indexes. Hash indexes will not be replay will not update these indexes. Hash indexes will not be
used for query plans during recovery. used for query plans during recovery.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Full knowledge of running transactions is required before snapshots Full knowledge of running transactions is required before snapshots
may be taken. Transactions that take use large numbers of subtransactions may be taken. Transactions that take use large numbers of subtransactions
(currently greater than 64) will delay the start of read only (currently greater than 64) will delay the start of read only
connections until the completion of the longest running write transaction. connections until the completion of the longest running write transaction.
If this situation occurs explanatory messages will be sent to server log. If this situation occurs explanatory messages will be sent to server log.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
Valid starting points for recovery connections are generated at each Valid starting points for recovery connections are generated at each
checkpoint on the master. If the standby is shutdown while the master checkpoint on the master. If the standby is shutdown while the master
is in a shutdown state it may not be possible to re-enter Hot Standby is in a shutdown state it may not be possible to re-enter Hot Standby
until the primary is started up so that it generates further starting until the primary is started up so that it generates further starting
points in the WAL logs. This is not considered a serious issue points in the WAL logs. This is not considered a serious issue
because the standby is usually switched into the primary role while because the standby is usually switched into the primary role while
the first node is taken down. the first node is taken down.
</para> </para>
</listitem> </listitem>
<listitem> <listitem>
<para> <para>
At the end of recovery, AccessExclusiveLocks held by prepared transactions At the end of recovery, AccessExclusiveLocks held by prepared transactions
will require twice the normal number of lock table entries. If you plan will require twice the normal number of lock table entries. If you plan
on running either a large number of concurrent prepared transactions on running either a large number of concurrent prepared transactions
that normally take AccessExclusiveLocks, or you plan on having one that normally take AccessExclusiveLocks, or you plan on having one
large transaction that takes many AccessExclusiveLocks then you are large transaction that takes many AccessExclusiveLocks then you are
advised to select a larger value of <varname>max_locks_per_transaction</>, advised to select a larger value of <varname>max_locks_per_transaction</>,
up to, but never more than twice the value of the parameter setting on up to, but never more than twice the value of the parameter setting on
the primary server in rare extremes. You need not consider this at all if the primary server in rare extremes. You need not consider this at all if
your setting of <varname>max_prepared_transactions</> is <literal>0</>. your setting of <varname>max_prepared_transactions</> is <literal>0</>.
</para> </para>
</listitem> </listitem>
</itemizedlist> </itemizedlist>
......
doc/src/sgml/config.sgml
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<chapter Id="runtime-config"> <chapter Id="runtime-config">
<title>Server Configuration</title> <title>Server Configuration</title>
...@@ -1755,15 +1755,15 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows ...@@ -1755,15 +1755,15 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
<term><varname>recovery_connections</varname> (<type>boolean</type>)</term> <term><varname>recovery_connections</varname> (<type>boolean</type>)</term>
<listitem> <listitem>
<para> <para>
Parameter has two roles. During recovery, specifies whether or not Parameter has two roles. During recovery, specifies whether or not
you can connect and run queries to enable <xref linkend="hot-standby">. you can connect and run queries to enable <xref linkend="hot-standby">.
During normal running, specifies whether additional information is written During normal running, specifies whether additional information is written
to WAL to allow recovery connections on a standby server that reads to WAL to allow recovery connections on a standby server that reads
WAL data generated by this server. The default value is WAL data generated by this server. The default value is
<literal>on</literal>. It is thought that there is little <literal>on</literal>. It is thought that there is little
measurable difference in performance from using this feature, so measurable difference in performance from using this feature, so
feedback is welcome if any production impacts are noticeable. feedback is welcome if any production impacts are noticeable.
It is likely that this parameter will be removed in later releases. It is likely that this parameter will be removed in later releases.
This parameter can only be set at server start. This parameter can only be set at server start.
</para> </para>
</listitem> </listitem>
...@@ -1773,15 +1773,15 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows ...@@ -1773,15 +1773,15 @@ archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
<term><varname>max_standby_delay</varname> (<type>string</type>)</term> <term><varname>max_standby_delay</varname> (<type>string</type>)</term>
<listitem> <listitem>
<para> <para>
When server acts as a standby, this parameter specifies a wait policy When server acts as a standby, this parameter specifies a wait policy
for queries that conflict with incoming data changes. Valid settings for queries that conflict with incoming data changes. Valid settings
are -1, meaning wait forever, or a wait time of 0 or more seconds. are -1, meaning wait forever, or a wait time of 0 or more seconds.
If a conflict should occur the server will delay up to this If a conflict should occur the server will delay up to this
amount before it begins trying to resolve things less amicably, as amount before it begins trying to resolve things less amicably, as
described in <xref linkend="hot-standby-conflict">. Typically, described in <xref linkend="hot-standby-conflict">. Typically,
this parameter makes sense only during replication, so when this parameter makes sense only during replication, so when
performing an archive recovery to recover from data loss a performing an archive recovery to recover from data loss a
parameter setting of 0 is recommended. The default is 30 seconds. parameter setting of 0 is recommended. The default is 30 seconds.
This parameter can only be set in the <filename>postgresql.conf</> This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line. file or on the server command line.
</para> </para>
...@@ -4226,17 +4226,17 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv; ...@@ -4226,17 +4226,17 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
<listitem> <listitem>
<para> <para>
Specifies the number of transactions by which <command>VACUUM</> and Specifies the number of transactions by which <command>VACUUM</> and
<acronym>HOT</> updates will defer cleanup of dead row versions. The <acronym>HOT</> updates will defer cleanup of dead row versions. The
default is 0 transactions, meaning that dead row versions will be default is 0 transactions, meaning that dead row versions will be
removed as soon as possible. You may wish to set this to a non-zero removed as soon as possible. You may wish to set this to a non-zero
value when planning or maintaining a <xref linkend="hot-standby"> value when planning or maintaining a <xref linkend="hot-standby">
configuration. The recommended value is <literal>0</> unless you have configuration. The recommended value is <literal>0</> unless you have
clear reason to increase it. The purpose of the parameter is to clear reason to increase it. The purpose of the parameter is to
allow the user to specify an approximate time delay before cleanup allow the user to specify an approximate time delay before cleanup
occurs. However, it should be noted that there is no direct link with occurs. However, it should be noted that there is no direct link with
any specific time delay and so the results will be application and any specific time delay and so the results will be application and
installation specific, as well as variable over time, depending upon installation specific, as well as variable over time, depending upon
the transaction rate (of writes only). the transaction rate (of writes only).
</para> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>
......
doc/src/sgml/func.sgml
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<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.494 2009/12/19 01:32:31 sriggs Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.495 2009/12/19 17:49:50 momjian Exp $ -->
<chapter id="functions"> <chapter id="functions">
<title>Functions and Operators</title> <title>Functions and Operators</title>
...@@ -13139,7 +13139,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup()); ...@@ -13139,7 +13139,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
<para> <para>
The functions shown in <xref The functions shown in <xref
linkend="functions-recovery-info-table"> provide information linkend="functions-recovery-info-table"> provide information
about the current status of Hot Standby. about the current status of Hot Standby.
These functions may be executed during both recovery and in normal running. These functions may be executed during both recovery and in normal running.
</para> </para>
...@@ -13158,7 +13158,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup()); ...@@ -13158,7 +13158,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
</entry> </entry>
<entry><type>bool</type></entry> <entry><type>bool</type></entry>
<entry>True if recovery is still in progress. <entry>True if recovery is still in progress.
</entry> </entry>
</row> </row>
</tbody> </tbody>
</tgroup> </tgroup>
......
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