On Fri, Oct 14, 2022 at 12:22:35PM +0100, Simon Riggs wrote:
> > > + <para>
> > > + The parent xid of each subxid is recorded in the
> > > + <filename>pg_subtrans</filename> directory. No entry is made for
> > > + top-level xids since they do not have a parent, nor is an entry made
> > > + for read-only subtransactions.
> > > + </para>
> >
> > Maybe say "the immediate parent xid of each ...", or is it too obvious?
>
> +1 to all of those suggestions
Attached is the merged patch from all the great comments I received. I
have also rebuilt the docs with the updated patch:
https://momjian.us/tmp/pgsql/
--
Bruce Momjian <br...@momjian.us> https://momjian.us
EDB https://enterprisedb.com
Indecision is a decision. Inaction is an action. Mark Batterson
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 66312b53b8..024a3c5101 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -7224,12 +7224,14 @@ local0.* /var/log/postgresql
</row>
<row>
<entry><literal>%v</literal></entry>
- <entry>Virtual transaction ID (backendID/localXID)</entry>
+ <entry>Virtual transaction ID (backendID/localXID); see
+ <xref linkend="transaction-id"/></entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%x</literal></entry>
- <entry>Transaction ID (0 if none is assigned)</entry>
+ <entry>Transaction ID (0 if none is assigned); see
+ <xref linkend="transaction-id"/></entry>
<entry>no</entry>
</row>
<row>
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index b030b36002..fdffba4442 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -4992,7 +4992,8 @@ WHERE ...
<structfield>xmin</structfield> and <structfield>xmax</structfield>. Transaction identifiers are 32-bit quantities.
In some contexts, a 64-bit variant <type>xid8</type> is used. Unlike
<type>xid</type> values, <type>xid8</type> values increase strictly
- monotonically and cannot be reused in the lifetime of a database cluster.
+ monotonically and cannot be reused in the lifetime of a database
+ cluster. See <xref linkend="transaction-id"/> for more details.
</para>
<para>
diff --git a/doc/src/sgml/filelist.sgml b/doc/src/sgml/filelist.sgml
index de450cd661..0d6be9a2fa 100644
--- a/doc/src/sgml/filelist.sgml
+++ b/doc/src/sgml/filelist.sgml
@@ -104,6 +104,7 @@
<!ENTITY protocol SYSTEM "protocol.sgml">
<!ENTITY sources SYSTEM "sources.sgml">
<!ENTITY storage SYSTEM "storage.sgml">
+<!ENTITY transaction SYSTEM "xact.sgml">
<!ENTITY tablesample-method SYSTEM "tablesample-method.sgml">
<!ENTITY generic-wal SYSTEM "generic-wal.sgml">
<!ENTITY custom-rmgr SYSTEM "custom-rmgr.sgml">
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index b5a2f94c4e..1e0d587932 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -24676,7 +24676,10 @@ SELECT collation for ('foo' COLLATE "de_DE");
<para>
Returns the current transaction's ID. It will assign a new one if the
current transaction does not have one already (because it has not
- performed any database updates).
+ performed any database updates); see <xref
+ linkend="transaction-id"/> for details. If executed in a
+ subtransaction this will return the top-level xid; see <xref
+ linkend="subxacts"/> for details.
</para></entry>
</row>
@@ -24693,6 +24696,7 @@ SELECT collation for ('foo' COLLATE "de_DE");
ID is assigned yet. (It's best to use this variant if the transaction
might otherwise be read-only, to avoid unnecessary consumption of an
XID.)
+ If executed in a subtransaction this will return the top-level xid.
</para></entry>
</row>
@@ -24736,6 +24740,8 @@ SELECT collation for ('foo' COLLATE "de_DE");
<para>
Returns a current <firstterm>snapshot</firstterm>, a data structure
showing which transaction IDs are now in-progress.
+ Only top-level xids are included in the snapshot; subxids are not
+ shown; see <xref linkend="subxacts"/> for details.
</para></entry>
</row>
@@ -24790,7 +24796,8 @@ SELECT collation for ('foo' COLLATE "de_DE");
Is the given transaction ID <firstterm>visible</firstterm> according
to this snapshot (that is, was it completed before the snapshot was
taken)? Note that this function will not give the correct answer for
- a subtransaction ID.
+ a subtransaction ID (subxid); see <xref linkend="subxacts"/> for
+ details.
</para></entry>
</row>
</tbody>
@@ -24802,8 +24809,9 @@ SELECT collation for ('foo' COLLATE "de_DE");
wraps around every 4 billion transactions. However,
the functions shown in <xref linkend="functions-pg-snapshot"/> use a
64-bit type <type>xid8</type> that does not wrap around during the life
- of an installation, and can be converted to <type>xid</type> by casting if
- required. The data type <type>pg_snapshot</type> stores information about
+ of an installation and can be converted to <type>xid</type> by casting if
+ required; see <xref linkend="transaction-id"/> for details.
+ The data type <type>pg_snapshot</type> stores information about
transaction ID visibility at a particular moment in time. Its components
are described in <xref linkend="functions-pg-snapshot-parts"/>.
<type>pg_snapshot</type>'s textual representation is
@@ -24849,7 +24857,7 @@ SELECT collation for ('foo' COLLATE "de_DE");
xmax</literal> and not in this list was already completed at the time
of the snapshot, and thus is either visible or dead according to its
commit status. This list does not include the transaction IDs of
- subtransactions.
+ subtransactions (subxids).
</entry>
</row>
</tbody>
diff --git a/doc/src/sgml/glossary.sgml b/doc/src/sgml/glossary.sgml
index d6d0a3a814..fe138a47bb 100644
--- a/doc/src/sgml/glossary.sgml
+++ b/doc/src/sgml/glossary.sgml
@@ -1710,7 +1710,8 @@
<literal>3</literal> (values under that are reserved) and the
epoch value is incremented by one.
In some contexts, the epoch and xid values are
- considered together as a single 64-bit value.
+ considered together as a single 64-bit value; see <xref
+ linkend="transaction-id"/> for more details.
</para>
<para>
For more information, see
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 342b20ebeb..3a9cf68578 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -918,7 +918,8 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<structfield>backend_xid</structfield> <type>xid</type>
</para>
<para>
- Top-level transaction identifier of this backend, if any.
+ Top-level transaction identifier of this backend, if any; see
+ <xref linkend="transaction-id"/>.
</para></entry>
</row>
@@ -1890,7 +1891,8 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
</row>
<row>
<entry><literal>virtualxid</literal></entry>
- <entry>Waiting to acquire a virtual transaction ID lock.</entry>
+ <entry>Waiting to acquire a virtual transaction ID lock; see
+ <xref linkend="transaction-id"/>.</entry>
</row>
</tbody>
</tgroup>
diff --git a/doc/src/sgml/pgrowlocks.sgml b/doc/src/sgml/pgrowlocks.sgml
index 2914bf6e6d..ad15cda668 100644
--- a/doc/src/sgml/pgrowlocks.sgml
+++ b/doc/src/sgml/pgrowlocks.sgml
@@ -57,7 +57,8 @@ pgrowlocks(text) returns setof record
<row>
<entry><structfield>locker</structfield></entry>
<entry><type>xid</type></entry>
- <entry>Transaction ID of locker, or multixact ID if multitransaction</entry>
+ <entry>Transaction ID of locker, or multixact ID if
+ multitransaction; see <xref linkend="transaction-id"/></entry>
</row>
<row>
<entry><structfield>multi</structfield></entry>
diff --git a/doc/src/sgml/postgres.sgml b/doc/src/sgml/postgres.sgml
index 73439c049e..2e271862fc 100644
--- a/doc/src/sgml/postgres.sgml
+++ b/doc/src/sgml/postgres.sgml
@@ -271,6 +271,7 @@ break is not needed in a wider output rendering.
&brin;
&hash;
&storage;
+ &transaction;
&bki;
&planstats;
&backup-manifest;
diff --git a/doc/src/sgml/ref/commit.sgml b/doc/src/sgml/ref/commit.sgml
index 5f244cdd3c..53d830998c 100644
--- a/doc/src/sgml/ref/commit.sgml
+++ b/doc/src/sgml/ref/commit.sgml
@@ -62,6 +62,9 @@ COMMIT [ WORK | TRANSACTION ] [ AND [ NO ] CHAIN ]
linkend="sql-set-transaction"/>) as the just finished one. Otherwise,
no new transaction is started.
</para>
+ <para>
+ The SQL Standard describes this as a chained transaction.
+ </para>
</listitem>
</varlistentry>
</variablelist>
diff --git a/doc/src/sgml/ref/release_savepoint.sgml b/doc/src/sgml/ref/release_savepoint.sgml
index daf8eb9a43..349091ce54 100644
--- a/doc/src/sgml/ref/release_savepoint.sgml
+++ b/doc/src/sgml/ref/release_savepoint.sgml
@@ -39,18 +39,19 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
</para>
<para>
- Destroying a savepoint makes it unavailable as a rollback point,
- but it has no other user visible behavior. It does not undo the
- effects of commands executed after the savepoint was established.
- (To do that, see <xref linkend="sql-rollback-to"/>.)
- Destroying a savepoint when
- it is no longer needed allows the system to reclaim some resources
- earlier than transaction end.
+ The full behavior is to subcommit the changes made within the current
+ subtransaction, remove the savepoint as a future rollback target, and
+ return to the context of the parent transaction or subtransaction. Note
+ that further changes made after <command>RELEASE SAVEPOINT</command>
+ will be in the same transaction, and have the same transaction id,
+ as changes made before the named savepoint was created.
</para>
<para>
- <command>RELEASE SAVEPOINT</command> also destroys all savepoints that were
- established after the named savepoint was established.
+ <command>RELEASE SAVEPOINT</command> also subcommits and destroys
+ all savepoints that were established after the named savepoint was
+ established. This means that any subtransactions of the named savepoint
+ will also be subcommitted and destroyed.
</para>
</refsect1>
@@ -78,7 +79,7 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
<para>
It is not possible to release a savepoint when the transaction is in
- an aborted state.
+ an aborted state, to do that use <xref linkend="sql-rollback-to"/>.
</para>
<para>
@@ -104,6 +105,38 @@ COMMIT;
</programlisting>
The above transaction will insert both 3 and 4.
</para>
+
+ <para>
+ A more complex example with multiple nested subtransactions:
+<programlisting>
+BEGIN;
+ INSERT INTO table1 VALUES (1);
+ SAVEPOINT sp1;
+ INSERT INTO table1 VALUES (2);
+ SAVEPOINT sp2;
+ INSERT INTO table1 VALUES (3);
+ RELEASE SAVEPOINT sp2;
+ INSERT INTO table1 VALUES (4))); -- generates an error
+</programlisting>
+ In this example, the application requests subcommit of the
+ subtransaction inserting 3 by requesting that savepoint
+ <literal>sp2</literal> is released. However, releasing
+ <literal>sp2</literal> returns the transaction context to
+ <literal>sp1</literal>, so when the statement attempting to insert
+ value 4 generates an error, the insertion of 2, 3 and 4 are lost
+ because values 2 and 4 are in the same, now-aborted subtransaction, and
+ value 3 is in a subcommitted transaction that is also now aborted. The
+ application can now only choose one of these two commands, since all other
+ commands will be ignored with a warning:
+<programlisting>
+ ROLLBACK;
+ ROLLBACK TO SAVEPOINT sp1;
+</programlisting>
+ Choosing <command>ROLLBACK</command> will abort everything, including
+ value 1, whereas <command>ROLLBACK TO SAVEPOINT sp1</command> will retain
+ value 1 and allow the transaction to continue.
+ </para>
+
</refsect1>
<refsect1>
diff --git a/doc/src/sgml/ref/rollback.sgml b/doc/src/sgml/ref/rollback.sgml
index 142f71e774..02b118fc04 100644
--- a/doc/src/sgml/ref/rollback.sgml
+++ b/doc/src/sgml/ref/rollback.sgml
@@ -56,11 +56,14 @@ ROLLBACK [ WORK | TRANSACTION ] [ AND [ NO ] CHAIN ]
<term><literal>AND CHAIN</literal></term>
<listitem>
<para>
- If <literal>AND CHAIN</literal> is specified, a new transaction is
+ If <literal>AND CHAIN</literal> is specified, a new unaborted transaction is
immediately started with the same transaction characteristics (see <xref
linkend="sql-set-transaction"/>) as the just finished one. Otherwise,
no new transaction is started.
</para>
+ <para>
+ The SQL Standard describes this as a chained transaction.
+ </para>
</listitem>
</varlistentry>
</variablelist>
diff --git a/doc/src/sgml/ref/rollback_to.sgml b/doc/src/sgml/ref/rollback_to.sgml
index 27fa95cd1b..32c1bb9723 100644
--- a/doc/src/sgml/ref/rollback_to.sgml
+++ b/doc/src/sgml/ref/rollback_to.sgml
@@ -35,8 +35,9 @@ ROLLBACK [ WORK | TRANSACTION ] TO [ SAVEPOINT ] <replaceable>savepoint_name</re
<para>
Roll back all commands that were executed after the savepoint was
- established. The savepoint remains valid and can be rolled back to
- again later, if needed.
+ established and then start a new subtransaction at the same transaction level.
+ The savepoint remains valid and can be rolled back to again later,
+ if needed.
</para>
<para>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 1ca7c3f9bf..6a37cb8b30 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1395,7 +1395,8 @@
</para>
<para>
Virtual ID of the transaction targeted by the lock,
- or null if the target is not a virtual transaction ID
+ or null if the target is not a virtual transaction ID; see
+ <xref linkend="transactions"/>
</para></entry>
</row>
@@ -1404,8 +1405,8 @@
<structfield>transactionid</structfield> <type>xid</type>
</para>
<para>
- ID of the transaction targeted by the lock,
- or null if the target is not a transaction ID
+ ID of the transaction targeted by the lock, or null if the target
+ is not a transaction ID; <xref linkend="transactions"/>
</para></entry>
</row>
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index 6a38b53744..f335305113 100644
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -4,8 +4,9 @@
<title>Reliability and the Write-Ahead Log</title>
<para>
- This chapter explains how the Write-Ahead Log is used to obtain
- efficient, reliable operation.
+ This chapter explains how to control the reliability of
+ <productname>PostgreSQL</productname>, including details about the
+ Write-Ahead Log.
</para>
<sect1 id="wal-reliability">
@@ -909,4 +910,36 @@
seem to be a problem in practice.
</para>
</sect1>
+
+ <sect1 id="two-phase">
+
+ <title>Two-Phase Transactions</title>
+
+ <para>
+ <productname>PostgreSQL</productname> supports a two-phase commit (2PC)
+ protocol that allows multiple distributed systems to work together
+ in a transactional manner. The commands are <command>PREPARE
+ TRANSACTION</command>, <command>COMMIT PREPARED</command> and
+ <command>ROLLBACK PREPARED</command>. Two-phase transactions
+ are intended for use by external transaction management systems.
+ <productname>PostgreSQL</productname> follows the features and model
+ proposed by the X/Open XA standard, but does not implement some less
+ often used aspects.
+ </para>
+
+ <para>
+ When the user executes <command>PREPARE TRANSACTION</command>, the
+ only possible next commands are <command>COMMIT PREPARED</command>
+ or <command>ROLLBACK PREPARED</command>. In general, this prepared
+ state is intended to be of very short duration, but external
+ availability issues might mean transactions stay in this state
+ for an extended interval. Short-lived prepared
+ transactions are stored only in shared memory and WAL.
+ Transactions that span checkpoints are recorded in the
+ <filename>pg_twophase</filename> directory. Currently-prepared
+ transactions can be inspected using <link
+ linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link>.
+ </para>
+ </sect1>
+
</chapter>
diff --git a/doc/src/sgml/xact.sgml b/doc/src/sgml/xact.sgml
new file mode 100644
index 0000000000..a14cc4c012
--- /dev/null
+++ b/doc/src/sgml/xact.sgml
@@ -0,0 +1,172 @@
+<!-- doc/src/sgml/mvcc.sgml -->
+
+ <chapter id="transactions">
+
+ <title>Transaction Management</title>
+
+ <para>
+ This chapter provides an overview of the internals of
+ <productname>PostgreSQL</productname>'s transaction management system.
+ The word transaction is often abbreviated as "xact".
+ </para>
+
+ <sect1 id="transaction-id">
+
+ <title>Transactions and Identifiers</title>
+
+ <para>
+ Transactions can be created explicitly using <command>BEGIN</command>
+ and <command>COMMIT</command>, which creates a transaction block.
+ An SQL statement outside of a transaction block automatically uses
+ a single-statement transaction.
+ </para>
+
+ <para>
+ Every transaction is identified by a unique
+ <literal>VirtualTransactionId</literal> (also called
+ <literal>virtualXID</literal> or <literal>vxid</literal>), which
+ is comprised of a backend ID (or <literal>backendID</literal>)
+ and a sequentially-assigned number local to each backend, known as
+ <literal>localXID</literal>. For example, the virtual transaction
+ ID <literal>4/12532</literal> has a <literal>backendID</literal>
+ of <literal>4</literal> and a <literal>localXID</literal> of
+ <literal>12532</literal>.
+ </para>
+
+ <para>
+ Once a transaction writes to the database, it is assigned a
+ non-virtual <literal>TransactionId</literal> (or <type>xid</type>),
+ e.g., <literal>278394</literal>. Xids are assigned sequentially
+ using a global counter used by all databases within the
+ <productname>PostgreSQL</productname> cluster. This property is used by
+ the transaction system to order transactions by their first database
+ write, i.e., lower-numbered xids started writing before higher-numbered
+ xids. Of course, transactions might start in a different order.
+ </para>
+
+ <para>
+ The internal transaction ID type <type>xid</type> is 32-bits wide
+ and <link linkend="vacuum-for-wraparound">wraps around</link> every
+ 4 billion transactions. A 32-bit epoch is incremented during each
+ wrap around. There is also a 64-bit type <type>xid8</type> which
+ includes this epoch and therefore does not wrap around during the
+ life of an installation and can be converted to xid by casting.
+ The functions in <xref linkend="functions-pg-snapshot"/>
+ return <type>xid8</type> values. Xids are used as the
+ basis for <productname>PostgreSQL</productname>'s <link
+ linkend="mvcc">MVCC</link> concurrency mechanism, <link
+ linkend="hot-standby">Hot Standby</link>, and Read Replica servers.
+ </para>
+
+ <para>
+ When a top-level transaction with a (non-virtual) xid commits,
+ it is marked as committed in the <filename>pg_xact</filename>
+ directory. Additional information is recorded in the
+ <filename>pg_commit_ts</filename> directory if <xref
+ linkend="guc-track-commit-timestamp"/> is enabled.
+ </para>
+
+ <para>
+ In addition to <literal>vxid</literal> and <type>xid</type>,
+ when a transaction is prepared for two-phase commit it
+ is also identified by a Global Transaction Identifier
+ (<acronym>GID</acronym>). GIDs are string literals up to 200
+ bytes long, which must be unique amongst other currently prepared
+ transactions. The mapping of GID to xid is shown in <link
+ linkend="view-pg-prepared-xacts"><structname>pg_prepared_xacts</structname></link>.
+ </para>
+ </sect1>
+
+ <sect1 id="xact-locking">
+
+ <title>Transactions and Locking</title>
+
+ <para>
+ Currently-executing transactions are shown in <link
+ linkend="view-pg-locks"><structname>pg_locks</structname></link>
+ in columns <structfield>virtualxid</structfield> and
+ <structfield>transactionid</structfield>. Read-only
+ transactions will have <structfield>virtualxid</structfield>s but
+ NULL <structfield>transactionid</structfield>s, while read-write
+ transactions will have both as non-NULL.
+ </para>
+
+ <para>
+ Lock waits on table-level locks are shown waiting for
+ <structfield>virtualxid</structfield>, while lock waits on row-level
+ locks are shown waiting for <structfield>transactionid</structfield>.
+ Row-level read and write locks are recorded directly in locked
+ rows and can be inspected using the <xref linkend="pgrowlocks"/>
+ extension. Row-level read locks might also require the assignment
+ of multixact IDs (<literal>mxid</literal>). Mxids are recorded in
+ the <filename>pg_multixact</filename> directory.
+ </para>
+ </sect1>
+
+ <sect1 id="subxacts">
+
+ <title>Subtransactions</title>
+
+ <para>
+ Subtransactions are started inside transactions, allowing large
+ transactions to be broken into smaller units. Subtransactions can
+ commit or abort without affecting their parent transactions, allowing
+ parent transactions to continue. This allows errors to be handled
+ more easily, which is a common application development pattern.
+ The word subtransaction is often abbreviated as
+ <literal>subxact</literal>.
+ </para>
+
+ <para>
+ Subtransactions can be started explicitly using the
+ <command>SAVEPOINT</command> command, but can also be started in
+ other ways, such as PL/pgSQL's <command>EXCEPTION</command> clause.
+ PL/Python and PL/TCL also support explicit subtransactions.
+ Subtransactions can also be started from other subtransactions.
+ The top-level transaction and its child subtransactions form a
+ hierarchy or tree, which is why we refer to the main transaction as
+ the top-level transaction.
+ </para>
+
+ <para>
+ If a subtransaction is assigned a non-virtual transaction ID,
+ its transaction ID is referred to as a <literal>subxid</literal>.
+ Read-only subtransactions are not assigned subxids, but once they
+ attempt to write, they will be assigned one. This also causes all of
+ a subxid's parents, up to and including the top-level transaction,
+ to be assigned non-virtual transaction ids. We ensure that a parent
+ xid is always lower than any of its child subxids.
+ </para>
+
+ <para>
+ The immediate parent xid of each subxid is recorded in the
+ <filename>pg_subtrans</filename> directory. No entry is made for
+ top-level xids since they do not have a parent, nor is an entry made
+ for read-only subtransactions.
+ </para>
+
+ <para>
+ When a subtransaction commits, all of its committed child
+ subtransactions with subxids will also be considered subcommitted
+ in that transaction. When a subtransaction aborts, all of its child
+ subtransactions will also be considered aborted.
+ </para>
+
+ <para>
+ When a top-level transaction with an xid commits, all of its
+ subcommitted child subtransactions are also persistently recorded
+ as committed in the <filename>pg_xact</filename> directory. If the
+ top-level transaction aborts, all its subtransactions are also aborted,
+ even if they were subcommitted.
+ </para>
+
+ <para>
+ The more subtransactions each transaction keeps open (not rolled back
+ or released), the greater the transaction management overhead. Up to
+ 64 open subxids are cached in shared memory for each backend; after
+ that point, the overhead increases significantly since we must look
+ up subxid entries in <filename>pg_subtrans</filename>.
+ </para>
+ </sect1>
+ </chapter>
+
