On Mon, Nov 7, 2022 at 10:58:05AM +0000, Simon Riggs wrote:
> What I've posted is the merged patch, i.e. your latest patch, plus
> changes to RELEASE SAVEPOINT from you on Oct 16, plus changes based on
> the later comments from Robert and I.
Thanks. I have two changes to your patch. First, I agree "destroy" is
the wrong word for this, but I don't think "subcommit" is good, for
three reasons:
1. Release merges the non-aborted changes into the previous transaction
_and_ frees their resources --- "subcommit" doesn't have both meanings,
which I think means if we need a single word, we should use "release"
and later define what that means.
2. The "subcommit" concept doesn't closely match the user-visible
behavior, even though we use subtransactions to accomplish this. Release
is more of a rollup/merge into the previously-active
transaction/savepoint.
3. "subcommit" is an implementation detail that I don't think we should
expose to users in the manual pages.
I adjusted the first paragraph of RELEASE SAVEPOINT to highlight the
above issues. My original patch had similar wording.
The first attachment shows my changes to your patch, and the second
attachment is my full patch.
--
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/ref/release_savepoint.sgml b/doc/src/sgml/ref/release_savepoint.sgml
index 00b57ffcbb..34299f5618 100644
--- a/doc/src/sgml/ref/release_savepoint.sgml
+++ b/doc/src/sgml/ref/release_savepoint.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refnamediv>
<refname>RELEASE SAVEPOINT</refname>
- <refpurpose>subcommit a previously defined savepoint</refpurpose>
+ <refpurpose>release a previously defined savepoint</refpurpose>
</refnamediv>
<refsynopsisdiv>
@@ -34,16 +34,13 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
<title>Description</title>
<para>
- <command>RELEASE SAVEPOINT</command> will subcommit the subtransaction
- established by the named savepoint, if one exists. This will release
- any resources held by the subtransaction. If there were any
- subtransactions of the named savepoint, these will also be subcommitted.
- </para>
-
- <para>
- 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.
+ <command>RELEASE SAVEPOINT</command> releases the named savepoint and
+ all active savepoints that were created after the named savepoint,
+ and frees their resources. All changes made since the creation of the
+ savepoint, excluding rolled back savepoints changes, are merged into
+ the transaction or savepoint that was active when the named savepoint
+ was created. Changes made after <command>RELEASE SAVEPOINT</command>
+ will also be part of this active transaction or savepoint.
</para>
</refsect1>
@@ -55,7 +52,7 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
<term><replaceable>savepoint_name</replaceable></term>
<listitem>
<para>
- The name of the savepoint to subcommit.
+ The name of the savepoint to release.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/rollback.sgml b/doc/src/sgml/ref/rollback.sgml
index 02b118fc04..e0e7c61560 100644
--- a/doc/src/sgml/ref/rollback.sgml
+++ b/doc/src/sgml/ref/rollback.sgml
@@ -61,9 +61,6 @@ ROLLBACK [ 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/config.sgml b/doc/src/sgml/config.sgml
index 559eb898a9..6df0092f4c 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -7225,12 +7225,14 @@ 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 6e0425cb3d..153587c4b8 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -24673,7 +24673,10 @@ 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>
@@ -24690,6 +24693,7 @@ 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>
@@ -24733,6 +24737,8 @@ 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>
@@ -24787,7 +24793,8 @@ 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>
@@ -24799,8 +24806,9 @@ 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
@@ -24846,7 +24854,7 @@ 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 e5d622d514..186d7ffa11 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..34299f5618 100644
--- a/doc/src/sgml/ref/release_savepoint.sgml
+++ b/doc/src/sgml/ref/release_savepoint.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refnamediv>
<refname>RELEASE SAVEPOINT</refname>
- <refpurpose>destroy a previously defined savepoint</refpurpose>
+ <refpurpose>release a previously defined savepoint</refpurpose>
</refnamediv>
<refsynopsisdiv>
@@ -34,23 +34,13 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
<title>Description</title>
<para>
- <command>RELEASE SAVEPOINT</command> destroys a savepoint previously defined
- in the current transaction.
- </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.
- </para>
-
- <para>
- <command>RELEASE SAVEPOINT</command> also destroys all savepoints that were
- established after the named savepoint was established.
+ <command>RELEASE SAVEPOINT</command> releases the named savepoint and
+ all active savepoints that were created after the named savepoint,
+ and frees their resources. All changes made since the creation of the
+ savepoint, excluding rolled back savepoints changes, are merged into
+ the transaction or savepoint that was active when the named savepoint
+ was created. Changes made after <command>RELEASE SAVEPOINT</command>
+ will also be part of this active transaction or savepoint.
</para>
</refsect1>
@@ -62,7 +52,7 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
<term><replaceable>savepoint_name</replaceable></term>
<listitem>
<para>
- The name of the savepoint to destroy.
+ The name of the savepoint to release.
</para>
</listitem>
</varlistentry>
@@ -78,7 +68,7 @@ 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>
@@ -93,7 +83,7 @@ RELEASE [ SAVEPOINT ] <replaceable>savepoint_name</replaceable>
<title>Examples</title>
<para>
- To establish and later destroy a savepoint:
+ To establish and later release a savepoint:
<programlisting>
BEGIN;
INSERT INTO table1 VALUES (3);
@@ -104,6 +94,36 @@ 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 the release of the savepoint
+ <literal>sp2</literal>, which inserted 3. This changes the insert's
+ transaction context to <literal>sp1</literal>. When the statement
+ attempting to insert value 4 generates an error, the insertion of 2 and
+ 4 are lost because they are in the same, now-rolled back savepoint,
+ and value 3 is in the same transaction context. 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..e0e7c61560 100644
--- a/doc/src/sgml/ref/rollback.sgml
+++ b/doc/src/sgml/ref/rollback.sgml
@@ -56,7 +56,7 @@ 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.
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 7c716fe327..4d8bc659f2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1416,7 +1416,8 @@
</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>
@@ -1425,8 +1426,8 @@
<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>
+
