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postgres/doc/src/sgml/ref/set_transaction.sgml
Heikki Linnakangas dafaa3efb7 Implement genuine serializable isolation level. 
Until now, our Serializable mode has in fact been what's called Snapshot
Isolation, which allows some anomalies that could not occur in any
serialized ordering of the transactions. This patch fixes that using a
method called Serializable Snapshot Isolation, based on research papers by
Michael J. Cahill (see README-SSI for full references). In Serializable
Snapshot Isolation, transactions run like they do in Snapshot Isolation,
but a predicate lock manager observes the reads and writes performed and
aborts transactions if it detects that an anomaly might occur. This method
produces some false positives, ie. it sometimes aborts transactions even
though there is no anomaly.

To track reads we implement predicate locking, see storage/lmgr/predicate.c.
Whenever a tuple is read, a predicate lock is acquired on the tuple. Shared
memory is finite, so when a transaction takes many tuple-level locks on a
page, the locks are promoted to a single page-level lock, and further to a
single relation level lock if necessary. To lock key values with no matching
tuple, a sequential scan always takes a relation-level lock, and an index
scan acquires a page-level lock that covers the search key, whether or not
there are any matching keys at the moment.

A predicate lock doesn't conflict with any regular locks or with another
predicate locks in the normal sense. They're only used by the predicate lock
manager to detect the danger of anomalies. Only serializable transactions
participate in predicate locking, so there should be no extra overhead for
for other transactions.

Predicate locks can't be released at commit, but must be remembered until
all the transactions that overlapped with it have completed. That means that
we need to remember an unbounded amount of predicate locks, so we apply a
lossy but conservative method of tracking locks for committed transactions.
If we run short of shared memory, we overflow to a new "pg_serial" SLRU
pool.

We don't currently allow Serializable transactions in Hot Standby mode.
That would be hard, because even read-only transactions can cause anomalies
that wouldn't otherwise occur.

Serializable isolation mode now means the new fully serializable level.
Repeatable Read gives you the old Snapshot Isolation level that we have
always had.

Kevin Grittner and Dan Ports, reviewed by Jeff Davis, Heikki Linnakangas and
Anssi Kääriäinen
2011-02-08 00:09:08 +02:00

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<!-- doc/src/sgml/ref/set_transaction.sgml -->
<refentry id="SQL-SET-TRANSACTION">
<refmeta>
<refentrytitle>SET TRANSACTION</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>SET TRANSACTION</refname>
<refpurpose>set the characteristics of the current transaction</refpurpose>
</refnamediv>
<indexterm zone="sql-set-transaction">
<primary>SET TRANSACTION</primary>
</indexterm>
<indexterm>
<primary>transaction isolation level</primary>
<secondary>setting</secondary>
</indexterm>
<indexterm>
<primary>read-only transaction</primary>
<secondary>setting</secondary>
</indexterm>
<indexterm>
<primary>deferrable transaction</primary>
<secondary>setting</secondary>
</indexterm>
<refsynopsisdiv>
<synopsis>
SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
<phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
[ NOT ] DEFERRABLE
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
The <command>SET TRANSACTION</command> command sets the
characteristics of the current transaction. It has no effect on any
subsequent transactions. <command>SET SESSION
CHARACTERISTICS</command> sets the default transaction
characteristics for subsequent transactions of a session. These
defaults can be overridden by <command>SET TRANSACTION</command>
for an individual transaction.
</para>
<para>
The available transaction characteristics are the transaction
isolation level, the transaction access mode (read/write or
read-only), and the deferrable mode.
</para>
<para>
The isolation level of a transaction determines what data the
transaction can see when other transactions are running concurrently:
<variablelist>
<varlistentry>
<term><literal>READ COMMITTED</literal></term>
<listitem>
<para>
A statement can only see rows committed before it began. This
is the default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>REPEATABLE READ</literal></term>
<listitem>
<para>
All statements of the current transaction can only see rows committed
before the first query or data-modification statement was executed in
this transaction.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SERIALIZABLE</literal></term>
<listitem>
<para>
All statements of the current transaction can only see rows committed
before the first query or data-modification statement was executed in
this transaction. If a pattern of reads and writes among concurrent
serializable transactions would create a situation which could not
have occurred for any serial (one-at-a-time) execution of those
transactions, one of them will be rolled back with a
<literal>serialization_failure</literal> <literal>SQLSTATE</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
The SQL standard defines one additional level, <literal>READ
UNCOMMITTED</literal>.
In <productname>PostgreSQL</productname> <literal>READ
UNCOMMITTED</literal> is treated as <literal>READ COMMITTED</literal>.
</para>
<para>
The transaction isolation level cannot be changed after the first query or
data-modification statement (<command>SELECT</command>,
<command>INSERT</command>, <command>DELETE</command>,
<command>UPDATE</command>, <command>FETCH</command>, or
<command>COPY</command>) of a transaction has been executed. See
<xref linkend="mvcc"> for more information about transaction
isolation and concurrency control.
</para>
<para>
The transaction access mode determines whether the transaction is
read/write or read-only. Read/write is the default. When a
transaction is read-only, the following SQL commands are
disallowed: <literal>INSERT</literal>, <literal>UPDATE</literal>,
<literal>DELETE</literal>, and <literal>COPY FROM</literal> if the
table they would write to is not a temporary table; all
<literal>CREATE</literal>, <literal>ALTER</literal>, and
<literal>DROP</literal> commands; <literal>COMMENT</literal>,
<literal>GRANT</literal>, <literal>REVOKE</literal>,
<literal>TRUNCATE</literal>; and <literal>EXPLAIN ANALYZE</literal>
and <literal>EXECUTE</literal> if the command they would execute is
among those listed. This is a high-level notion of read-only that
does not prevent all writes to disk.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
If <command>SET TRANSACTION</command> is executed without a prior
<command>START TRANSACTION</command> or <command>BEGIN</command>,
it will appear to have no effect, since the transaction will immediately
end.
</para>
<para>
It is possible to dispense with <command>SET TRANSACTION</command>
by instead specifying the desired <replaceable
class="parameter">transaction_modes</replaceable> in
<command>BEGIN</command> or <command>START TRANSACTION</command>.
</para>
<para>
The session default transaction modes can also be set by setting the
configuration parameters <xref linkend="guc-default-transaction-isolation">,
<xref linkend="guc-default-transaction-read-only">, and
<xref linkend="guc-default-transaction-deferrable">.
(In fact <command>SET SESSION CHARACTERISTICS</command> is just a
verbose equivalent for setting these variables with <command>SET</>.)
This means the defaults can be set in the configuration file, via
<command>ALTER DATABASE</>, etc. Consult <xref linkend="runtime-config">
for more information.
</para>
</refsect1>
<refsect1 id="R1-SQL-SET-TRANSACTION-3">
<title>Compatibility</title>
<para>
Both commands are defined in the <acronym>SQL</acronym> standard.
<literal>SERIALIZABLE</literal> is the default transaction
isolation level in the standard. In
<productname>PostgreSQL</productname> the default is ordinarily
<literal>READ COMMITTED</literal>, but you can change it as
mentioned above.
</para>
<para>
In the SQL standard, there is one other transaction characteristic
that can be set with these commands: the size of the diagnostics
area. This concept is specific to embedded SQL, and therefore is
not implemented in the <productname>PostgreSQL</productname> server.
</para>
<para>
The <literal>DEFERRABLE</literal>
<replaceable class="parameter">transaction_mode</replaceable>
is a <productname>PostgreSQL</productname> language extension.
</para>
<para>
The SQL standard requires commas between successive <replaceable
class="parameter">transaction_modes</replaceable>, but for historical
reasons <productname>PostgreSQL</productname> allows the commas to be
omitted.
</para>
</refsect1>
</refentry>
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