dafaa3efb7
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
248 lines
8.9 KiB
Plaintext
248 lines
8.9 KiB
Plaintext
<!--
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doc/src/sgml/ref/lock.sgml
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PostgreSQL documentation
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-->
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<refentry id="SQL-LOCK">
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<refmeta>
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<refentrytitle>LOCK</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>LOCK</refname>
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<refpurpose>lock a table</refpurpose>
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</refnamediv>
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<indexterm zone="sql-lock">
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<primary>LOCK</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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LOCK [ TABLE ] [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ IN <replaceable class="PARAMETER">lockmode</replaceable> MODE ] [ NOWAIT ]
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<phrase>where <replaceable class="PARAMETER">lockmode</replaceable> is one of:</phrase>
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ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
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| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>LOCK TABLE</command> obtains a table-level lock, waiting
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if necessary for any conflicting locks to be released. If
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<literal>NOWAIT</literal> is specified, <command>LOCK
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TABLE</command> does not wait to acquire the desired lock: if it
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cannot be acquired immediately, the command is aborted and an
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error is emitted. Once obtained, the lock is held for the
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remainder of the current transaction. (There is no <command>UNLOCK
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TABLE</command> command; locks are always released at transaction
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end.)
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</para>
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<para>
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When acquiring locks automatically for commands that reference
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tables, <productname>PostgreSQL</productname> always uses the least
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restrictive lock mode possible. <command>LOCK TABLE</command>
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provides for cases when you might need more restrictive locking.
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For example, suppose an application runs a transaction at the
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Read Committed isolation level and needs to ensure that data in a
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table remains stable for the duration of the transaction. To
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achieve this you could obtain <literal>SHARE</> lock mode over the
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table before querying. This will prevent concurrent data changes
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and ensure subsequent reads of the table see a stable view of
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committed data, because <literal>SHARE</> lock mode conflicts with
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the <literal>ROW EXCLUSIVE</> lock acquired by writers, and your
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<command>LOCK TABLE <replaceable
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class="PARAMETER">name</replaceable> IN SHARE MODE</command>
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statement will wait until any concurrent holders of <literal>ROW
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EXCLUSIVE</literal> mode locks commit or roll back. Thus, once you
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obtain the lock, there are no uncommitted writes outstanding;
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furthermore none can begin until you release the lock.
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</para>
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<para>
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To achieve a similar effect when running a transaction at the
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<literal>REPEATABLE READ</> or <literal>SERIALIZABLE</>
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isolation level, you have to execute the <command>LOCK TABLE</> statement
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before executing any <command>SELECT</> or data modification statement.
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A <literal>REPEATABLE READ</> or <literal>SERIALIZABLE</> transaction's
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view of data will be frozen when its first
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<command>SELECT</> or data modification statement begins. A <command>LOCK
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TABLE</> later in the transaction will still prevent concurrent writes
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— but it won't ensure that what the transaction reads corresponds to
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the latest committed values.
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</para>
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<para>
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If a transaction of this sort is going to change the data in the
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table, then it should use <literal>SHARE ROW EXCLUSIVE</> lock mode
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instead of <literal>SHARE</> mode. This ensures that only one
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transaction of this type runs at a time. Without this, a deadlock
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is possible: two transactions might both acquire <literal>SHARE</>
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mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</>
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mode to actually perform their updates. (Note that a transaction's
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own locks never conflict, so a transaction can acquire <literal>ROW
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EXCLUSIVE</> mode when it holds <literal>SHARE</> mode — but not
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if anyone else holds <literal>SHARE</> mode.) To avoid deadlocks,
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make sure all transactions acquire locks on the same objects in the
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same order, and if multiple lock modes are involved for a single
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object, then transactions should always acquire the most
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restrictive mode first.
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</para>
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<para>
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More information about the lock modes and locking strategies can be
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found in <xref linkend="explicit-locking">.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><replaceable class="PARAMETER">name</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of an existing table to
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lock. If <literal>ONLY</> is specified, only that table is
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locked. If <literal>ONLY</> is not specified, the table and all
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its descendant tables (if any) are locked.
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</para>
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<para>
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The command <literal>LOCK TABLE a, b;</> is equivalent to
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<literal>LOCK TABLE a; LOCK TABLE b;</>. The tables are locked
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one-by-one in the order specified in the <command>LOCK
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TABLE</command> command.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">lockmode</replaceable></term>
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<listitem>
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<para>
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The lock mode specifies which locks this lock conflicts with.
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Lock modes are described in <xref linkend="explicit-locking">.
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</para>
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<para>
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If no lock mode is specified, then <literal>ACCESS
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EXCLUSIVE</literal>, the most restrictive mode, is used.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>NOWAIT</literal></term>
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<listitem>
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<para>
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Specifies that <command>LOCK TABLE</command> should not wait for
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any conflicting locks to be released: if the specified lock(s)
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cannot be acquired immediately without waiting, the transaction
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is aborted.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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<literal>LOCK TABLE ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
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privileges on the target table. All other forms of <command>LOCK</>
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require table-level <literal>UPDATE</>, <literal>DELETE</>, or
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<literal>TRUNCATE</> privileges.
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</para>
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<para>
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<command>LOCK TABLE</> is useless outside a transaction block: the lock
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would remain held only to the completion of the statement. Therefore
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<productname>PostgreSQL</productname> reports an error if <command>LOCK</>
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is used outside a transaction block.
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Use
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<xref linkend="sql-begin"> and
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<xref linkend="sql-commit">
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(or <xref linkend="sql-rollback">)
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to define a transaction block.
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</para>
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<para>
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<command>LOCK TABLE</> only deals with table-level locks, and so
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the mode names involving <literal>ROW</> are all misnomers. These
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mode names should generally be read as indicating the intention of
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the user to acquire row-level locks within the locked table. Also,
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<literal>ROW EXCLUSIVE</> mode is a sharable table lock. Keep in
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mind that all the lock modes have identical semantics so far as
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<command>LOCK TABLE</> is concerned, differing only in the rules
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about which modes conflict with which. For information on how to
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acquire an actual row-level lock, see <xref linkend="locking-rows">
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and the <xref linkend="sql-for-update-share"
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endterm="sql-for-update-share-title"> in the <command>SELECT</command>
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reference documentation.
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</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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Obtain a <literal>SHARE</> lock on a primary key table when going to perform
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inserts into a foreign key table:
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<programlisting>
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BEGIN WORK;
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LOCK TABLE films IN SHARE MODE;
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SELECT id FROM films
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WHERE name = 'Star Wars: Episode I - The Phantom Menace';
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-- Do ROLLBACK if record was not returned
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INSERT INTO films_user_comments VALUES
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(_id_, 'GREAT! I was waiting for it for so long!');
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COMMIT WORK;
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</programlisting>
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</para>
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<para>
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Take a <literal>SHARE ROW EXCLUSIVE</> lock on a primary key table when going to perform
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a delete operation:
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<programlisting>
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BEGIN WORK;
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LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
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DELETE FROM films_user_comments WHERE id IN
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(SELECT id FROM films WHERE rating < 5);
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DELETE FROM films WHERE rating < 5;
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COMMIT WORK;
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</programlisting>
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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There is no <command>LOCK TABLE</command> in the SQL standard,
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which instead uses <command>SET TRANSACTION</command> to specify
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concurrency levels on transactions. <productname>PostgreSQL</productname> supports that too;
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see <xref linkend="SQL-SET-TRANSACTION"> for details.
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</para>
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<para>
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Except for <literal>ACCESS SHARE</>, <literal>ACCESS EXCLUSIVE</>,
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and <literal>SHARE UPDATE EXCLUSIVE</> lock modes, the
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<productname>PostgreSQL</productname> lock modes and the
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<command>LOCK TABLE</command> syntax are compatible with those
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present in <productname>Oracle</productname>.
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</para>
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</refsect1>
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</refentry>
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