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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
98 lines
2.9 KiB
Plaintext
98 lines
2.9 KiB
Plaintext
<!--
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doc/src/sgml/ref/start_transaction.sgml
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PostgreSQL documentation
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-->
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<refentry id="SQL-START-TRANSACTION">
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<refmeta>
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<refentrytitle>START TRANSACTION</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>START TRANSACTION</refname>
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<refpurpose>start a transaction block</refpurpose>
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</refnamediv>
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<indexterm zone="sql-start-transaction">
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<primary>START TRANSACTION</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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START TRANSACTION [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ]
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<phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
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ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
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READ WRITE | READ ONLY
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[ NOT ] DEFERRABLE
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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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This command begins a new transaction block. If the isolation level,
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read/write mode, or deferrable mode is specified, the new transaction has those
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characteristics, as if <xref linkend="sql-set-transaction"> was executed. This is the same
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as the <xref linkend="sql-begin"> command.
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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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<para>
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Refer to <xref linkend="sql-set-transaction"> for information on the meaning
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of the parameters to this statement.
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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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In the standard, it is not necessary to issue <command>START TRANSACTION</>
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to start a transaction block: any SQL command implicitly begins a block.
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<productname>PostgreSQL</productname>'s behavior can be seen as implicitly
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issuing a <command>COMMIT</command> after each command that does not
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follow <command>START TRANSACTION</> (or <command>BEGIN</command>),
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and it is therefore often called <quote>autocommit</>.
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Other relational database systems might offer an autocommit feature
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as a convenience.
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</para>
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<para>
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The <literal>DEFERRABLE</literal>
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<replaceable class="parameter">transaction_mode</replaceable>
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is a <productname>PostgreSQL</productname> language extension.
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</para>
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<para>
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The SQL standard requires commas between successive <replaceable
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class="parameter">transaction_modes</replaceable>, but for historical
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reasons <productname>PostgreSQL</productname> allows the commas to be
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omitted.
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</para>
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<para>
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See also the compatibility section of <xref linkend="sql-set-transaction">.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-begin"></member>
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<member><xref linkend="sql-commit"></member>
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<member><xref linkend="sql-rollback"></member>
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<member><xref linkend="sql-savepoint"></member>
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<member><xref linkend="sql-set-transaction"></member>
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</simplelist>
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</refsect1>
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</refentry>
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