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docs: add "serialization anomaly" to transaction isolation table

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Also distinguish between SQL-standard and Postgres behavior.

Report by David G. Johnston
This commit is contained in:
Bruce Momjian 2015-05-11 12:02:10 -04:00
parent c71e273402
commit 23c33198b9
1 changed files with 48 additions and 19 deletions
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67
doc/src/sgml/mvcc.sgml
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@ -143,6 +143,20 @@
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
serialization anomaly
<indexterm><primary>serialization anomaly</primary></indexterm>
</term>
<listitem>
<para>
The result of successfully committing a group of transactions
is inconsistent with all possible orderings of running those
transactions one at a time.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
@ -150,13 +164,13 @@
<indexterm>
<primary>transaction isolation level</primary>
</indexterm>
The four transaction isolation levels and the corresponding
behaviors are described in <xref linkend="mvcc-isolevel-table">.
The SQL standard and PostgreSQL-implemented transaction isolation levels
are described in <xref linkend="mvcc-isolevel-table">.
</para>
<table tocentry="1" id="mvcc-isolevel-table">
<title>Standard <acronym>SQL</acronym> Transaction Isolation Levels</title>
<tgroup cols="4">
<title>Transaction Isolation Levels</title>
<tgroup cols="5">
<thead>
<row>
<entry>
@ -171,6 +185,9 @@
<entry>
Phantom Read
</entry>
<entry>
Serialization Anomaly
</entry>
</row>
</thead>
<tbody>
@ -178,6 +195,9 @@
<entry>
Read uncommitted
</entry>
<entry>
Allowed, but not in PG
</entry>
<entry>
Possible
</entry>
@ -202,6 +222,9 @@
<entry>
Possible
</entry>
<entry>
Possible
</entry>
</row>
<row>
@ -214,6 +237,9 @@
<entry>
Not possible
</entry>
<entry>
Allowed, but not in PG
</entry>
<entry>
Possible
</entry>
@ -232,27 +258,30 @@
<entry>
Not possible
</entry>
<entry>
Not possible
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
In <productname>PostgreSQL</productname>, you can request any of the
four standard transaction isolation levels. But internally, there are
only three distinct isolation levels, which correspond to the levels Read
Committed, Repeatable Read, and Serializable. When you select the level Read
Uncommitted you really get Read Committed, and phantom reads are not possible
in the <productname>PostgreSQL</productname> implementation of Repeatable
Read, so the actual
isolation level might be stricter than what you select. This is
permitted by the SQL standard: the four isolation levels only
define which phenomena must not happen, they do not define which
phenomena must happen. The reason that <productname>PostgreSQL</>
only provides three isolation levels is that this is the only
sensible way to map the standard isolation levels to the multiversion
concurrency control architecture. The behavior of the available
isolation levels is detailed in the following subsections.
In <productname>PostgreSQL</productname>, you can request any of
the four standard transaction isolation levels, but internally only
three distinct isolation levels are implemented, i.e. PostgreSQL's
Read Uncommitted mode behaves like Read Committed. This is because
it is the only sensible way to map the standard isolation levels to
PostgreSQL's multiversion concurrency control architecture.
</para>
<para>
The table also shows that PostgreSQL's Repeatable Read implementation
does not allow phantom reads. Stricter behavior is permitted by the
SQL standard: the four isolation levels only define which phenomena
must not happen, not which phenomena <emphasis>must</> happen.
The behavior of the available isolation levels is detailed in the
following subsections.
</para>
<para>