regress.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

    </para>
   </sect2>
    
   <sect2>
    <title>Date and time differences</title>

    <para>
     Most of the date and time results are dependent on the time zone
     environment.  The reference files are generated for time zone
     <literal>PST8PDT</literal> (Berkeley, California), and there will be
     apparent failures if the tests are not run with that time zone setting.
     The regression test driver sets environment variable
     <envar>PGTZ</envar> to <literal>PST8PDT</literal>, which normally
     ensures proper results.
    </para>
   </sect2>
    
   <sect2>
    <title>Floating-point differences</title>
      
    <para>
     Some of the tests involve computing 64-bit floating-point numbers (<type>double
     precision</type>) from table columns. Differences in
     results involving mathematical functions of <type>double
     precision</type> columns have been observed.  The <literal>float8</> and
     <literal>geometry</> tests are particularly prone to small differences
     across platforms, or even with different compiler optimization options.
     Human eyeball comparison is needed to determine the real
     significance of these differences which are usually 10 places to
     the right of the decimal point.
    </para>

    <para>
     Some systems display minus zero as <literal>-0</>, while others
     just show <literal>0</>.
    </para>

    <para>
     Some systems signal errors from <function>pow()</function> and
     <function>exp()</function> differently from the mechanism
     expected by the current <productname>PostgreSQL</productname>
     code.
    </para>
   </sect2>

   <sect2>
    <title>Row ordering differences</title>
      
    <para>
You might see differences in which the same rows are output in a
different order than what appears in the expected file.  In most cases
this is not, strictly speaking, a bug.  Most of the regression test
scripts are not so pedantic as to use an <literal>ORDER BY</> for every single
<literal>SELECT</>, and so their result row orderings are not well-defined
according to the letter of the SQL specification.  In practice, since we are
looking at the same queries being executed on the same data by the same
software, we usually get the same result ordering on all platforms, and
so the lack of <literal>ORDER BY</> isn't a problem.  Some queries do exhibit
cross-platform ordering differences, however.  When testing against an
already-installed server, ordering differences can also be caused by
non-C locale settings or non-default parameter settings, such as custom values
of <varname>work_mem</> or the planner cost parameters.
    </para>

    <para>
Therefore, if you see an ordering difference, it's not something to
worry about, unless the query does have an <literal>ORDER BY</> that your
result is violating.  But please report it anyway, so that we can add an
<literal>ORDER BY</> to that particular query and thereby eliminate the bogus
<quote>failure</quote> in future releases.
    </para>

    <para>
You might wonder why we don't order all the regression test queries explicitly
to get rid of this issue once and for all.  The reason is that that would
make the regression tests less useful, not more, since they'd tend
to exercise query plan types that produce ordered results to the
exclusion of those that don't.
    </para>
   </sect2>

   <sect2>
    <title>Insufficient stack depth</title>
      
    <para>
     If the <literal>errors</literal> test results in a server crash
     at the <literal>select infinite_recurse()</> command, it means that
     the platform's limit on process stack size is smaller than the
     <xref linkend="guc-max-stack-depth"> parameter indicates.  This
     can be fixed by running the postmaster under a higher stack
     size limit (4MB is recommended with the default value of 
     <varname>max_stack_depth</>).  If you are unable to do that, an
     alternative is to reduce the value of <varname>max_stack_depth</>.
    </para>
   </sect2>

   <sect2>
    <title>The <quote>random</quote> test</title>
      
    <para>
     The <literal>random</literal> test script is intended to produce 
     random results.   In rare cases, this causes the random regression
     test to fail.  Typing
<programlisting>
diff results/random.out expected/random.out
</programlisting>
     should produce only one or a few lines of differences.  You need
     not worry unless the random test fails repeatedly.
    </para>
   </sect2>
  </sect1>

<!-- We might want to move the following section into the developer's guide. -->
  <sect1 id="regress-variant">
   <title>Variant Comparison Files</title>

   <para>
    Since some of the tests inherently produce environment-dependent
    results, we have provided ways to specify alternative <quote>expected</>
    result files.  Each regression test can have several comparison files
    showing possible results on different platforms.  There are two
    independent mechanisms for determining which comparison file is used
    for each test.
   </para>

   <para>
    The first mechanism allows comparison files to be selected for
    specific platforms.  There is a mapping file,
    <filename>src/test/regress/resultmap</filename>, that defines
    which comparison file to use for each platform.
    To eliminate bogus test <quote>failures</quote> for a particular platform,
    you first choose or make a variant result file, and then add a line to the
    <filename>resultmap</filename> file.
   </para>

   <para>
    Each line in the mapping file is of the form
<synopsis>
testname/platformpattern=comparisonfilename
</synopsis>
    The test name is just the name of the particular regression test
    module.  The platform pattern is a pattern in the style of the Unix
    tool <command>expr</> (that is, a regular expression with an implicit
    <literal>^</literal> anchor
    at the start).  It is matched against the platform name as printed
    by <command>config.guess</command> followed by
    <literal>:gcc</literal> or <literal>:cc</literal>, depending on
    whether you use the GNU compiler or the system's native compiler
    (on systems where there is a difference).  The comparison file
    name is the base name of the substitute result comparison file.
   </para>

   <para>
    For example: some systems interpret very small floating-point values
    as zero, rather than reporting an underflow error.  This causes a
    few differences in the <filename>float8</> regression test.
    Therefore, we provide a variant comparison file,
    <filename>float8-small-is-zero.out</filename>, which includes
    the results to be expected on these systems.  To silence the bogus
    <quote>failure</quote> message on <systemitem>OpenBSD</systemitem>
    platforms, <filename>resultmap</filename> includes
<programlisting>
float8/i.86-.*-openbsd=float8-small-is-zero
</programlisting>
    which will trigger on any machine for which the output of
    <command>config.guess</command> matches <literal>i.86-.*-openbsd</literal>.
    Other lines
    in <filename>resultmap</> select the variant comparison file for other
    platforms where it's appropriate.
   </para>

   <para>
    The second selection mechanism for variant comparison files is
    much more automatic: it simply uses the <quote>best match</> among
    several supplied comparison files.  The regression test driver
    script considers both the standard comparison file for a test,
    <literal><replaceable>testname</>.out</>, and variant files named
    <literal><replaceable>testname</>_<replaceable>digit</>.out</>
    (where the <replaceable>digit</> is any single digit
    <literal>0</>-<literal>9</>).  If any such file is an exact match,
    the test is considered to pass; otherwise, the one that generates
    the shortest diff is used to create the failure report.  (If
    <filename>resultmap</filename> includes an entry for the particular
    test, then the base <replaceable>testname</> is the substitute
    name given in <filename>resultmap</filename>.)
   </para>

   <para>
    For example, for the <literal>char</literal> test, the comparison file
    <filename>char.out</filename> contains results that are expected
    in the <literal>C</> and <literal>POSIX</> locales, while
    the file <filename>char_1.out</filename> contains results sorted as
    they appear in many other locales.
   </para>

   <para>
    The best-match mechanism was devised to cope with locale-dependent
    results, but it can be used in any situation where the test results
    cannot be predicted easily from the platform name alone.  A limitation of
    this mechanism is that the test driver cannot tell which variant is
    actually <quote>correct</> for the current environment; it will just pick
    the variant that seems to work best.  Therefore it is safest to use this
    mechanism only for variant results that you are willing to consider
    equally valid in all contexts.
   </para>
    
  </sect1>
  
</chapter>

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