Some properly installed and fully functional
    PostgreSQL installations can
    “fail” some of these regression tests due to
    platform-specific artifacts such as varying floating-point representation
    and message wording. The tests are currently evaluated using a simple
    diff comparison against the outputs
    generated on a reference system, so the results are sensitive to
    small system differences.  When a test is reported as
    “failed”, always examine the differences between
    expected and actual results; you might find that the
    differences are not significant.  Nonetheless, we still strive to
    maintain accurate reference files across all supported platforms,
    so it can be expected that all tests pass.
   
    The actual outputs of the regression tests are in files in the
    src/test/regress/results directory. The test
    script uses diff to compare each output
    file against the reference outputs stored in the
    src/test/regress/expected directory.  Any
    differences are saved for your inspection in
    src/test/regress/regression.diffs.
    (When running a test suite other than the core tests, these files
    of course appear in the relevant subdirectory,
    not src/test/regress.)
   
    If you don't
    like the diff options that are used by default, set the
    environment variable PG_REGRESS_DIFF_OPTS, for
    instance PG_REGRESS_DIFF_OPTS='-c'.  (Or you
    can run diff yourself, if you prefer.)
   
If for some reason a particular platform generates a “failure” for a given test, but inspection of the output convinces you that the result is valid, you can add a new comparison file to silence the failure report in future test runs. See Section 33.3 for details.
Some of the regression tests involve intentional invalid input values. Error messages can come from either the PostgreSQL code or from the host platform system routines. In the latter case, the messages can vary between platforms, but should reflect similar information. These differences in messages will result in a “failed” regression test that can be validated by inspection.
If you run the tests against a server that was initialized with a collation-order locale other than C, then there might be differences due to sort order and subsequent failures. The regression test suite is set up to handle this problem by providing alternate result files that together are known to handle a large number of locales.
     To run the tests in a different locale when using the
     temporary-installation method, pass the appropriate
     locale-related environment variables on
     the make command line, for example:
make check LANG=de_DE.utf8
     (The regression test driver unsets LC_ALL, so it
     does not work to choose the locale using that variable.)  To use
     no locale, either unset all locale-related environment variables
     (or set them to C) or use the following
     special invocation:
make check NO_LOCALE=1
     When running the tests against an existing installation, the
     locale setup is determined by the existing installation.  To
     change it, initialize the database cluster with a different
     locale by passing the appropriate options
     to initdb.
    
In general, it is advisable to try to run the regression tests in the locale setup that is wanted for production use, as this will exercise the locale- and encoding-related code portions that will actually be used in production. Depending on the operating system environment, you might get failures, but then you will at least know what locale-specific behaviors to expect when running real applications.
     Most of the date and time results are dependent on the time zone
     environment.  The reference files are generated for time zone
     America/Los_Angeles, and there will be
     apparent failures if the tests are not run with that time zone setting.
     The regression test driver sets environment variable
     PGTZ to America/Los_Angeles,
     which normally ensures proper results.
    
     Some of the tests involve computing 64-bit floating-point numbers (double
     precision) from table columns. Differences in
     results involving mathematical functions of double
     precision columns have been observed.  The float8 and
     geometry tests are particularly prone to small differences
     across platforms, or even with different compiler optimization settings.
     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.
    
     Some systems display minus zero as -0, while others
     just show 0.
    
     Some systems signal errors from pow() and
     exp() differently from the mechanism
     expected by the current PostgreSQL
     code.
    
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 ORDER BY for every single
SELECT, and so their result row orderings are not well-defined
according to 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,
so the lack of ORDER BY is not 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 work_mem or the planner cost parameters.
    
Therefore, if you see an ordering difference, it's not something to
worry about, unless the query does have an ORDER BY that your
result is violating.  However, please report it anyway, so that we can add an
ORDER BY to that particular query to eliminate the bogus
“failure” in future releases.
    
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.
     If the errors test results in a server crash
     at the select infinite_recurse() command, it means that
     the platform's limit on process stack size is smaller than the
     max_stack_depth parameter indicates.  This
     can be fixed by running the server under a higher stack
     size limit (4MB is recommended with the default value of
     max_stack_depth).  If you are unable to do that, an
     alternative is to reduce the value of max_stack_depth.
    
     On platforms supporting getrlimit(), the server should
     automatically choose a safe value of max_stack_depth;
     so unless you've manually overridden this setting, a failure of this
     kind is a reportable bug.
    
     The random test script is intended to produce
     random results.   In very rare cases, this causes that regression
     test to fail.  Typing:
diff results/random.out expected/random.out
should produce only one or a few lines of differences. You need not worry unless the random test fails repeatedly.
     When running the tests against an existing installation, some non-default
     parameter settings could cause the tests to fail.  For example, changing
     parameters such as enable_seqscan or
     enable_indexscan could cause plan changes that would
     affect the results of tests that use EXPLAIN.