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14.3. Test Evaluation

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 time zone support. 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 may well 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. (Or you can run diff yourself, if you prefer.)

14.3.1. Error message differences

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 may 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.

14.3.2. Locale differences

The tests expect to run in plain "C" locale. This should not cause any problems when you run the tests against a temporary installation, since the regression test driver takes care to start the server in C locale. However, if you run the tests against an already-installed server that is using non-C locale settings, you may see differences caused by varying rules for string sort order, formatting of numeric and monetary values, and so forth.

In some locales the resulting differences are small and easily checked by inspection. However, in a locale that changes the rules for formatting of numeric values (typically by swapping the usage of commas and decimal points), entry of some data values will fail, resulting in extensive differences later in the tests where the missing data values are supposed to be used.

14.3.3. Date and time differences

Some of the queries in the timestamp test will fail if you run the test on the day of a daylight-saving time changeover, or the day before or after one. These queries assume that the intervals between midnight yesterday, midnight today and midnight tomorrow are exactly twenty-four hours -- which is wrong if daylight-saving time went into or out of effect meanwhile.

Most of the date and time results are dependent on the time zone environment. The reference files are generated for time zone PST8PDT (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 PGTZ to PST8PDT, which normally ensures proper results. However, your system must provide library support for the PST8PDT time zone, or the time zone-dependent tests will fail. To verify that your machine does have this support, type the following:

$ env TZ=PST8PDT date

The command above should have returned the current system time in the PST8PDT time zone. If the PST8PDT database is not available, then your system may have returned the time in GMT. If the PST8PDT time zone is not available, you can set the time zone rules explicitly:

PGTZ='PST8PDT7,M04.01.0,M10.05.03'; export PGTZ

There appear to be some systems that do not accept the recommended syntax for explicitly setting the local time zone rules; you may need to use a different PGTZ setting on such machines.

Some systems using older time zone libraries fail to apply daylight-saving corrections to dates before 1970, causing pre-1970 PDT times to be displayed in PST instead. This will result in localized differences in the test results.

14.3.4. Floating-point differences

Some of the tests involve computing 64-bit (double precision) numbers 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 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.

Some systems signal errors from pow() and exp() differently from the mechanism expected by the current PostgreSQL code.

14.3.5. Polygon differences

Several of the tests involve operations on geographic data about the Oakland/Berkeley, California street map. The map data is expressed as polygons whose vertices are represented as pairs of double precision numbers (decimal latitude and longitude). Initially, some tables are created and loaded with geographic data, then some views are created that join two tables using the polygon intersection operator (##), then a select is done on the view.

When comparing the results from different platforms, differences occur in the 2nd or 3rd place to the right of the decimal point. The SQL statements where these problems occur are the following:

SELECT * from street;
SELECT * from iexit;

14.3.6. Row ordering differences

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 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 ORDER BY isn't a problem. Some queries do exhibit cross-platform ordering differences, however. (Ordering differences can also be triggered by non-C locale settings.)

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. But please report it anyway, so that we can add an ORDER BY to that particular query and thereby eliminate the bogus "failure" in future releases.

You might wonder why we don't order all the regress 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.

14.3.7. The "random" test

There is at least one case in the "random" test script that is intended to produce random results. This causes random to fail the regression test once in a while (perhaps once in every five to ten trials). 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 always fails in repeated attempts. (On the other hand, if the random test is never reported to fail even in many trials of the regression tests, you probably should worry.)

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