PostgreSQL 7.4.11 Documentation
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# 26.2. 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.)

## 26.2.1. 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.

## 26.2.2. Locale differences

If you run the tests against an already-installed server that was initialized with a collation-order locale other than C, then there may be differences due to sort order and follow-up failures. The regression test suite is set up to handle this problem by providing alternative result files that together are known to handle a large number of locales. For example, for the `char` test, the expected file `char.out` handles the `C` and `POSIX` locales, and the file `char_1.out` handles many other locales. The regression test driver will automatically pick the best file to match against when checking for success and for computing failure differences. (This means that the regression tests cannot detect whether the results are appropriate for the configured locale. The tests will simply pick the one result file that works best.)

If for some reason the existing expected files do not cover some locale, you can add a new file. The naming scheme is ``testname`_`digit`.out`. The actual digit is not significant. Remember that the regression test driver will consider all such files to be equally valid test results. If the test results are platform-specific, the technique described in Section 26.3 should be used instead.

## 26.2.3. Date and time differences

A few of the queries in the `horology` test will fail if you run the test on the day of a daylight-saving time changeover, or the day after one. These queries expect 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.

> Note: Because USA daylight-saving time rules are used, this problem always occurs on the first Sunday of April, the last Sunday of October, and their following Mondays, regardless of when daylight-saving time is in effect where you live. Also note that the problem appears or disappears at midnight Pacific time (UTC-7 or UTC-8), not midnight your local time. Thus the failure may appear late on Saturday or persist through much of Tuesday, depending on where you live.

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 operating system must provide 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` time zone is not available, then your system may have returned the time in UTC. If the `PST8PDT` time zone is missing, 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.

## 26.2.4. Floating-point differences

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 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 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.

## 26.2.5. 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 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.

## 26.2.6. 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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