PostgreSQL
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# Regression Analysis

The actual outputs of the regression tests are in files in the `./results` directory. The test script uses diff to compare each output file against the reference outputs stored in the `./expected` directory. Any differences are saved for your inspection in `./regression.diffs`. (Or you can run diff yourself, if you prefer.)

The files might not compare exactly. The test script will report any difference as a "failure", but the difference might be due to small cross-system differences in error message wording, math library behavior, etc. "Failures" of this type do not indicate a problem with Postgres.

Thus, it is necessary to examine the actual differences for each "failed" test to determine whether there is really a problem. The following paragraphs attempt to provide some guidance in determining whether a difference is significant or not.

## Error message differences

Some of the regression tests involve intentional invalid input values. Error messages can come from either the Postgres 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 which can be validated by inspection.

## Date and time differences

Most of the date and time results are dependent on timezone environment. The reference files are generated for timezone PST8PDT (Berkeley, California) and there will be apparent failures if the tests are not run with that timezone setting. The regression test driver sets environment variable PGTZ to PST8PDT to ensure proper results.

Some of the queries in the "timestamp" test will fail if you run the test on the day of a daylight-savings 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-savings time went into or out of effect meanwhile.

There appear to be some systems which 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 timezone libraries fail to apply daylight-savings corrections to pre-1970 dates, causing pre-1970 PDT times to be displayed in PST instead. This will result in localized differences in the test results.

## Floating point differences

Some of the tests involve computing 64-bit (float8) numbers from table columns. Differences in results involving mathematical functions of float8 columns have been observed. The float8 and geometry tests are particularly prone to small differences across platforms. 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 Postgres code.

## Polygon differences

Several of the tests involve operations on geographic date about the Oakland/Berkley CA street map. The map data is expressed as polygons whose vertices are represented as pairs of float8 numbers (decimal latitude and longitude). Initially, some tables are created and loaded with geographic data, then some views are created which 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:

    	  QUERY: SELECT * from street;
    	  QUERY: SELECT * from iexit;

## Random differences

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 regress tests, you probably should worry.)

## The "expected" files

The `./expected/*.out` files were adapted from the original monolithic `expected.input` file provided by Jolly Chen et al. Newer versions of these files generated on various development machines have been substituted after careful (?) inspection. Many of the development machines are running a Unix OS variant (FreeBSD, Linux, etc) on Ix86 hardware. The original `expected.input` file was created on a SPARC Solaris 2.4 system using the `postgres5-1.02a5.tar.gz` source tree. It was compared with a file created on an I386 Solaris 2.4 system and the differences were only in the floating point polygons in the 3rd digit to the right of the decimal point. The original `sample.regress.out` file was from the postgres-1.01 release constructed by Jolly Chen. It may have been created on a DEC ALPHA machine as the `Makefile.global` in the postgres-1.01 release has PORTNAME=alpha.

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