From b2cbced9eef20692b51a84d68d469627f4fc43ac Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 16 Oct 2014 15:22:10 -0400 Subject: Support timezone abbreviations that sometimes change. Up to now, PG has assumed that any given timezone abbreviation (such as "EDT") represents a constant GMT offset in the usage of any particular region; we had a way to configure what that offset was, but not for it to be changeable over time. But, as with most things horological, this view of the world is too simplistic: there are numerous regions that have at one time or another switched to a different GMT offset but kept using the same timezone abbreviation. Almost the entire Russian Federation did that a few years ago, and later this month they're going to do it again. And there are similar examples all over the world. To cope with this, invent the notion of a "dynamic timezone abbreviation", which is one that is referenced to a particular underlying timezone (as defined in the IANA timezone database) and means whatever it currently means in that zone. For zones that use or have used daylight-savings time, the standard and DST abbreviations continue to have the property that you can specify standard or DST time and get that time offset whether or not DST was theoretically in effect at the time. However, the abbreviations mean what they meant at the time in question (or most recently before that time) rather than being absolutely fixed. The standard abbreviation-list files have been changed to use this behavior for abbreviations that have actually varied in meaning since 1970. The old simple-numeric definitions are kept for abbreviations that have not changed, since they are a bit faster to resolve. While this is clearly a new feature, it seems necessary to back-patch it into all active branches, because otherwise use of Russian zone abbreviations is going to become even more problematic than it already was. This change supersedes the changes in commit 513d06ded et al to modify the fixed meanings of the Russian abbreviations; since we've not shipped that yet, this will avoid an undesirably incompatible (not to mention incorrect) change in behavior for timestamps between 2011 and 2014. This patch makes some cosmetic changes in ecpglib to keep its usage of datetime lookup tables as similar as possible to the backend code, but doesn't do anything about the increasingly obsolete set of timezone abbreviation definitions that are hard-wired into ecpglib. Whatever we do about that will likely not be appropriate material for back-patching. Also, a potential free() of a garbage pointer after an out-of-memory failure in ecpglib has been fixed. This patch also fixes pre-existing bugs in DetermineTimeZoneOffset() that caused it to produce unexpected results near a timezone transition, if both the "before" and "after" states are marked as standard time. We'd only ever thought about or tested transitions between standard and DST time, but that's not what's happening when a zone simply redefines their base GMT offset. In passing, update the SGML documentation to refer to the Olson/zoneinfo/ zic timezone database as the "IANA" database, since it's now being maintained under the auspices of IANA. --- doc/src/sgml/config.sgml | 6 +++--- doc/src/sgml/datatype.sgml | 39 ++++++++++++++++++++++++++++----------- doc/src/sgml/datetime.sgml | 35 ++++++++++++++++++++--------------- doc/src/sgml/installation.sgml | 2 +- 4 files changed, 52 insertions(+), 30 deletions(-) (limited to 'doc/src') diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 949443931cd..6ee17d84772 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -6003,9 +6003,9 @@ SET XML OPTION { DOCUMENT | CONTENT }; Sets the collection of time zone abbreviations that will be accepted by the server for datetime input. The default is 'Default', which is a collection that works in most of the world; there are - also 'Australia' and 'India', and other collections can be defined - for a particular installation. See for more information. + also 'Australia' and 'India', + and other collections can be defined for a particular installation. + See for more information. diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml index 3e83dbbe4c0..223ba6ade8e 100644 --- a/doc/src/sgml/datatype.sgml +++ b/doc/src/sgml/datatype.sgml @@ -2325,7 +2325,7 @@ January 8 04:05:06 1999 PST but continue to be prone to arbitrary changes, particularly with respect to daylight-savings rules. PostgreSQL uses the widely-used - zoneinfo (Olson) time zone database for information about + IANA (Olson) time zone database for information about historical time zone rules. For times in the future, the assumption is that the latest known rules for a given time zone will continue to be observed indefinitely far into the future. @@ -2390,8 +2390,8 @@ January 8 04:05:06 1999 PST The recognized time zone names are listed in the pg_timezone_names view (see ). - PostgreSQL uses the widely-used - zoneinfo time zone data for this purpose, so the same + PostgreSQL uses the widely-used IANA + time zone data for this purpose, so the same time zone names are also recognized by much other software. @@ -2427,7 +2427,7 @@ January 8 04:05:06 1999 PST When a daylight-savings zone abbreviation is present, it is assumed to be used according to the same daylight-savings transition rules used in the - zoneinfo time zone database's posixrules entry. + IANA time zone database's posixrules entry. In a standard PostgreSQL installation, posixrules is the same as US/Eastern, so that POSIX-style time zone specifications follow USA daylight-savings @@ -2438,9 +2438,25 @@ January 8 04:05:06 1999 PST In short, this is the difference between abbreviations - and full names: abbreviations always represent a fixed offset from - UTC, whereas most of the full names imply a local daylight-savings time - rule, and so have two possible UTC offsets. + and full names: abbreviations represent a specific offset from UTC, + whereas many of the full names imply a local daylight-savings time + rule, and so have two possible UTC offsets. As an example, + 2014-06-04 12:00 America/New_York represents noon local + time in New York, which for this particular date was Eastern Daylight + Time (UTC-4). So 2014-06-04 12:00 EDT specifies that + same time instant. But 2014-06-04 12:00 EST specifies + noon Eastern Standard Time (UTC-5), regardless of whether daylight + savings was nominally in effect on that date. + + + + To complicate matters, some jurisdictions have used the same timezone + abbreviation to mean different UTC offsets at different times; for + example, in Moscow MSK has meant UTC+3 in some years and + UTC+4 in others. PostgreSQL interprets such + abbreviations according to whatever they meant (or had most recently + meant) on the specified date; but, as with the EST example + above, this is not necessarily the same as local civil time on that date. @@ -2457,13 +2473,14 @@ January 8 04:05:06 1999 PST - In all cases, timezone names are recognized case-insensitively. - (This is a change from PostgreSQL versions - prior to 8.2, which were case-sensitive in some contexts but not others.) + In all cases, timezone names and abbreviations are recognized + case-insensitively. (This is a change from PostgreSQL + versions prior to 8.2, which were case-sensitive in some contexts but + not others.) - Neither full names nor abbreviations are hard-wired into the server; + Neither timezone names nor abbreviations are hard-wired into the server; they are obtained from configuration files stored under .../share/timezone/ and .../share/timezonesets/ of the installation directory diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml index 444b0ec2b93..ffd07151282 100644 --- a/doc/src/sgml/datetime.sgml +++ b/doc/src/sgml/datetime.sgml @@ -374,22 +374,27 @@ these formats: -time_zone_name offset -time_zone_name offset D +zone_abbreviation offset +zone_abbreviation offset D +zone_abbreviation time_zone_name @INCLUDE file_name @OVERRIDE - A time_zone_name is just the abbreviation - being defined. The offset is the zone's + A zone_abbreviation is just the abbreviation + being defined. The offset is the equivalent offset in seconds from UTC, positive being east from Greenwich and negative being west. For example, -18000 would be five hours west of Greenwich, or North American east coast standard time. D - indicates that the zone name represents local daylight-savings time - rather than standard time. Since all known time zone offsets are on - 15 minute boundaries, the number of seconds has to be a multiple of 900. + indicates that the zone name represents local daylight-savings time rather + than standard time. Alternatively, a time_zone_name can + be given, in which case that time zone definition is consulted, and the + abbreviation's meaning in that zone is used. This alternative is + recommended only for abbreviations whose meaning has historically varied, + as looking up the meaning is noticeably more expensive than just using + a fixed integer value. @@ -400,9 +405,9 @@ The @OVERRIDE syntax indicates that subsequent entries in the - file can override previous entries (i.e., entries obtained from included - files). Without this, conflicting definitions of the same timezone - abbreviation are considered an error. + file can override previous entries (typically, entries obtained from + included files). Without this, conflicting definitions of the same + timezone abbreviation are considered an error. @@ -410,14 +415,14 @@ all the non-conflicting time zone abbreviations for most of the world. Additional files Australia and India are provided for those regions: these files first include the - Default file and then add or modify timezones as needed. + Default file and then add or modify abbreviations as needed. For reference purposes, a standard installation also contains files Africa.txt, America.txt, etc, containing information about every time zone abbreviation known to be in use - according to the zoneinfo timezone database. The zone name + according to the IANA timezone database. The zone name definitions found in these files can be copied and pasted into a custom configuration file as needed. Note that these files cannot be directly referenced as timezone_abbreviations settings, because of @@ -426,9 +431,9 @@ - If an error occurs while reading the time zone data sets, no new value is - applied but the old set is kept. If the error occurs while starting the - database, startup fails. + If an error occurs while reading the time zone abbreviation set, no new + value is applied and the old set is kept. If the error occurs while + starting the database, startup fails. diff --git a/doc/src/sgml/installation.sgml b/doc/src/sgml/installation.sgml index a3c9bea3665..68931d25b66 100644 --- a/doc/src/sgml/installation.sgml +++ b/doc/src/sgml/installation.sgml @@ -1108,7 +1108,7 @@ su - postgres PostgreSQL includes its own time zone database, which it requires for date and time operations. This time zone - database is in fact compatible with the zoneinfo time zone + database is in fact compatible with the IANA time zone database provided by many operating systems such as FreeBSD, Linux, and Solaris, so it would be redundant to install it again. When this option is used, the system-supplied time zone database -- cgit v1.2.3