LightDB supports the full set of SQL date and time types, shown in Table 8.9. The operations available on these data types are described in Section 9.9. Dates are counted according to the Gregorian calendar, even in years before that calendar was introduced (see Section B.6 for more information).
Table 8.9. Date/Time Types
Name | Storage Size | Description | Low Value | High Value | Resolution |
---|---|---|---|---|---|
timestamp [ ( | 8 bytes | both date and time (no time zone) | 4713 BC | 294276 AD | 1 microsecond |
timestamp [ ( | 8 bytes | both date and time, with time zone | 4713 BC | 294276 AD | 1 microsecond |
date | 4 bytes | date (no time of day) | 4713 BC | 5874897 AD | 1 day |
time [ ( | 8 bytes | time of day (no date) | 00:00:00 | 24:00:00 | 1 microsecond |
time [ ( | 12 bytes | time of day (no date), with time zone | 00:00:00+1559 | 24:00:00-1559 | 1 microsecond |
interval [ | 16 bytes | time interval | -178000000 years | 178000000 years | 1 microsecond |
The SQL standard requires that writing just timestamp
be equivalent to timestamp without time
zone
, and LightDB honors that
behavior. timestamptz
is accepted as an
abbreviation for timestamp with time zone
; this is a
LightDB extension.
time
, timestamp
, and
interval
accept an optional precision value
p
which specifies the number of
fractional digits retained in the seconds field. By default, there
is no explicit bound on precision. The allowed range of
p
is from 0 to 6.
The interval
type has an additional option, which is
to restrict the set of stored fields by writing one of these phrases:
YEAR MONTH DAY HOUR MINUTE SECOND YEAR TO MONTH DAY TO HOUR DAY TO MINUTE DAY TO SECOND HOUR TO MINUTE HOUR TO SECOND MINUTE TO SECOND
Note that if both fields
and
p
are specified, the
fields
must include SECOND
,
since the precision applies only to the seconds.
The type time with time zone
is defined by the SQL
standard, but the definition exhibits properties which lead to
questionable usefulness. In most cases, a combination of
date
, time
, timestamp without time
zone
, and timestamp with time zone
should
provide a complete range of date/time functionality required by
any application.
Date and time input is accepted in almost any reasonable format, including
ISO 8601, SQL-compatible,
traditional POSTGRES, and others.
For some formats, ordering of day, month, and year in date input is
ambiguous and there is support for specifying the expected
ordering of these fields. Set the DateStyle parameter
to MDY
to select month-day-year interpretation,
DMY
to select day-month-year interpretation, or
YMD
to select year-month-day interpretation.
LightDB is more flexible in handling date/time input than the SQL standard requires. See Appendix B for the exact parsing rules of date/time input and for the recognized text fields including months, days of the week, and time zones.
Remember that any date or time literal input needs to be enclosed in single quotes, like text strings. Refer to Section 4.1.2.7 for more information. SQL requires the following syntax
type
[ (p
) ] 'value
'
where p
is an optional precision
specification giving the number of
fractional digits in the seconds field. Precision can be
specified for time
, timestamp
, and
interval
types, and can range from 0 to 6.
If no precision is specified in a constant specification,
it defaults to the precision of the literal value (but not
more than 6 digits).
Table 8.10 shows some possible
inputs for the date
type.
Table 8.10. Date Input
Example | Description |
---|---|
1999-01-08 | ISO 8601; January 8 in any mode (recommended format) |
January 8, 1999 | unambiguous in any datestyle input mode |
1/8/1999 | January 8 in MDY mode;
August 1 in DMY mode |
1/18/1999 | January 18 in MDY mode;
rejected in other modes |
01/02/03 | January 2, 2003 in MDY mode;
February 1, 2003 in DMY mode;
February 3, 2001 in YMD mode
|
1999-Jan-08 | January 8 in any mode |
Jan-08-1999 | January 8 in any mode |
08-Jan-1999 | January 8 in any mode |
99-Jan-08 | January 8 in YMD mode, else error |
08-Jan-99 | January 8, except error in YMD mode |
Jan-08-99 | January 8, except error in YMD mode |
19990108 | ISO 8601; January 8, 1999 in any mode |
990108 | ISO 8601; January 8, 1999 in any mode |
1999.008 | year and day of year |
J2451187 | Julian date |
January 8, 99 BC | year 99 BC |
The time-of-day types are time [
(
and
p
) ] without time zonetime [ (
. p
) ] with time
zonetime
alone is equivalent to
time without time zone
.
Valid input for these types consists of a time of day followed
by an optional time zone. (See Table 8.11
and Table 8.12.) If a time zone is
specified in the input for time without time zone
,
it is silently ignored. You can also specify a date but it will
be ignored, except when you use a time zone name that involves a
daylight-savings rule, such as
America/New_York
. In this case specifying the date
is required in order to determine whether standard or daylight-savings
time applies. The appropriate time zone offset is recorded in the
time with time zone
value.
Table 8.11. Time Input
Example | Description |
---|---|
04:05:06.789 | ISO 8601 |
04:05:06 | ISO 8601 |
04:05 | ISO 8601 |
040506 | ISO 8601 |
04:05 AM | same as 04:05; AM does not affect value |
04:05 PM | same as 16:05; input hour must be <= 12 |
04:05:06.789-8 | ISO 8601 |
04:05:06-08:00 | ISO 8601 |
04:05-08:00 | ISO 8601 |
040506-08 | ISO 8601 |
04:05:06 PST | time zone specified by abbreviation |
2003-04-12 04:05:06 America/New_York | time zone specified by full name |
Table 8.12. Time Zone Input
Example | Description |
---|---|
PST | Abbreviation (for Pacific Standard Time) |
America/New_York | Full time zone name |
PST8PDT | POSIX-style time zone specification |
-8:00 | ISO-8601 offset for PST |
-800 | ISO-8601 offset for PST |
-8 | ISO-8601 offset for PST |
zulu | Military abbreviation for UTC |
z | Short form of zulu |
Refer to Section 8.5.3 for more information on how to specify time zones.
Valid input for the time stamp types consists of the concatenation
of a date and a time, followed by an optional time zone,
followed by an optional AD
or BC
.
(Alternatively, AD
/BC
can appear
before the time zone, but this is not the preferred ordering.)
Thus:
1999-01-08 04:05:06
and:
1999-01-08 04:05:06 -8:00
are valid values, which follow the ISO 8601 standard. In addition, the common format:
January 8 04:05:06 1999 PST
is supported.
The SQL standard differentiates
timestamp without time zone
and timestamp with time zone
literals by the presence of a
“+” or “-” symbol and time zone offset after
the time. Hence, according to the standard,
TIMESTAMP '2004-10-19 10:23:54'
is a timestamp without time zone
, while
TIMESTAMP '2004-10-19 10:23:54+02'
is a timestamp with time zone
.
LightDB never examines the content of a
literal string before determining its type, and therefore will treat
both of the above as timestamp without time zone
. To
ensure that a literal is treated as timestamp with time
zone
, give it the correct explicit type:
TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'
In a literal that has been determined to be timestamp without time
zone
, LightDB will silently ignore
any time zone indication.
That is, the resulting value is derived from the date/time
fields in the input value, and is not adjusted for time zone.
For timestamp with time zone
, the internally stored
value is always in UTC (Universal
Coordinated Time, traditionally known as Greenwich Mean Time,
GMT). An input value that has an explicit
time zone specified is converted to UTC using the appropriate offset
for that time zone. If no time zone is stated in the input string,
then it is assumed to be in the time zone indicated by the system's
TimeZone parameter, and is converted to UTC using the
offset for the timezone
zone.
When a timestamp with time
zone
value is output, it is always converted from UTC to the
current timezone
zone, and displayed as local time in that
zone. To see the time in another time zone, either change
timezone
or use the AT TIME ZONE
construct
(see Section 9.9.3).
Conversions between timestamp without time zone
and
timestamp with time zone
normally assume that the
timestamp without time zone
value should be taken or given
as timezone
local time. A different time zone can
be specified for the conversion using AT TIME ZONE
.
LightDB supports several
special date/time input values for convenience, as shown in Table 8.13. The values
infinity
and -infinity
are specially represented inside the system and will be displayed
unchanged; but the others are simply notational shorthands
that will be converted to ordinary date/time values when read.
(In particular, now
and related strings are converted
to a specific time value as soon as they are read.)
All of these values need to be enclosed in single quotes when used
as constants in SQL commands.
Table 8.13. Special Date/Time Inputs
Input String | Valid Types | Description |
---|---|---|
epoch | date , timestamp | 1970-01-01 00:00:00+00 (Unix system time zero) |
infinity | date , timestamp | later than all other time stamps |
-infinity | date , timestamp | earlier than all other time stamps |
now | date , time , timestamp | current transaction's start time |
today | date , timestamp | midnight (00:00 ) today |
tomorrow | date , timestamp | midnight (00:00 ) tomorrow |
yesterday | date , timestamp | midnight (00:00 ) yesterday |
allballs | time | 00:00:00.00 UTC |
The following SQL-compatible functions can also
be used to obtain the current time value for the corresponding data
type:
CURRENT_DATE
, CURRENT_TIME
,
CURRENT_TIMESTAMP
, LOCALTIME
,
LOCALTIMESTAMP
. (See Section 9.9.4.) Note that these are
SQL functions and are not recognized in data input strings.
While the input strings now
,
today
, tomorrow
,
and yesterday
are fine to use in interactive SQL
commands, they can have surprising behavior when the command is
saved to be executed later, for example in prepared statements,
views, and function definitions. The string can be converted to a
specific time value that continues to be used long after it becomes
stale. Use one of the SQL functions instead in such contexts.
For example, CURRENT_DATE + 1
is safer than
'tomorrow'::date
.
The output format of the date/time types can be set to one of the four
styles ISO 8601,
SQL (Ingres), traditional POSTGRES
(Unix date format), or
German. The default
is the ISO format. (The
SQL standard requires the use of the ISO 8601
format. The name of the “SQL” output format is a
historical accident.) Table 8.14 shows examples of each
output style. The output of the date
and
time
types is generally only the date or time part
in accordance with the given examples. However, the
POSTGRES style outputs date-only values in
ISO format.
Table 8.14. Date/Time Output Styles
Style Specification | Description | Example |
---|---|---|
ISO | ISO 8601, SQL standard | 1997-12-17 07:37:16-08 |
SQL | traditional style | 12/17/1997 07:37:16.00 PST |
Postgres | original style | Wed Dec 17 07:37:16 1997 PST |
German | regional style | 17.12.1997 07:37:16.00 PST |
ISO 8601 specifies the use of uppercase letter T
to separate
the date and time. LightDB accepts that format on
input, but on output it uses a space rather than T
, as shown
above. This is for readability and for consistency with RFC 3339 as
well as some other database systems.
In the SQL and POSTGRES styles, day appears before month if DMY field ordering has been specified, otherwise month appears before day. (See Section 8.5.1 for how this setting also affects interpretation of input values.) Table 8.15 shows examples.
Table 8.15. Date Order Conventions
datestyle Setting | Input Ordering | Example Output |
---|---|---|
SQL, DMY | day /month /year | 17/12/1997 15:37:16.00 CET |
SQL, MDY | month /day /year | 12/17/1997 07:37:16.00 PST |
Postgres, DMY | day /month /year | Wed 17 Dec 07:37:16 1997 PST |
The date/time style can be selected by the user using the
SET datestyle
command, the DateStyle parameter in the
lightdb.conf
configuration file, or the
LTDATESTYLE
environment variable on the server or
client.
The formatting function to_char
(see Section 9.8) is also available as
a more flexible way to format date/time output.
Time zones, and time-zone conventions, are influenced by political decisions, not just earth geometry. Time zones around the world became somewhat standardized during the 1900s, but continue to be prone to arbitrary changes, particularly with respect to daylight-savings rules. LightDB uses the widely-used 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.
LightDB endeavors to be compatible with the SQL standard definitions for typical usage. However, the SQL standard has an odd mix of date and time types and capabilities. Two obvious problems are:
Although the date
type
cannot have an associated time zone, the
time
type can.
Time zones in the real world have little meaning unless
associated with a date as well as a time,
since the offset can vary through the year with daylight-saving
time boundaries.
The default time zone is specified as a constant numeric offset from UTC. It is therefore impossible to adapt to daylight-saving time when doing date/time arithmetic across DST boundaries.
To address these difficulties, we recommend using date/time types
that contain both date and time when using time zones. We
do not recommend using the type time with
time zone
(though it is supported by
LightDB for legacy applications and
for compliance with the SQL standard).
LightDB assumes
your local time zone for any type containing only date or time.
All timezone-aware dates and times are stored internally in UTC. They are converted to local time in the zone specified by the TimeZone configuration parameter before being displayed to the client.
LightDB allows you to specify time zones in three different forms:
A full time zone name, for example America/New_York
.
The recognized time zone names are listed in the
pg_timezone_names
view (see Section 51.93).
LightDB uses the widely-used IANA
time zone data for this purpose, so the same time zone
names are also recognized by other software.
A time zone abbreviation, for example PST
. Such a
specification merely defines a particular offset from UTC, in
contrast to full time zone names which can imply a set of daylight
savings transition rules as well. The recognized abbreviations
are listed in the pg_timezone_abbrevs
view (see Section 51.92). You cannot set the
configuration parameters TimeZone or
log_timezone to a time
zone abbreviation, but you can use abbreviations in
date/time input values and with the AT TIME ZONE
operator.
In addition to the timezone names and abbreviations, LightDB will accept POSIX-style time zone specifications, as described in Section B.5. This option is not normally preferable to using a named time zone, but it may be necessary if no suitable IANA time zone entry is available.
In short, this is the difference between abbreviations
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. LightDB 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.
In all cases, timezone names and abbreviations are recognized case-insensitively. (This is a change from LightDB versions prior to 8.2, which were case-sensitive in some contexts but not others.)
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
(see Section B.4).
The TimeZone configuration parameter can
be set in the file lightdb.conf
, or in any of the
other standard ways described in Chapter 17.
There are also some special ways to set it:
The SQL command SET TIME ZONE
sets the time zone for the session. This is an alternative spelling
of SET TIMEZONE TO
with a more SQL-spec-compatible syntax.
The LTTZ
environment variable is used by
libpq clients
to send a SET TIME ZONE
command to the server upon connection.
interval
values can be written using the following
verbose syntax:
[@]quantity
unit
[quantity
unit
...] [direction
]
where quantity
is a number (possibly signed);
unit
is microsecond
,
millisecond
, second
,
minute
, hour
, day
,
week
, month
, year
,
decade
, century
, millennium
,
or abbreviations or plurals of these units;
direction
can be ago
or
empty. The at sign (@
) is optional noise. The amounts
of the different units are implicitly added with appropriate
sign accounting. ago
negates all the fields.
This syntax is also used for interval output, if
IntervalStyle is set to
postgres_verbose
.
Quantities of days, hours, minutes, and seconds can be specified without
explicit unit markings. For example, '1 12:59:10'
is read
the same as '1 day 12 hours 59 min 10 sec'
. Also,
a combination of years and months can be specified with a dash;
for example '200-10'
is read the same as '200 years
10 months'
. (These shorter forms are in fact the only ones allowed
by the SQL standard, and are used for output when
IntervalStyle
is set to sql_standard
.)
Interval values can also be written as ISO 8601 time intervals, using either the “format with designators” of the standard's section 4.4.3.2 or the “alternative format” of section 4.4.3.3. The format with designators looks like this:
Pquantity
unit
[quantity
unit
...] [ T [quantity
unit
...]]
The string must start with a P
, and may include a
T
that introduces the time-of-day units. The
available unit abbreviations are given in Table 8.16. Units may be
omitted, and may be specified in any order, but units smaller than
a day must appear after T
. In particular, the meaning of
M
depends on whether it is before or after
T
.
Table 8.16. ISO 8601 Interval Unit Abbreviations
Abbreviation | Meaning |
---|---|
Y | Years |
M | Months (in the date part) |
W | Weeks |
D | Days |
H | Hours |
M | Minutes (in the time part) |
S | Seconds |
In the alternative format:
P [years
-months
-days
] [ Thours
:minutes
:seconds
]
the string must begin with P
, and a
T
separates the date and time parts of the interval.
The values are given as numbers similar to ISO 8601 dates.
When writing an interval constant with a fields
specification, or when assigning a string to an interval column that was
defined with a fields
specification, the interpretation of
unmarked quantities depends on the fields
. For
example INTERVAL '1' YEAR
is read as 1 year, whereas
INTERVAL '1'
means 1 second. Also, field values
“to the right” of the least significant field allowed by the
fields
specification are silently discarded. For
example, writing INTERVAL '1 day 2:03:04' HOUR TO MINUTE
results in dropping the seconds field, but not the day field.
According to the SQL standard all fields of an interval
value must have the same sign, so a leading negative sign applies to all
fields; for example the negative sign in the interval literal
'-1 2:03:04'
applies to both the days and hour/minute/second
parts. LightDB allows the fields to have different
signs, and traditionally treats each field in the textual representation
as independently signed, so that the hour/minute/second part is
considered positive in this example. If IntervalStyle
is
set to sql_standard
then a leading sign is considered
to apply to all fields (but only if no additional signs appear).
Otherwise the traditional LightDB interpretation is
used. To avoid ambiguity, it's recommended to attach an explicit sign
to each field if any field is negative.
In the verbose input format, and in some fields of the more compact
input formats, field values can have fractional parts; for example
'1.5 week'
or '01:02:03.45'
. Such input is
converted to the appropriate number of months, days, and seconds
for storage. When this would result in a fractional number of
months or days, the fraction is added to the lower-order fields
using the conversion factors 1 month = 30 days and 1 day = 24 hours.
For example, '1.5 month'
becomes 1 month and 15 days.
Only seconds will ever be shown as fractional on output.
Table 8.17 shows some examples
of valid interval
input.
Table 8.17. Interval Input
Example | Description |
---|---|
1-2 | SQL standard format: 1 year 2 months |
3 4:05:06 | SQL standard format: 3 days 4 hours 5 minutes 6 seconds |
1 year 2 months 3 days 4 hours 5 minutes 6 seconds | Traditional Postgres format: 1 year 2 months 3 days 4 hours 5 minutes 6 seconds |
P1Y2M3DT4H5M6S | ISO 8601 “format with designators”: same meaning as above |
P0001-02-03T04:05:06 | ISO 8601 “alternative format”: same meaning as above |
Internally interval
values are stored as months, days,
and seconds. This is done because the number of days in a month
varies, and a day can have 23 or 25 hours if a daylight savings
time adjustment is involved. The months and days fields are integers
while the seconds field can store fractions. Because intervals are
usually created from constant strings or timestamp
subtraction,
this storage method works well in most cases, but can cause unexpected
results:
SELECT EXTRACT(hours from '80 minutes'::interval); date_part ----------- 1 SELECT EXTRACT(days from '80 hours'::interval); date_part ----------- 0
Functions justify_days
and
justify_hours
are available for adjusting days
and hours that overflow their normal ranges.
The output format of the interval type can be set to one of the
four styles sql_standard
, postgres
,
postgres_verbose
, or iso_8601
,
using the command SET intervalstyle
.
The default is the postgres
format.
Table 8.18 shows examples of each
output style.
The sql_standard
style produces output that conforms to
the SQL standard's specification for interval literal strings, if
the interval value meets the standard's restrictions (either year-month
only or day-time only, with no mixing of positive
and negative components). Otherwise the output looks like a standard
year-month literal string followed by a day-time literal string,
with explicit signs added to disambiguate mixed-sign intervals.
The output of the postgres
style matches the output of
postgres releases prior to 8.4 when the
DateStyle parameter was set to ISO
.
The output of the postgres_verbose
style matches the output of
postgres releases prior to 8.4 when the
DateStyle
parameter was set to non-ISO
output.
The output of the iso_8601
style matches the “format
with designators” described in section 4.4.3.2 of the
ISO 8601 standard.
Table 8.18. Interval Output Style Examples
Style Specification | Year-Month Interval | Day-Time Interval | Mixed Interval |
---|---|---|---|
sql_standard | 1-2 | 3 4:05:06 | -1-2 +3 -4:05:06 |
postgres | 1 year 2 mons | 3 days 04:05:06 | -1 year -2 mons +3 days -04:05:06 |
postgres_verbose | @ 1 year 2 mons | @ 3 days 4 hours 5 mins 6 secs | @ 1 year 2 mons -3 days 4 hours 5 mins 6 secs ago |
iso_8601 | P1Y2M | P3DT4H5M6S | P-1Y-2M3DT-4H-5M-6S |