DATE and DATETIME data types

Informix®

Informix provides two data types to store date and time information:

DATE = for year, month and day storage.
DATETIME = for year to fraction (1-5) storage.

The DATE type is stored as an INTEGER with the number of days since 1899/12/31.

The DATETIME type can be defined with various time units, by specifying a start and end qualifier. For example, you can define a datetime to store an hour-to-second time value with DATETIME HOUR TO SECOND.

The values of Informix DATETIME can be represented with a character string literal, or as DATETIME() literals:

'2017-12-24 15:45:12.345'  -- a DATETIME YEAR TO FRACTION(3)
'15:45'   -- a DATETIME HOUR TO MINUTE
DATETIME(2017-12-24 12:45) YEAR TO MINUTE
DATETIME(12:45:56.333) HOUR TO FRACTION(3)

Informix is able to convert quoted strings to DATE / DATETIME data, if the string contains matching environment parameters. The string to date conversion rules for DATE is defined by the DBDATE environment variable. The string to datetime format for DATETIME is defined by the GL_DATETIME environment variable.

Note: Within Genero programs, the string representation for DATETIME values is always ISO (YYYY-MM-DD hh:mm:ss.fffff)

Informix supports date arithmetic on DATE and DATETIME values. The result of an arithmetic expression involving dates/times is an INTEGER number of days when only DATE values are used, and an INTERVAL value if a DATETIME is used in the expression.

Informix automatically converts an INTEGER to a DATE when the integer is used to set a value of a date column.

Informix provides the

CURRENT [ q1 TO
q2 ]

operator, to get the system date/time on the server where the current database is located. When no qualifiers are specified, CURRENT returns a

DATETIME
YEAR TO FRACTION(3)

. Informix also supports the SYSDATE operator, which returns the current system time as a DATETIME YEAR TO FRACTION(5).

Note: The USEOSTIME configuration parameter must be set to 1 in order to get the subsecond precision in CURRENT and SYSDATE operators. See Informix documentation for more details.

PostgreSQL

PostgreSQL supports the following data types to store date/time values:

Table 1. PostgreSQL date/time data types
PostgreSQL data type	Description
`DATE`	For year, month, day storage
`TIME [(p)] [WITHOUT TIME ZONE]`	For hour, minute, second and fraction of second storage (with `0<=p<=6`)
`TIME [(p)] WITH TIME ZONE`	For hour, minute, second and fraction of second storage (with `0<=p<=6`), applying time zone conversions.
`TIMESTAMP [(p)] [WITHOUT TIME ZONE]`	For year, month, day, hour, minute, second and fraction of second storage (with `0<=p<=6`)
`TIMESTAMP [(p)] WITH TIME ZONE`	For year, month, day, hour, minute, second and fraction of second storage (with `0<=p<=6`), applying time zone conversions.

The result of an arithmetic expression involving PostgreSQL DATE values is an INTEGER representing a number of days.

When the precision p is not specified in a PostgreSQL TIME/TIMESTAMP type, the precision of the fractional part is not fixed.

PostgreSQL can convert quoted strings to date/time data depending on the DateStyle session parameter. PostgreSQL always accepts ISO date time strings. The date format can be changed with SET DateStyle PostgreSQL command. However, it is better to use the default ISO DateStyle format.

The WITH TIME ZONE option enables automatic storage of date/time data in UTC. This option is not to store time zone offset along with the date/time value: It enables conversion from/to UTC to/from a date/time in current PostgreSQL system TimeZone or a specific time zone: The value stored in the PostgreSQL database is always in UTC. See PostgreSQL documentation for more details.

Solution

Use the following conversion rules to map Informix date/time types to PostgreSQL date/time types:

Table 2. Informix data types and PostgreSQL equivalents
Informix data type	PostgreSQL data type
`DATE`	`DATE`
`DATETIME HOUR TO MINUTE`	`TIME(0) WITHOUT TIME ZONE`
`DATETIME HOUR TO SECOND`	`TIME(0) WITHOUT TIME ZONE`
`DATETIME HOUR TO FRACTION(p)`	`TIME(p) WITHOUT TIME ZONE`
`DATETIME YEAR TO MONTH`	`TIMESTAMP(0) WITHOUT TIME ZONE`
`DATETIME YEAR TO DAY`	`TIMESTAMP(0) WITHOUT TIME ZONE`
`DATETIME YEAR TO HOUR`	`TIMESTAMP(0) WITHOUT TIME ZONE`
`DATETIME YEAR TO MINUTE`	`TIMESTAMP(0) WITHOUT TIME ZONE`
`DATETIME YEAR TO SECOND`	`TIMESTAMP(0) WITHOUT TIME ZONE`
`DATETIME YEAR TO FRACTION(p)`	`TIMESTAMP(p) WITHOUT TIME ZONE`

When the PostgreSQL TIME/TIMESTAMP precision p is not specified, the number of digits in the fractional part is variable. This prevents a proper type mapping, for example when extracting column types with fgldbsch from a PostgreSQL database. When creating table columns in PostgreSQL, always use TIME(p)/TIMESTAMP(p) types, and specify 0 (zero) if there are no fraction of seconds to be stored.

The DATE and DATETIME types translation can be controlled with the following FGLPROFILE entries:

dbi.database.dsname.ifxemul.datatype.date = { true | false }
dbi.database.dsname.ifxemul.datatype.datetime = { true | false }

For more details see IBM Informix emulation parameters in FGLPROFILE.

PostgreSQL and Informix DATE types are equivalent and store year, month, day values.

The PostgreSQL ODI driver can detect the current DateSyle used in the SQL session. It can be ISO, SQL and German. The DateStyle "Postgres" is not supported for TIMESTAMP types. Consider using the ISO default DateStyle format.

PostgreSQL TIME(n) WITHOUT TIME ZONE data type can be used to store Informix DATETIME HOUR TO FRACTION(n), DATETIME HOUR TO SECOND, DATETIME HOUR TO MINUTE values, and any other DATETIME type with qualifiers HOUR, MINUTE, SECOND and FRACTION(n). Missing time parts default to 00:00:00.0. For example, when using a DATETIME MINUTE TO FRACTION(3) with the value of "45:23.999", the PostgreSQL TIME(3) WITHOUT TIME ZONE value will be "00:45:23.999".

Informix DATETIME values with any qualifiers from YEAR to FRACTION(5) can be stored in PostgreSQL TIMESTAMP(n) WITHOUT TIME ZONE columns. Missing date or time parts default to 1900-01-01 00:00:00.0. For example, when using a DATETIME DAY TO MINUTE with the value of "23 11:45", the PostgreSQL TIMESTAMP(0) WITHOUT TIME ZONE value will be "1900-01-23 11:45:00".

The TIME WITH TIME ZONE or TIMESTAMP WITH TIME ZONE PostgreSQL data types must be used with care and are not recommended for SQL portability: When passing date/time data from FGL variables for TIME WITH TIME ZONE or TIMESTAMP WITH TIME ZONE columns, the value provided by the ODI driver has no time zone offset. However, it is considered by PostgreSQL as a date/time in the current PostgreSQL system TimeZone and will be converted to UTC before it is stored. When fetching the value back from the SQL column into an FGL variable, PostgreSQL will automatically convert from UTC to a date/time in the PostgreSQL system TimeZone. For example, insert of FGL value 2023-08-16 17:45:33 in the time zone GMT+1H and with +1H for Daylight Saving Time (DST), will be stored as UTC 2023-08-16 15:45:33 because 17H - 2H = 15H. When fetching the value back, UTC 2023-08-16 15:45:33 is converted to a date/time in the current system TimeZone: 15H + 2H = 17H. PostgreSQL adds the TZ offset +02 in the raw data string, but it is ignored by the ODI driver. As result, the FGL variable value is set to 2023-08-16 17:45:33. If the PostgreSQL system TimeZone is changed, the value in the FGL variable will be different. If you need to store date/time data in UTC, do the conversions at the BDL level before using the UTC date/time values in SQL statements. See util.Datetime.toUTC().

Complex DATETIME expressions (involving INTERVAL values for example) are Informix-specific and have no equivalent in PostgreSQL.