9.8. Data Type Formatting Functions

The PostgreSQL formatting functions provide a powerful set of tools for converting various data types (date/time, integer, floating point, numeric) to formatted strings and for converting from formatted strings to specific data types. Table 9-20 lists them. These functions all follow a common calling convention: the first argument is the value to be formatted and the second argument is a template that defines the output or input format.

The to_timestamp function can also take a single double precision argument to convert from Unix epoch to timestamp with time zone. (Integer Unix epochs are implicitly cast to double precision.)

In an output template string (for to_char), there are certain patterns that are recognized and replaced with appropriately-formatted data from the value to be formatted. Any text that is not a template pattern is simply copied verbatim. Similarly, in an input template string (for anything but to_char), template patterns identify the parts of the input data string to be looked at and the values to be found there.

Table 9-21 shows the template patterns available for formatting date and time values.

Certain modifiers can be applied to any template pattern to alter its behavior. For example, FMMonth is the Month pattern with the FM modifier. Table 9-22 shows the modifier patterns for date/time formatting.

Usage notes for date/time formatting:

• FM suppresses leading zeroes and trailing blanks that would otherwise be added to make the output of a pattern be fixed-width.

• TM does not include trailing blanks.

• to_timestamp and to_date skip multiple blank spaces in the input string if the FX option is not used. FX must be specified as the first item in the template. For example to_timestamp('2000    JUN', 'YYYY MON') is correct, but to_timestamp('2000    JUN', 'FXYYYY MON') returns an error, because to_timestamp expects one space only.

• Ordinary text is allowed in to_char templates and will be output literally. You can put a substring in double quotes to force it to be interpreted as literal text even if it contains pattern key words. For example, in '"Hello Year "YYYY', the YYYY will be replaced by the year data, but the single Y in Year will not be.

• If you want to have a double quote in the output you must precede it with a backslash, for example E'\\"YYYY Month\\"'. (Two backslashes are necessary because the backslash already has a special meaning when using the escape string syntax.)

• The YYYY conversion from string to timestamp or date has a restriction if you use a year with more than 4 digits. You must use some non-digit character or template after YYYY, otherwise the year is always interpreted as 4 digits. For example (with the year 20000): to_date('200001131', 'YYYYMMDD') will be interpreted as a 4-digit year; instead use a non-digit separator after the year, like to_date('20000-1131', 'YYYY-MMDD') or to_date('20000Nov31', 'YYYYMonDD').

• In conversions from string to timestamp or date, the CC field is ignored if there is a YYY, YYYY or Y,YYY field. If CC is used with YY or Y then the year is computed as (CC-1)*100+YY.

• An ISO week date (as distinct from a Gregorian date) can be specified to to_timestamp and to_date in one of two ways:

  • Year, week and weekday, for example to_date('2006-42-4', 'IYYY-IW-ID') returns the date 2006-10-19. If you omit the weekday it is assumed to be 1 (Monday).

  • Year and day of year, for example to_date('2006-291', 'IYYY-IDDD') also returns 2006-10-19.

  Attempting to construct a date using a mixture of ISO week and Gregorian date fields is nonsensical, and could yield unexpected results. In the context of an ISO year, the concept of a 'month' or 'day of month' has no meaning. In the context of a Gregorian year, the ISO week has no meaning. Users should take care to keep Gregorian and ISO date specifications separate.

• Millisecond (MS) and microsecond (US) values in a conversion from string to timestamp are used as part of the seconds after the decimal point. For example to_timestamp('12:3', 'SS:MS') is not 3 milliseconds, but 300, because the conversion counts it as 12 + 0.3 seconds. This means for the format SS:MS, the input values 12:3, 12:30, and 12:300 specify the same number of milliseconds. To get three milliseconds, one must use 12:003, which the conversion counts as 12 + 0.003 = 12.003 seconds.

  Here is a more complex example: to_timestamp('15:12:02.020.001230', 'HH:MI:SS.MS.US') is 15 hours, 12 minutes, and 2 seconds + 20 milliseconds + 1230 microseconds = 2.021230 seconds.

• to_char(..., 'ID')'s day of the week numbering matches the extract(isodow from ...) function, but to_char(..., 'D')'s does not match extract(dow from ...)'s day numbering.

• to_char(interval) formats HH and HH12 as hours in a single day, while HH24 can output hours exceeding a single day, e.g. >24.

Table 9-23 shows the template patterns available for formatting numeric values.

Usage notes for numeric formatting:

• A sign formatted using SG, PL, or MI is not anchored to the number; for example, to_char(-12, 'S9999') produces '  -12', but to_char(-12, 'MI9999') produces '-  12'. The Oracle implementation does not allow the use of MI ahead of 9, but rather requires that 9 precede MI.

• 9 results in a value with the same number of digits as there are 9s. If a digit is not available it outputs a space.

• TH does not convert values less than zero and does not convert fractional numbers.

• PL, SG, and TH are PostgreSQL extensions.

• V effectively multiplies the input values by 10^n, where n is the number of digits following V. to_char does not support the use of V combined with a decimal point. (E.g., 99.9V99 is not allowed.)

Table 9-24 shows some examples of the use of the to_char function.