doc:  interval spill method for units greater than months
Units are _truncated_ to months, but only in back branches since the
recent commit.
Reported-by: Bryn Llewellyn
Discussion:
Backpatch-through: 9.6 to 14
@@ -2710,15 +2710,18 @@ P <optional> <replaceable>years</replaceable>-<replaceable>months</replaceable>-
</para>
<para>
- In the verbose input format, and in some fields of the more compact
- input formats, field values can have fractional parts; for example
- <literal>'1.5 week'</literal> or <literal>'01:02:03.45'</literal>. 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, <literal>'1.5 month'</literal> becomes 1 month and 15 days.
- Only seconds will ever be shown as fractional on output.
+ Field values can have fractional parts: for example, <literal>'1.5
+ weeks'</literal> or <literal>'01:02:03.45'</literal>. However,
+ because interval internally stores only three integer units (months,
+ days, microseconds), fractional units must be spilled to smaller
+ units. Fractional parts of units greater than months is truncated to
+ be an integer number of months, e.g. <literal>'1.5 years'</literal>
+ becomes <literal>'1 year 6 mons'</literal>. Fractional parts of
+ weeks and days are computed to be an integer number of days and
+ microseconds, assuming 30 days per month and 24 hours per day, e.g.,
+ <literal>'1.75 months'</literal> becomes <literal>1 mon 22 days
+ 12:00:00</literal>. Only seconds will ever be shown as fractional
+ on output.
</para>
<para>
@@ -2762,10 +2765,10 @@ P <optional> <replaceable>years</replaceable>-<replaceable>months</replaceable>-
<para>
Internally <type>interval</type> values are stored as months, days,
- and seconds. This is done because the number of days in a month
+ and microseconds. 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
+ while the microseconds field can store fractional seconds. Because intervals are
usually created from constant strings or <type>timestamp</type> subtraction,
this storage method works well in most cases, but can cause unexpected
results:
