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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: https://postgr.es/m/BDAE4B56-3337-45A2-AC8A-30593849D6C0@yugabyte.com Backpatch-through: 9.6 to 14
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@ -2699,15 +2699,18 @@ P <optional> <replaceable>years</replaceable>-<replaceable>months</replaceable>-
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</para>
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<para>
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In the verbose input format, and in some fields of the more compact
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input formats, field values can have fractional parts; for example
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<literal>'1.5 week'</literal> or <literal>'01:02:03.45'</literal>. Such input is
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converted to the appropriate number of months, days, and seconds
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for storage. When this would result in a fractional number of
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months or days, the fraction is added to the lower-order fields
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using the conversion factors 1 month = 30 days and 1 day = 24 hours.
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For example, <literal>'1.5 month'</literal> becomes 1 month and 15 days.
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Only seconds will ever be shown as fractional on output.
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Field values can have fractional parts: for example, <literal>'1.5
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weeks'</literal> or <literal>'01:02:03.45'</literal>. However,
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because interval internally stores only three integer units (months,
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days, microseconds), fractional units must be spilled to smaller
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units. Fractional parts of units greater than months is truncated to
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be an integer number of months, e.g. <literal>'1.5 years'</literal>
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becomes <literal>'1 year 6 mons'</literal>. Fractional parts of
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weeks and days are computed to be an integer number of days and
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microseconds, assuming 30 days per month and 24 hours per day, e.g.,
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<literal>'1.75 months'</literal> becomes <literal>1 mon 22 days
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12:00:00</literal>. Only seconds will ever be shown as fractional
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on output.
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</para>
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<para>
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@ -2751,10 +2754,10 @@ P <optional> <replaceable>years</replaceable>-<replaceable>months</replaceable>-
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<para>
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Internally <type>interval</type> values are stored as months, days,
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and seconds. This is done because the number of days in a month
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and microseconds. This is done because the number of days in a month
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varies, and a day can have 23 or 25 hours if a daylight savings
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time adjustment is involved. The months and days fields are integers
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while the seconds field can store fractions. Because intervals are
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while the microseconds field can store fractional seconds. Because intervals are
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usually created from constant strings or <type>timestamp</type> subtraction,
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this storage method works well in most cases, but can cause unexpected
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results:
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