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doc: show how interval's 3 unit buckets behave using EXTRACT()
This clarifies when justify_days() and justify_hours() are useful. Paragraph moved too. Reported-by: vodevsh@gmail.com Discussion: https://postgr.es/m/152698651482.26744.15456677499485530703@wrigleys.postgresql.org Backpatch-through: 9.3
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@ -2688,19 +2688,6 @@ P <optional> <replaceable>years</>-<replaceable>months</>-<replaceable>days</> <
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to each field if any field is negative.
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</para>
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<para>
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Internally <type>interval</> 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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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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usually created from constant strings or <type>timestamp</> subtraction,
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this storage method works well in most cases. Functions
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<function>justify_days</> and <function>justify_hours</> are
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available for adjusting days and hours that overflow their normal
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ranges.
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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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@ -2752,6 +2739,33 @@ P <optional> <replaceable>years</>-<replaceable>months</>-<replaceable>days</> <
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</tgroup>
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</table>
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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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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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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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<programlisting>
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SELECT EXTRACT(hours from '80 minutes'::interval);
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date_part
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-----------
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1
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SELECT EXTRACT(days from '80 hours'::interval);
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date_part
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-----------
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0
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</programlisting>
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Functions <function>justify_days</function> and
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<function>justify_hours</function> are available for adjusting days
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and hours that overflow their normal ranges.
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</para>
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</sect2>
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<sect2 id="datatype-interval-output">
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