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https://issues.apache.org/jira/browse/CALCITE-5155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17602583#comment-17602583
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Julian Hyde commented on CALCITE-5155:
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Thanks for the feedback, [~tdsilva]. That all makes sense.
I realized that there's an interesting corner case, '7 hour', which cannot roll
up to day (because 7 hours do not divide 24 hours) but can roll up to week
(because 7 hours divides 168 hours).
And (an even rarer case) if a time unit is not aligned with its parent (i.e.
start when its parent starts), then the question arises: what does it align
with? You could imagine a 'year-aligned week' time unit where each week starts
on a Thursday if the year starts on a Thursday. But of course the vast majority
of time units are aligned on the epoch.
> Extensible time units
> ---------------------
>
> Key: CALCITE-5155
> URL: https://issues.apache.org/jira/browse/CALCITE-5155
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Priority: Major
>
> Allow a type system to define its own time units and how they are rolled up.
> Currently, time units are used in the {{EXTRACT}}, {{FLOOR}}, {{TRUNC}}
> functions, and include {{YEAR}}, {{QUARTER}}, {{MONTH}}, {{HOUR}},
> {{MINUTE}}, {{NANOSECOND}}. For example {{FLOOR(t TO HOUR)}} is valid.
> A type system would be allowed to define extra time units. Once a time unit
> is defined the {{EXTRACT}}, {{FLOOR}} and {{TRUNC}} functions should just
> work.
> The definition of might consist of a base unit and multiplier. So
> {{MINUTE15}} would be based on {{MINUTE}} with a multiplier of 15.
> Various rules know that you can roll up {{FLOOR(t TO DAY)}} to {{FLOOR(t TO
> MONTH)}} but you cannot roll {{FLOOR(t TO WEEK)}} to {{FLOOR(t TO MONTH)}}.
> When you define a new time unit, the type system can deduce that full set of
> time units that it can roll up to, and which can roll up to it.
> Should we support time units that do not evenly divide the next largest time
> unit? For example the number of seconds since the top of the hour modulo 7.
> 60 and 3,600 are not a multiples of 7, so {{SecondOfHourMod7}} would be
> different from {{SecondOfMinuteMod7}}.
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