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  1. IMPALA
  2. IMPALA-7472

Consider removing TimestampValue::FromSubsecondUnixTime

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    Details

    • Type: Bug
    • Status: Open
    • Priority: Minor
    • Resolution: Unresolved
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: Backend
    • Labels:
    • Epic Color:
      ghx-label-1

      Description

      TimestampValue::FromSubsecondUnixTime converts a double interpreted as unix time in seconds to TimestampValue. Impala uses it in two cases:
      1: double <-> timestamp casting
      2: aggregate function AVG on timestamps - it converts timestamp to double, calculates the average on doubles, and converts the average back to timestamp.

      My concern about this is double's varying's precision depending on the distance from 0 (1970-01-01) - subsec timestamps around 2018 are rounded/distorted with several nanosecs, which can add up if a large number of timestamps are averaged.

      I think that decimal (with nanosec precision) could be used for both cases:
      1. Decimal could used as an intermediate stage - double < - >decimal and decimal < - > timestamp conversions have well defined rounding rules since decimal V2.
      2. Decimal could be used as aggregation state - this would probably make the AVG slower, but it would be much more precise.

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              • Assignee:
                Unassigned
                Reporter:
                csringhofer Csaba Ringhofer
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                Dates

                • Created:
                  Updated: