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https://issues.apache.org/jira/browse/CALCITE-1588?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15828664#comment-15828664
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Gian Merlino commented on CALCITE-1588:
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In Druid's SQL dialect, for distinct-count, even if you disable
useApproximateCountDistinct you can still get the approximate algo by doing
APPROX_COUNT_DISTINCT(col). We chose that since it seems to be the unofficial
standard (bigquery, oracle, memsql all call it that).
We don't have anything in the dialect to get approximate topNs if you disabled
the useApproximateTopN property though. Or, for that matter, to get exact ones
if you enabled the property.
> Add SQL syntax to allow approximate LIMIT and distinct-COUNT
> ------------------------------------------------------------
>
> Key: CALCITE-1588
> URL: https://issues.apache.org/jira/browse/CALCITE-1588
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: Julian Hyde
>
> Add SQL syntax to allow approximate LIMIT and distinct-COUNT. These will set
> the properties specified in CALCITE-1587. By default the properties are
> false, so the query will return exact results.
> Exact syntax is to be decided. It could be at the top of the query (therefore
> affecting every LIMIT or aggregate in the query) or it could be more
> localized (e.g. {{COUNT(DISTINCT customerId) APPROXIMATE (WITHIN 10
> PERCENT)}}).
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