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https://issues.apache.org/jira/browse/SPARK-18000?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Zhenhua Wang updated SPARK-18000:
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Description:
The MapAggregate function Computes frequency for each distinct non-null value
of a column.
It returns: 1. null if the table is empty or all values of the column are null.
2. (distinct non-null value, frequency) pairs if the number of distinct
non-null values is
less than or equal to the specified threshold.
3. an empty result if the number of distinct non-null values exceeds that
threshold.
was:
For a column, we will generate a equi-width or equi-height histogram, depending
on if its ndv is large than the maximum number of bins allowed in one histogram
(denoted as numBins).
The agg function for a column returns bins - (distinct value, frequency) pairs
of equi-width histogram when the number of distinct values is less than or
equal to numBins. Otherwise, 1) for column of string type, it returns an empty
map; 2) for column of numeric type (including DateType and TimestampType), it
returns endpoints of equi-height histogram - approximate percentiles at
percentages 0.0, 1/numBins, 2/numBins, ..., (numBins-1)/numBins, 1.0.
> Aggregation function for computing endpoints for histograms
> -----------------------------------------------------------
>
> Key: SPARK-18000
> URL: https://issues.apache.org/jira/browse/SPARK-18000
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Affects Versions: 2.1.0
> Reporter: Zhenhua Wang
>
> The MapAggregate function Computes frequency for each distinct non-null value
> of a column.
> It returns: 1. null if the table is empty or all values of the column are
> null.
> 2. (distinct non-null value, frequency) pairs if the number of distinct
> non-null values is
> less than or equal to the specified threshold.
> 3. an empty result if the number of distinct non-null values exceeds that
> threshold.
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