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https://issues.apache.org/jira/browse/PHOENIX-3390?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15598761#comment-15598761
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Swapna Kasula commented on PHOENIX-3390:
----------------------------------------

The function APPROX_SUM is very similar to the existing aggregate function we 
have is 'SUM(X)', to allow in the select statements.
However we pass col to the SUM(X), APPROX_SUM works the same way as :
select APPROX_SUM(col) from table;

This is how I visualize:

Example:
              
rowkey        col1          col2
google         hll1           hll2
yahoo          hll3           hll4
msn             hll5           hll6

select APPROX_SUM(col1) from table;  //returns  union of (hll1 U hll3 U hll5) 
for all rows
select APPROX_SUM(col1) from table where rowkey ='google'  or rowkey ='yahoo'  
// this returns unions of (hll1 U hll3) 


> Custom UDAF for HyperLogLogPlus
> -------------------------------
>
>                 Key: PHOENIX-3390
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-3390
>             Project: Phoenix
>          Issue Type: New Feature
>            Reporter: Swapna Kasula
>            Priority: Minor
>
> With ref # PHOENIX-2069
> Custome UDAF to aggregate/union of Hyperloglog's of a column and returns a 
> Hyperloglog.
> select hllUnion(col1) from table;  //returns a Hyperloglog, which is the 
> union of all hyperloglog's from all rows for column 'col1'



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