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https://issues.apache.org/jira/browse/PHOENIX-3773?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16035310#comment-16035310
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Loknath Priyatham Teja Singamsetty commented on PHOENIX-3773:
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bq. You might want to check out ArrayAppendFunctionIT which exercises
ARRAY_APPEND
Thanks for the pointer James. Was looking into the same and was understanding
how things were working.
[~jamestaylor] Looks like I found the reason. The
PArrayDataType.appendItemToArray can be used when you already have an Array
serialized to bytes with atleast one element in it. We cannot leverage this
without having an array pre-constructed.
In our case, the requirement is to convert the multiple <T>PDataType to single
<T>PArrayDataType. There is no such util method which can construct the Array
from scratch given element one by one to array.
We have to perform serialization/deserialization for one element in order to
construct the Array, post which we can make use of
PArrayDataType.appendItemToArray. This would save serialization/deserialization
cost on the rest of items in first values array result set.
Let me know if this approach is fine with you.
> Implement FIRST_VALUES aggregate function
> -----------------------------------------
>
> Key: PHOENIX-3773
> URL: https://issues.apache.org/jira/browse/PHOENIX-3773
> Project: Phoenix
> Issue Type: New Feature
> Reporter: James Taylor
> Assignee: Loknath Priyatham Teja Singamsetty
> Labels: SFDC
> Fix For: 4.11.0
>
> Attachments: PHOENIX-3773_4.x-HBase-0.98.patch,
> PHOENIX-3773_master.patch, PHOENIX-3773.patch, PHOENIX-3773.v2.patch,
> PHOENIX-3773.v3.patch
>
>
> Similar to FIRST_VALUE, but would allow the user to specify how many values
> to keep. This could use a MinMaxPriorityQueue under the covers and be much
> more efficient than using multiple NTH_VALUE calls to do the same like this:
> {code}
> SELECT entity_id,
> NTH_VALUE(user_id,1) WITHIN GROUP (ORDER BY last_read_date DESC) as
> nth1_user_id,
> NTH_VALUE(user_id,2) WITHIN GROUP (ORDER BY last_read_date DESC) as
> nth2_user_id,
> NTH_VALUE(user_id,3) WITHIN GROUP (ORDER BY last_read_date DESC) as
> nth3_user_id,
> count(*)
> FROM MY_TABLE
> WHERE tenant_id='00Dx0000000XXXX'
> AND entity_id in ('0D5x000000ABCD','0D5x000000ABCE')
> GROUP BY entity_id;
> {code}
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