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From "Ashutosh Chauhan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-20536) Add Surrogate Keys function to Hive
Date Mon, 17 Sep 2018 20:59:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-20536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16618145#comment-16618145
] 

Ashutosh Chauhan commented on HIVE-20536:
-----------------------------------------

Can you create a review board request on https://reviews.apache.org/ It makes reviewing code
easier.

> Add Surrogate Keys function to Hive
> -----------------------------------
>
>                 Key: HIVE-20536
>                 URL: https://issues.apache.org/jira/browse/HIVE-20536
>             Project: Hive
>          Issue Type: Task
>          Components: Hive
>            Reporter: Miklos Gergely
>            Assignee: Miklos Gergely
>            Priority: Major
>         Attachments: HIVE-20536.01.patch, HIVE-20536.02.patch, HIVE-20536.03.patch, HIVE-20536.04.patch
>
>
> Surrogate keys is an ability to generate and use unique integers for each row in a table.
If we have that ability then in conjunction with default clause we can get surrogate keys
functionality. Consider following ddl:
> create table t1 (a string, b bigint default unique_long());
> We already have default clause wherein you can specify a function to provide values.
So, what we need is udf which can generate unique longs for each row across queries for a
table. 
> Idea is to use write_id . This is a column in metastore table TXN_COMPONENTS whose value
is determined at compile time to be used during query execution. Each query execution generates
a new write_id. So, we can seed udf with this value during compilation.
> Then we statically allocate ranges for each task from which it can draw next long. So,
lets say 64-bit write_id we divy up such that last 24 bits belong to original usage of it
that is txns. Next 16 bits are used for task_attempts and last 24 bits to generate new long
for each row. This implies we can allow 17M txns, 65K tasks and 17M rows in a task. If you
hit any of those limits we can fail the query.
> Implementation wise: serialize write_id in initialize() of udf. Then during execute()
we find out what task_attempt current task is and use it along with write_id() to get starting
long and give a new value on each invocation of execute().
> Here we are assuming write_id can be determined at compile time, which should be the
case but we need to figure out how to get handle to it.



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