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https://issues.apache.org/jira/browse/HIVE-20536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16619928#comment-16619928
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Vineet Garg commented on HIVE-20536:
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Oh it skipped my mind! Unfortunately MERGE doesn't allow column schema
specification with the INSERT therefore DEFAULT isn't supported.
> 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, HIVE-20536.05.patch,
> HIVE-20536.06.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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