Miklos Gergely created HIVE-20536:
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Summary: 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
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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