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https://issues.apache.org/jira/browse/KUDU-3353?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17568976#comment-17568976
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Yingchun Lai commented on KUDU-3353:
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Let me clarify some use cases:
A user profile table in Kudu has a column "first_login_ts", it represent the
first login time to the website. The data in the table is upsert by user event
log, the log contains user's id, some attributes, and "first_login_ts". The
first_login_ts is filled by the log produced time, that means for a specified
user, his/her event logs have a different (higher and higher) "first_login_ts",
but only the first one could be set, and the following logs should not update
it.
The updated design:
1. Add a column attribute to define a column as IMMUTABLE, means the column
cell value can not be updated after it's been written during inserting the row.
2. Use UPDATE_IGNORE and add UPSERT_IGNORE, for UPDATE and UPSERT ops but
ignore update-errors on IMMUTABLE columns.
> Support setnx semantic on column
> --------------------------------
>
> Key: KUDU-3353
> URL: https://issues.apache.org/jira/browse/KUDU-3353
> Project: Kudu
> Issue Type: New Feature
> Components: api, server
> Reporter: Yingchun Lai
> Assignee: Yingchun Lai
> Priority: Major
>
> h1. motivation
> In some usage scenarios, Kudu table has a column with semantic of "create
> time", which means it represent the create timestamp of the row. The other
> columns have the similar semantic as before, for example, the user properties
> like age, address, and etc.
> Upstream and Kudu user doesn't know whether a row is exist or not, and every
> cell data is the lastest ingested from, for example, event stream.
> If without the "create time" column, Kudu user can use UPSERT operations to
> write data to the table, every columns with data will overwrite the old data.
> But if with the "create time" column, the cell data will be overwrote by the
> following UPSERT ops, which is not what we expect.
> To achive the goal, we have to read the column out to judge whether the
> column is NULL or not, if it's NULL, we can fill the row with the cell, if
> not NULL, we will drop it from the data before UPSERT, to avoid overwite
> "create time".
> It's expensive, is there a way to avoid a read from Kudu?
> h1. Resolvation
> We can implement column schema with semantic of "update if null". That means
> cell data in changelist will update the base data if the latter is NULL, and
> will ignore updates if it is not NULL.
> So we can use Kudu similarly as before, but only defined the column as
> "update if null" when create table or add column.
>
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