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https://issues.apache.org/jira/browse/FLINK-41363?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated FLINK-41363:
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Labels: pull-request-available (was: )
> Support IF NOT EXISTS for ALTER TABLE ADD COLUMN
> ------------------------------------------------
>
> Key: FLINK-41363
> URL: https://issues.apache.org/jira/browse/FLINK-41363
> Project: Flink
> Issue Type: Improvement
> Components: Table SQL / API
> Reporter: Muhammad
> Priority: Major
> Labels: pull-request-available
>
> h3. Problem
> The addition of _ALTER TABLE <table> ADD COLUMN_ to a Flink job makes the
> job kind of non-idempotent. This means the job will run fine for the first
> time as the column doesn't exist but later it'll fail with an exception until
> the ALTER TABLE command is removed.
> {code:java}
> org.apache.flink.table.api.ValidationException: Failed to execute ALTER TABLE
> statement.
> Column `col_1` already exists in the table.
> {code}
> h3. Solution
> A broader idea is to support something like ALTER TABLE <table> ADD [COLUMN]
> IF NOT EXISTS pattern. This pattern is pretty common with many datastores
> like Postgres, Cassandra, Clickhouse etc. The scope of this ticket is to keep
> it restricted to COLUMN, but can be extended to ADD PARTITIONS etc
> This involves the following work.
> * The first is about extending Flink’s SQL grammar to support the IF NOT
> EXISTS pattern for column addition. The `COLUMN` keyword following the `ADD`
> construct is optional, so both of those patterns needs to be handled.
> * This also involves preserving the guard (IF NOT EXISTS) in the parsed SQL
> representation.
> * Also, it requires touching the planner. This means to read the table's
> current schema and decide what operations are needed.
> * Concurrent operations for `ADD column IF NOT EXISTS` is not much different
> from the existing`ADD column`. If the 2 concurrent jobs both commit the
> schema change together, one of them will fail. This case doesn't appear to be
> legit and will not be handled.
> * Unit tests and documentation for the above solution.
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