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https://issues.apache.org/jira/browse/FLINK-24311?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Martijn Visser updated FLINK-24311:
-----------------------------------
Parent: (was: FLINK-9477)
Issue Type: New Feature (was: Sub-task)
> Support expression reduction for JSON construction functions
> ------------------------------------------------------------
>
> Key: FLINK-24311
> URL: https://issues.apache.org/jira/browse/FLINK-24311
> Project: Flink
> Issue Type: New Feature
> Components: Table SQL / API
> Reporter: Ingo Bürk
> Priority: Major
>
> For JSON construction functions such as JSON_OBJECT, we currently disable
> expression reduction. This is because these functions have special semantics
> where they behave differently depending on their call context. For example,
> JSON_OBJECT returns a JSON string. This means that a call like
> {code:java}
> JSON_OBJECT('A' VALUE JSON_OBJECT('B' VALUE 'C')){code}
> would result in
> {code:java}
> {"A": "{\"B\": \"C\"}"}{code}
> However, this is not user-friendly, and thus such nested calls are treated
> differently and instead result in the likely more intended outcome
> {code:java}
> {"A": {"B": "C"}}{code}
> To make this work, the function looks at its operands and checks whether each
> operand is another RexCall to such a JSON construction function. If it is, it
> inserts it as a raw node instead.
> This creates a problem during expression reduction. The RexCall will be
> replaced with a RexLiteral carrying the JSON string value. The function
> looking at the operands now cannot determine that this originated from such a
> RexCall anymore, and yields the unintended result once again. To prevent
> this, we currently disable expression reduction for these functions.
> We should aim to once again allow such expressions to be reduced while still
> preserving the intended behavior. See [this
> comment|https://github.com/apache/flink/pull/17186#issuecomment-920783089]
> for a rough idea of how this could be achieved.
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