Ramin Gharib created FLINK-40826:
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Summary: Support casting constructed types to VARIANT
Key: FLINK-40826
URL: https://issues.apache.org/jira/browse/FLINK-40826
Project: Flink
Issue Type: Sub-task
Components: Table SQL / API
Reporter: Ramin Gharib
Assignee: Ramin Gharib
Adds \{{CAST}} from primitive types to VARIANT. This ticket adds ARRAY, MAP,
ROW, and STRUCTURED sources.
h3. Rule
A constructed type casts to VARIANT only if every leaf type does. Each level
maps to its VARIANT shape, and each leaf is cast by the primitive rule. A leaf
without a VARIANT kind rejects the whole cast at validation time.
||Source type||Stored VARIANT shape||
|ARRAY<T>|array, each element cast from T|
|MAP<K, V>|object, K must be a character string, each value cast from V|
|ROW, STRUCTURED|object keyed by field name|
{code:sql}
CAST(ARRAY[1, 2, 3] AS VARIANT) -- [1,2,3]
CAST(ARRAY[1, NULL] AS VARIANT) -- [1,null], a NULL element
becomes a variant null
CAST(MAP['a', 1, 'b', 2] AS VARIANT) -- \{"a":1,"b":2}
CAST(r AS VARIANT) -- \{"id":7,"name":"ada"} for
r ROW<id INT, name STRING>
CAST(r AS VARIANT) -- \{"id":1,"tags":[1,2]} for
r ROW<id INT, tags VARIANT>
CAST(MAP[1, 'a'] AS VARIANT) -- fails at validation, keys
are not stringified
CAST(ARRAY[INTERVAL '1' DAY] AS VARIANT) -- fails at validation, the
leaf has no VARIANT kind
CAST(MULTISET[1, 1] AS VARIANT) -- fails at validation
{code}
* A NULL element, map value, or field becomes a variant null, so the array
length and the object shape are kept. A NULL top-level value stays SQL NULL.
* A nested VARIANT is embedded as is.
* Field order is not kept, since a variant object is ordered by key.
* An empty array, map, or row becomes an empty array or object.
This can be delivered in three PRs, ARRAY, then MAP, then ROW and STRUCTURED,
like the opposite direction in FLINK-37926.
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