Ramin Gharib created FLINK-40580:
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Summary: Cast from VARIANT to a character string should render an
object or array as JSON
Key: FLINK-40580
URL: https://issues.apache.org/jira/browse/FLINK-40580
Project: Flink
Issue Type: Improvement
Reporter: Ramin Gharib
Assignee: Ramin Gharib
Casting a VARIANT to CHAR/VARCHAR/STRING renders a stored scalar the way a
regular SQL cast of that kind would (a boolean becomes TRUE, a timestamp uses
the SQL format, a binary value is read as UTF-8). A VARIANT that holds an
object or an array has no scalar form, so today the cast fails:
{code}Cannot cast a VARIANT OBJECT value to a character string. Use the
JSON_STRING function to obtain its JSON representation.\{code}
Redirecting users to JSON_STRING for the container case is surprising: the
value already has a well-defined textual form, its JSON representation, which
both JSON_STRING and the result-print path already produce.
Proposal: make CAST(v AS STRING) render an object or array as its JSON
representation (via Variant#toJson()) instead of failing. Scalar rendering is
unchanged, so a top-level scalar string still casts unquoted while a string
nested inside a rendered container stays quoted, matching JSON_STRING and the
print path.
{code:sql}
CAST(PARSE_JSON('["a","b"]') AS STRING) -- ["a","b"] (was: runtime error)
CAST(PARSE_JSON('\{"a":1}') AS STRING) -- \{"a":1} (was: runtime error)
CAST(PARSE_JSON('"foo"') AS STRING) -- foo (unchanged)
{code}
A bounded CHAR(n)/VARCHAR(n) target trims the rendered JSON like any other
over-length value. TRY_CAST returns the same JSON rather than NULL for a
container. A VARIANT storing a JSON null still casts to SQL NULL.
Scope: only OBJECT and ARRAY change here. Scalar kinds whose string rendering
is still unsupported (TIME and nanosecond timestamps, tracked in FLINK-40492)
keep failing.
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