eldenmoon opened a new pull request, #70359:
URL: https://github.com/apache/doris/pull/70359
### What problem does this PR solve?
Problem Summary: with the default `enable_variant_v2 = false`, a CAST from a
NOT NULL Variant column to a non-Variant type fails when the legacy Variant
kernel produces NULL for a non-NULL Variant input:
```sql
CREATE TABLE t (k int NOT NULL, v variant NOT NULL) DUPLICATE KEY(k)
DISTRIBUTED BY HASH(k) BUCKETS 1 PROPERTIES("replication_num" = "1");
INSERT INTO t VALUES (1, 'null'), (2, 'null');
SELECT CAST(v AS STRING) FROM t;
```
```
ERROR 1105 (HY000): errCode = 2, detailMessage =
(127.0.0.1)[INTERNAL_ERROR]Expr
(CAST v(Variant(max subcolumns count = 2048, enable doc mode = false)) TO
String)
return column type mismatch: declared=String, actual=Nullable(String)
```
`Cast.castNullable` only declared a Variant -> non-Variant cast nullable
when the Variant type used
the V2 execution representation (`isExecutionV2()`). For the legacy
representation it returned
false, so the `CastExpr` was serialized with `is_nullable = false` and the
BE slot type became a
non-nullable String. The legacy kernel (`cast_from_variant_impl` in
`be/src/exprs/function/cast/cast_to_variant.h`) serializes a Variant through
the root/WRAP path
(or through the all-default-values path) and legitimately yields
`ColumnNullable` with NULLs for
a non-NULL Variant input, so `VExpr::execute_column` rejects the column
against the non-nullable
slot. The same mismatch also hits non-string targets, e.g. `CAST(v AS INT)`
on a Variant column
that contains Variant nulls.
Fix: declare Variant -> non-Variant casts nullable for both execution
representations, which is
what master already does (master no longer has the legacy representation).
The values produced by
the cast are unchanged; only the declared nullability of the output slot is
relaxed, and `VExpr`
already wraps a non-nullable column when the declaration is nullable.
### Release note
Fix `return column type mismatch: declared=String, actual=Nullable(String)`
when casting a NOT
NULL Variant column to STRING (and other non-Variant types) with the legacy
Variant representation.
### Check List (For Author)
- Test: Unit Test + Manual test
- `CastTest#testCastFromVariant` (updated): fails before the change,
passes after it
- Manual end-to-end: 4.2 FE+BE built from this branch, minimal table
above
- before: the `CAST(v AS STRING)` query fails with the error above
- after: the query returns `NULL` for the Variant-null rows, and
`CAST(v AS INT)` returns
`1`, `NULL`, `3` on `(1, 1), (2, 'null'), (3, 3)`
- Behavior changed: No (cast results are unchanged; only the declared
nullability of the output
slot changes)
- Does this need documentation: No
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