hozan23 opened a new pull request, #26888:
URL: https://github.com/apache/datafusion/pull/26888
## Which issue does this PR close?
- Closes #26887.
## Rationale for this change
MySQL's `CAST` does not accept `SMALLINT` or the `... UNSIGNED` integer
types as target types, but `arrow_dtype_to_ast_dtype` hard-codes them for
`Int16` and `UInt8`..`UInt64`. As a result, `MySqlDialect` cannot override them
and the generated SQL fails with `ERROR 1064`.
See
https://dev.mysql.com/doc/refman/8.0/en/cast-functions.html#function_cast
Checked against MySQL 8.0.46 and 8.4.13 with `SELECT CAST('1' AS <type>)`:
every type in the issue table fails with `ERROR 1064`, while `SIGNED` and
`UNSIGNED` succeed.
## What changes are included in this PR?
- New `Dialect` hooks `int16_cast_dtype` and
`uint8`/`uint16`/`uint32`/`uint64_cast_dtype`. Their defaults return the
previous types.
- `MySqlDialect` returns `SIGNED` for `Int16` and `UNSIGNED` for the
unsigned types.
This builds on #26883.
## What is the testing strategy for this PR?
New unit tests in `datafusion/sql/src/unparser/expr.rs`:
- `mysql_int16_and_unsigned_casts`: the new MySQL output
- `default_dialect_int16_and_unsigned_casts`: the default output is
unchanged
- `custom_dialect_int16_and_unsigned_casts`: the builder setters
## Are there any user-facing changes?
With `MySqlDialect`, `Int16` casts now unparse as `SIGNED` and unsigned
integer casts as `UNSIGNED`.
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