slinkydeveloper commented on a change in pull request #18813:
URL: https://github.com/apache/flink/pull/18813#discussion_r808999013
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File path: docs/content/docs/dev/table/types.md
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@@ -1487,6 +1479,86 @@ Not supported.
{{< /tab >}}
{{< /tabs >}}
+Casting
+-------
+
+Flink SQL can perform casting between a defined tuple `from` type and `target`
type.
+While some casting operations can always succeed no matter the input value,
others can fail at runtime,
+that is for some input values there is no way to construct an instance of the
target type.
+For example, it's always possible to convert `INT` to `STRING`, but viceversa
is not true,
+as you can provide an input `STRING` which cannot be converted to `INT`, like
`'abc'`.
+
+Flink SQL validates and rejects queries which cast tuples are invalid,
+e.g. when trying to cast a `TIMESTAMP` to an `INTERVAL`,
+but the user needs to handle manually cases where a cast tuple is valid, but
can fail at runtime.
+
+In Flink SQL casting can be performed using one of the two following built-in
functions:
+
+* `CAST`: The regular cast function defined by the SQL standard. It can fail
the job if the cast operation is fallible and the provided input is not valid.
The type inference will preserve the nullability of the input type.
+* `TRY_CAST`: An extension to the regular cast function which returns `NULL`
in case the cast operation fails. Its return type is always nullable.
+
+For example:
+
+```sql
+CAST('42' AS INT) --- returns 42 of type VARCHAR NOT NULL
+CAST(NULL AS VARCHAR) --- returns NULL of type VARCHAR
+CAST('non-number' AS INT) --- throws an exception and fails the job
+
+TRY_CAST('42' AS INT) --- returns 42 of type VARCHAR
+TRY_CAST(NULL AS VARCHAR) --- returns NULL of type VARCHAR
+TRY_CAST('non-number' AS INT) --- returns NULL of type INT
+COALESCE(TRY_CAST('non-number' AS INT), 0) --- returns 0 of type INT NOT NULL
+```
+
+The matrix below describes the supported cast tuples, where "Y" means
supported, "!" means fallible, "N" means unsupported:
+
+| From\Target |
`CHAR`¹/<br/>`VARCHAR`¹/<br/>`STRING` |
`BINARY`¹/<br/>`VARBINARY`¹/<br/>`BYTES` | `BOOLEAN` | `DECIMAL` | `TINYINT` |
`SMALLINT` | `INTEGER` | `BIGINT` | `FLOAT` | `DOUBLE` | `DATE` | `TIME` |
`TIMESTAMP` | `TIMESTAMP_LTZ` | `INTERVAL` | `ARRAY` | `MULTISET` | `MAP` |
`ROW` | `RAW` | `STRUCTURED` |
+|:---------------------------------------|:-------------------------------------:|:----------------------------------------:|:---------:|:---------:|:---------:|:----------:|:---------:|:--------:|:-------:|:--------:|:------:|:------:|:-----------:|:---------------:|:----------:|:-------:|:----------:|:-----:|:-----:|:-----:|:------------:|
+| `CHAR`/<br/>`VARCHAR`/<br/>`STRING` | Y
| ! | ! | ! | !
| ! | ! | ! | ! | ! | ! | ! |
! | ! | N | N | N | N | N
| N | N |
+| `BINARY`/<br/>`VARBINARY`/<br/>`BYTES` | Y
| Y | N | N | N
| N | N | N | N | N | N | N |
N | N | N | N | N | N | N
| N | N |
+| `BOOLEAN` | Y
| N | Y | Y | Y
| Y | Y | Y | Y | Y | N | N |
N | N | N | N | N | N | N
| N | N |
+| `DECIMAL` | Y
| N | N | Y | Y
| Y | Y | Y | Y | Y | N | N |
N | N | N | N | N | N | N
| N | N |
+| `TINYINT` | Y
| N | Y | Y | Y
| Y | Y | Y | Y | Y | N | N |
N² | N² | N | N | N | N | N
| N | N |
+| `SMALLINT` | Y
| N | Y | Y | Y
| Y | Y | Y | Y | Y | N | N |
N² | N² | N | N | N | N | N
| N | N |
+| `INTEGER` | Y
| N | Y | Y | Y
| Y | Y | Y | Y | Y | N | N |
N² | N² | Y⁵ | N | N | N | N
| N | N |
+| `BIGINT` | Y
| N | Y | Y | Y
| Y | Y | Y | Y | Y | N | N |
N² | N² | Y⁶ | N | N | N | N
| N | N |
+| `FLOAT` | Y
| N | N | Y | Y
| Y | Y | Y | Y | Y | N | N |
N | N | N | N | N | N | N
| N | N |
+| `DOUBLE` | Y
| N | N | Y | Y
| Y | Y | Y | Y | Y | N | N |
N | N | N | N | N | N | N
| N | N |
+| `DATE` | Y
| N | N | N | N
| N | N | N | N | N | Y | N |
Y | Y | N | N | N | N | N
| N | N |
+| `TIME` | Y
| N | N | N | N
| N | N | N | N | N | N | Y |
Y | Y | N | N | N | N | N
| N | N |
+| `TIMESTAMP` | Y
| N | N | N | N
| N | N | N | N | N | Y | Y |
Y | Y | N | N | N | N | N
| N | N |
+| `TIMESTAMP_LTZ` | Y
| N | N | N | N
| N | N | N | N | N | Y | Y |
Y | Y | N | N | N | N | N
| N | N |
+| `INTERVAL` | Y
| N | N | N | N
| N | Y⁵ | Y⁶ | N | N | N | N |
N | N | Y | N | N | N | N
| N | N |
+| `ARRAY` | Y
| N | N | N | N
| N | N | N | N | N | N | N |
N | N | N | !³ | N | N | N
| N | N |
+| `MULTISET` | Y
| N | N | N | N
| N | N | N | N | N | N | N |
N | N | N | N | !³ | N | N
| N | N |
+| `MAP` | Y
| N | N | N | N
| N | N | N | N | N | N | N |
N | N | N | N | N | !³ | N
| N | N |
+| `ROW` | Y
| N | N | N | N
| N | N | N | N | N | N | N |
N | N | N | N | N | N | !³
| N | N |
+| `RAW` | Y
| ! | N | N | N
| N | N | N | N | N | N | N |
N | N | N | N | N | N | N
| Y⁴ | N |
+| `STRUCTURED` | Y
| N | N | N | N
| N | N | N | N | N | N | N |
N | N | N | N | N | N | N
| N | !³ |
+
+1. All the casting to constant length or variable length will also trim and
pad accordingly to the type definition.
+2. `TO_TIMESTAMP` and `TO_TIMESTAMP_LTZ` must be used instead of
`CAST`/`TRY_CAST`.
+3. Supported iff the children type tuples are supported. Fallible iff the
children type tuples are fallible.
Review comment:
yep
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