fix: PostgreSQL dialect can not support tinyint type (#21445)

## Which issue does this PR close?

- No linked issue.

## Rationale for this change

DataFusion's PostgreSQL unparser should emit PostgreSQL-compatible SQL
for integer casts.`Int8` was still being rendered as `TINYINT`, which is
not valid PostgreSQL syntax. This change makes PostgreSQL output
`SMALLINT` instead.

## What changes are included in this PR?

- Added an `int8_cast_dtype` hook to the SQL unparser dialect
abstraction.
- Updated `PostgreSqlDialect` to map `Int8` to `SMALLINT`.
- Routed `DataType::Int8` unparsing through the dialect hook.
- Added a regression test covering `select cast(3 as tinyint)]` with
PostgreSQL unparser output.

## Are these changes tested?

- Yes. Ran:
- `cargo test -p datafusion-sql --test sql_integration
cases::plan_to_sql::test_cast_to_tinyint -- --exact`
- The test passes.

## Are there any user-facing changes?

- Yes. PostgreSQL dialect SQL generation now renders `CAST(... AS
SMALLINT)` for `Int8` values instead of `CAST(... AS TINYINT)`.

Author: xiedeyantu

datafusion/sql/src/unparser/dialect.rs

@@ -100,6 +100,12 @@ pub trait Dialect: Send + Sync {
         ast::DataType::BigInt(None)
     }
 
+    /// The SQL type to use for Arrow Int8 unparsing
+    /// Most dialects use TinyInt, but PostgreSQL prefers SmallInt
+    fn int8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::TinyInt(None)
+    }
+
     /// The SQL type to use for Arrow Int32 unparsing
     /// Most dialects use Integer, but some, like MySQL, require SIGNED
     fn int32_cast_dtype(&self) -> ast::DataType {
@@ -345,6 +351,10 @@ impl Dialect for PostgreSqlDialect {
         ast::DataType::DoublePrecision
     }
 
+    fn int8_cast_dtype(&self) -> ast::DataType {
+        ast::DataType::SmallInt(None)
+    }
+
     fn scalar_function_to_sql_overrides(
         &self,
         unparser: &Unparser,
@@ -664,6 +674,7 @@ pub struct CustomDialect {
     large_utf8_cast_dtype: ast::DataType,
     date_field_extract_style: DateFieldExtractStyle,
     character_length_style: CharacterLengthStyle,
+    int8_cast_dtype: ast::DataType,
     int64_cast_dtype: ast::DataType,
     int32_cast_dtype: ast::DataType,
     timestamp_cast_dtype: ast::DataType,
@@ -689,6 +700,7 @@ impl Default for CustomDialect {
             large_utf8_cast_dtype: ast::DataType::Text,
             date_field_extract_style: DateFieldExtractStyle::DatePart,
             character_length_style: CharacterLengthStyle::CharacterLength,
+            int8_cast_dtype: ast::DataType::TinyInt(None),
             int64_cast_dtype: ast::DataType::BigInt(None),
             int32_cast_dtype: ast::DataType::Integer(None),
             timestamp_cast_dtype: ast::DataType::Timestamp(None, TimezoneInfo::None),
@@ -748,6 +760,10 @@ impl Dialect for CustomDialect {
         self.int64_cast_dtype.clone()
     }
 
+    fn int8_cast_dtype(&self) -> ast::DataType {
+        self.int8_cast_dtype.clone()
+    }
+
     fn int32_cast_dtype(&self) -> ast::DataType {
         self.int32_cast_dtype.clone()
     }
@@ -839,6 +855,7 @@ pub struct CustomDialectBuilder {
     large_utf8_cast_dtype: ast::DataType,
     date_field_extract_style: DateFieldExtractStyle,
     character_length_style: CharacterLengthStyle,
+    int8_cast_dtype: ast::DataType,
     int64_cast_dtype: ast::DataType,
     int32_cast_dtype: ast::DataType,
     timestamp_cast_dtype: ast::DataType,
@@ -870,6 +887,7 @@ impl CustomDialectBuilder {
             large_utf8_cast_dtype: ast::DataType::Text,
             date_field_extract_style: DateFieldExtractStyle::DatePart,
             character_length_style: CharacterLengthStyle::CharacterLength,
+            int8_cast_dtype: ast::DataType::TinyInt(None),
             int64_cast_dtype: ast::DataType::BigInt(None),
             int32_cast_dtype: ast::DataType::Integer(None),
             timestamp_cast_dtype: ast::DataType::Timestamp(None, TimezoneInfo::None),
@@ -898,6 +916,7 @@ impl CustomDialectBuilder {
             large_utf8_cast_dtype: self.large_utf8_cast_dtype,
             date_field_extract_style: self.date_field_extract_style,
             character_length_style: self.character_length_style,
+            int8_cast_dtype: self.int8_cast_dtype,
             int64_cast_dtype: self.int64_cast_dtype,
             int32_cast_dtype: self.int32_cast_dtype,
             timestamp_cast_dtype: self.timestamp_cast_dtype,
@@ -952,6 +971,12 @@ impl CustomDialectBuilder {
         self
     }
 
+    /// Customize the dialect with a specific SQL type for Int8 casting: TinyInt, SmallInt, etc.
+    pub fn with_int8_cast_dtype(mut self, int8_cast_dtype: ast::DataType) -> Self {
+        self.int8_cast_dtype = int8_cast_dtype;
+        self
+    }
+
     /// Customize the dialect with a specific SQL type for Float64 casting: DOUBLE, DOUBLE PRECISION, etc.
     pub fn with_float64_ast_dtype(mut self, float64_ast_dtype: ast::DataType) -> Self {
         self.float64_ast_dtype = float64_ast_dtype;

datafusion/sql/src/unparser/expr.rs

@@ -1734,7 +1734,7 @@ impl Unparser<'_> {
                 not_impl_err!("Unsupported DataType: conversion: {data_type}")
             }
             DataType::Boolean => Ok(ast::DataType::Bool),
-            DataType::Int8 => Ok(ast::DataType::TinyInt(None)),
+            DataType::Int8 => Ok(self.dialect.int8_cast_dtype()),
             DataType::Int16 => Ok(ast::DataType::SmallInt(None)),
             DataType::Int32 => Ok(self.dialect.int32_cast_dtype()),
             DataType::Int64 => Ok(self.dialect.int64_cast_dtype()),

datafusion/sql/tests/cases/plan_to_sql.rs

@@ -1937,6 +1937,28 @@ fn test_without_offset() {
     )
 }
 
+#[test]
+fn test_cast_to_tinyint() -> Result<(), DataFusionError> {
+    roundtrip_statement_with_dialect_helper!(
+        sql: "select cast(3 as tinyint)",
+        parser_dialect: GenericDialect {},
+        unparser_dialect: UnparserPostgreSqlDialect {},
+        expected: @"SELECT CAST(3 AS SMALLINT)",
+    );
+    Ok(())
+}
+
+#[test]
+fn test_cast_to_tinyint_default_dialect() -> Result<(), DataFusionError> {
+    roundtrip_statement_with_dialect_helper!(
+        sql: "select cast(3 as tinyint)",
+        parser_dialect: GenericDialect {},
+        unparser_dialect: UnparserDefaultDialect {},
+        expected: @"SELECT CAST(3 AS TINYINT)",
+    );
+    Ok(())
+}
+
 #[test]
 fn test_with_offset0() {
     let statement = generate_round_trip_statement(MySqlDialect {}, "select 1 offset 0");