ClickHouse: `Display` implementation converts types to uppercase, causing `UNKNOWN_TYPE` errors

首先检查 DataType Display 的实现，以及 issue 中列出的 AST 类型，包括 Statement、Query、Expr、ColumnDef 和相关节点。跟踪序列化如何处理嵌套数据类型，然后评估提议的、面向方言的 to_sql API 在整个层次结构中的适用性。当 ClickHouse 示例使用 PascalCase 类型完成往返，并且现有 Display 行为仍保持向后兼容时，即表示完成。

Problem

ClickHouse data types are case-sensitive and require PascalCase (e.g., String, Int32, Nullable). However, the sqlparser-rs library's Display implementation for DataType converts certain types to uppercase, causing UNKNOWN_TYPE errors when round-tripping SQL through ClickHouse.

Example

use sqlparser::dialect::ClickHouseDialect;
use sqlparser::parser::Parser;

let sql = "CREATE TABLE t (col Nullable(String))";
let dialect = ClickHouseDialect {};
let ast = Parser::parse_sql(&dialect, sql).unwrap();

// Round-trip: parse and convert back to string
let regenerated = ast[0].to_string();
// Result: "CREATE TABLE t (col Nullable(STRING))"
//                                       ^^^^^^ uppercase!

When this regenerated SQL is executed against ClickHouse, it fails with:

Code: 47. DB::Exception: Unknown type STRING. (UNKNOWN_TYPE)

Affected Types

Type	Current Output	ClickHouse Requires
DataType::Int8	INT8	Int8
DataType::Int64	INT64	Int64
DataType::Float64	FLOAT64	Float64
DataType::String	STRING	String
DataType::Bool	BOOL	Bool
DataType::Date	DATE	Date
DataType::Datetime	DATETIME	DateTime

Types already correct (PascalCase):

• Int16, Int32, Int128, Int256
• UInt8, UInt16, UInt32, UInt64, UInt128, UInt256
• Float32
• Nullable, LowCardinality, Array, Map, Tuple, Nested

Root Cause

The Display trait implementation for DataType uses uppercase for type names (e.g., write!(f, "STRING")), which is standard for most SQL dialects but incorrect for ClickHouse.

The challenge is that Display doesn't have access to dialect context, so it can't conditionally format based on the active dialect.

The Problem with Display

Most users serialize SQL by calling Display on top-level AST types:

let ast = Parser::parse_sql(&dialect, sql).unwrap();
let regenerated = ast[0].to_string();  // Uses Display on Statement
// or
let regenerated = format!("{}", query);  // Uses Display on Query

The Display trait signature doesn't allow passing context:

fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result

Proposed Solution

To solve this, we need dialect-aware serialization throughout the AST hierarchy:

Add to_sql(&dyn Dialect) to all AST types

• Add to_sql(&dyn Dialect) -> String method to Statement, Query, Expr, ColumnDef, and other AST types
• Each type's implementation calls to_sql() on its children, propagating the dialect
• Keep existing Display implementations unchanged for backwards compatibility

// Example usage after fix:
let ast = Parser::parse_sql(&dialect, sql).unwrap();
let regenerated = ast[0].to_sql(&dialect);  // Correct PascalCase for ClickHouse

Implementation Scope

The affected types include (non-exhaustive):

• Statement (top-level)
• Query, SetExpr, Select
• Expr (especially Cast, TryCast, SafeCast)
• ColumnDef, ColumnOption
• TableConstraint
• AlterTableOperation
• FunctionArg, FunctionArgExpr

This is a significant change but provides the cleanest API and maintains backwards compatibility.

Workarounds

Currently, users must post-process the SQL string output to fix casing. See 514-labs/moosestack#3152 for an example regex-based workaround.

References

• ClickHouse Data Types Documentation
• Related workaround: 514-labs/moosestack#3152