Add PreparedStatement::get_arrow_schema (no execution)

include/lbug_arrow.h

@@ -10,6 +10,7 @@
 namespace lbug_arrow {
 
 ArrowSchema query_result_get_arrow_schema(const lbug::main::QueryResult& result);
+ArrowSchema prepared_statement_get_arrow_schema(const lbug::main::PreparedStatement& stmt);
 bool query_result_has_next_arrow_chunk(lbug::main::QueryResult& result);
 ArrowArray query_result_get_next_arrow_chunk(lbug::main::QueryResult& result, uint64_t chunkSize);
 ArrowArray query_result_get_csr_indptr(const lbug::main::QueryResult& result);

src/connection.rs

@@ -56,6 +56,19 @@ impl PreparedStatement {
     pub fn is_read_only(&self) -> bool {
         ffi::prepared_statement_is_read_only(&self.statement)
     }
+
+    #[cfg(feature = "arrow")]
+    /// Returns the query's result schema as Arrow without executing it.
+    ///
+    /// The schema comes from the bound and planned statement, so this is
+    /// cheap even for expensive queries.
+    ///
+    /// *Requires the `arrow` feature*
+    pub fn get_arrow_schema(&self) -> Result<arrow::ffi::FFI_ArrowSchema, Error> {
+        Ok(
+            crate::ffi::arrow::ffi_arrow::prepared_statement_get_arrow_schema(&self.statement)?.0,
+        )
+    }
 }
 
 /// Connections are used to interact with a Database instance.

src/ffi/arrow.rs

@@ -29,6 +29,9 @@ pub(crate) mod ffi_arrow {
 
         #[namespace = "lbug::main"]
         type QueryResult<'db> = crate::ffi::ffi::QueryResult<'db>;
+
+        #[namespace = "lbug::main"]
+        type PreparedStatement = crate::ffi::ffi::PreparedStatement;
     }
 
     unsafe extern "C++" {
@@ -61,6 +64,11 @@ pub(crate) mod ffi_arrow {
 
         #[namespace = "lbug_arrow"]
         fn query_result_get_arrow_schema<'db>(result: &QueryResult<'db>) -> Result<ArrowSchema>;
+
+        #[namespace = "lbug_arrow"]
+        fn prepared_statement_get_arrow_schema(
+            statement: &PreparedStatement,
+        ) -> Result<ArrowSchema>;
     }
 
     #[namespace = "lbug_rs"]

src/lbug_arrow.cpp

@@ -4,6 +4,8 @@
 #include <stdexcept>
 #include <vector>
 
+#include "common/arrow/arrow_converter.h"
+
 namespace lbug {
 namespace main {
 
@@ -69,6 +71,12 @@ ArrowSchema query_result_get_arrow_schema(const lbug::main::QueryResult& result)
     return *result.getArrowSchema();
 }
 
+ArrowSchema prepared_statement_get_arrow_schema(const lbug::main::PreparedStatement& stmt) {
+    // Schema only: names/types come from the bound and planned statement, no execution.
+    return *lbug::common::ArrowConverter::toArrowSchema(stmt.getColumnTypes(),
+        stmt.getColumnNames(), false /* fallbackExtensionTypes */);
+}
+
 bool query_result_has_next_arrow_chunk(lbug::main::QueryResult& result) {
     return result.hasNextArrowChunk();
 }