refactor(mdb): use SQLExecDirect for infer_schema (simpler than prepare+execute)

remote-table/src/connection/mdb/mod.rs

@@ -190,22 +190,20 @@ impl Connection for MdbConnection {
         let sql = RemoteDbType::Mdb.limit_1_query_if_possible(source)?;
         debug!("[remote-table] inferring mdb schema with: {sql}");
         let conn = self.conn.lock().await;
-        let mut prepared = conn.prepare(&sql).map_err(|e| {
-            DataFusionError::Plan(format!(
-                "Failed to prepare query for schema inference on mdb: {e:?}, sql: {sql}"
-            ))
-        })?;
-        // mdbtools does not populate result-set metadata (SQLNumResultCols /
-        // SQLDescribeCol) until after SQLExecute. Prepared::execute(()) is the
-        // safe wrapper around SQLExecute; we discard the returned cursor since
-        // column metadata is then available on `prepared` itself.
-        prepared.execute(()).map_err(|e| {
+        let cursor_opt = conn.execute(&sql, (), None).map_err(|e| {
             DataFusionError::Plan(format!(
                 "Failed to execute query for schema inference on mdb: {e:?}, sql: {sql}"
             ))
         })?;
-        let remote_schema = Arc::new(build_remote_schema(&mut prepared)?);
-        Ok(remote_schema)
+        match cursor_opt {
+            None => Err(DataFusionError::Plan(
+                "No rows returned to infer schema".to_string(),
+            )),
+            Some(cursor) => {
+                let remote_schema = Arc::new(build_remote_schema(cursor)?);
+                Ok(remote_schema)
+            }
+        }
     }
 
     async fn query(

remote-table/src/connection/mdb/schema.rs

@@ -4,7 +4,7 @@ use crate::RemoteField;
 use crate::RemoteSchema;
 use crate::RemoteType;
 use datafusion_common::DataFusionError;
-use odbc_api::Prepared;
+use odbc_api::CursorImpl;
 use odbc_api::ResultSetMetadata;
 use odbc_api::handles::{AsStatementRef, ColumnDescription, Statement, StatementImpl};
 
@@ -26,10 +26,8 @@ fn sql_chars_to_string_lossy(name: &[u16]) -> String {
     String::from_utf16_lossy(name)
 }
 
-pub(super) fn build_remote_schema(
-    prepared: &mut Prepared<StatementImpl<'_>>,
-) -> DFResult<RemoteSchema> {
-    let col_count = prepared
+pub(super) fn build_remote_schema(mut cursor: CursorImpl<StatementImpl>) -> DFResult<RemoteSchema> {
+    let col_count = cursor
         .num_result_cols()
         .map_err(|e| DataFusionError::External(Box::new(e)))? as u16;
     let mut remote_fields = vec![];
@@ -39,7 +37,7 @@ pub(super) fn build_remote_schema(
         // col_nullability return NoDiagnostics), so we go through the
         // low-level SQLDescribeCol path.
         let mut col_desc = ColumnDescription::default();
-        let describe_result = prepared.as_stmt_ref().describe_col(i, &mut col_desc);
+        let describe_result = cursor.as_stmt_ref().describe_col(i, &mut col_desc);
         if describe_result.is_err() {
             return Err(DataFusionError::Plan(format!(
                 "describe_col failed for column {i} on mdb"