Use a single transaction per batch

src/main/java/org/duckdb/DuckDBPreparedStatement.java

@@ -124,6 +124,10 @@ private void prepare(String sql) throws SQLException {
 
     @Override
     public boolean execute() throws SQLException {
+        return execute(true);
+    }
+
+    private boolean execute(boolean startTransaction) throws SQLException {
         if (isClosed()) {
             throw new SQLException("Statement was closed");
         }
@@ -138,7 +142,9 @@ public boolean execute() throws SQLException {
         select_result = null;
 
         try {
-            startTransaction();
+            if (startTransaction) {
+                startTransaction();
+            }
             result_ref = DuckDBNative.duckdb_jdbc_execute(stmt_ref, params);
             DuckDBResultSetMetaData result_meta = DuckDBNative.duckdb_jdbc_query_result_meta(result_ref);
             select_result = new DuckDBResultSet(this, result_meta, result_ref, conn.conn_ref);
@@ -476,19 +482,23 @@ public int[] executeBatch() throws SQLException {
 
     private int[] executeBatchedPreparedStatement() throws SQLException {
         int[] updateCounts = new int[this.batchedParams.size()];
+
+        startTransaction();
         for (int i = 0; i < this.batchedParams.size(); i++) {
             params = this.batchedParams.get(i);
-            execute();
+            execute(false);
             updateCounts[i] = getUpdateCount();
         }
         return updateCounts;
     }
 
     private int[] executeBatchedStatements() throws SQLException {
         int[] updateCounts = new int[this.batchedStatements.size()];
+
+        startTransaction();
         for (int i = 0; i < this.batchedStatements.size(); i++) {
             prepare(this.batchedStatements.get(i));
-            execute();
+            execute(false);
             updateCounts[i] = getUpdateCount();
         }
         return updateCounts;