set_condition should not reset entire instance state

apis/python/src/tiledbsoma/managed_query.cc

@@ -98,7 +98,7 @@ void load_managed_query(py::module& m) {
                              .ptr()
                              .get();
                 }
-                mq.reset();
+                mq.reset_columns();
                 mq.select_columns(column_names);
 
                 // Release python GIL after we're done accessing python

libtiledbsoma/src/soma/managed_query.cc

@@ -129,6 +129,10 @@ void ManagedQuery::select_columns(
     }
 }
 
+void ManagedQuery::reset_columns() {
+    columns_.clear();
+}
+
 void ManagedQuery::setup_read() {
     // If the query is complete, return so we do not submit it again
     auto status = query_->query_status();

libtiledbsoma/src/soma/managed_query.h

@@ -135,12 +135,20 @@ class ManagedQuery {
      * list of selected columns is empty. This prevents a `select_columns` call
      * from changing an empty list (all columns) to a subset of columns.
      *
+     * NB: you may only select a given column once. Selecting twice will generate
+     * an error in read_next.
+     *
      * @param names Vector of column names
      * @param if_not_empty Prevent changing an "empty" selection of all columns
      */
     void select_columns(
         const std::vector<std::string>& names, bool if_not_empty = false);
 
+    /**
+     * @brief Reset column selection to none, aka "all".
+     */
+    void reset_columns();
+
     /**
      * @brief Returns the column names set by the query.
      *