cql3: fix fetching clustering key columns for filtering

When a column is not present in the select clause, but used for
filtering, it usually needs to be fetched from replicas.
Sometimes it can be avoided, e.g. if primary key columns form a valid
prefix - then, they will be optimized out before filtering itself.
However, clustering key prefix can only be qualified for this
optimization if the whole partition key is restricted - otherwise
the clustering columns still need to be present for filtering.

Fixes scylladb#4541

cql3/restrictions/statement_restrictions.cc

@@ -392,8 +392,9 @@ std::vector<const column_definition*> statement_restrictions::get_column_defs_fo
                 }
             }
         }
-        if (_clustering_columns_restrictions->needs_filtering(*_schema)) {
-            column_id first_filtering_id = _schema->clustering_key_columns().begin()->id +
+        const bool pk_has_unrestricted_components = _partition_key_restrictions->has_unrestricted_components(*_schema);
+        if (pk_has_unrestricted_components || _clustering_columns_restrictions->needs_filtering(*_schema)) {
+            column_id first_filtering_id = pk_has_unrestricted_components ? 0 : _schema->clustering_key_columns().begin()->id +
                     _clustering_columns_restrictions->num_prefix_columns_that_need_not_be_filtered();
             for (auto&& cdef : _clustering_columns_restrictions->get_column_defs()) {
                 if (cdef->id >= first_filtering_id && !column_uses_indexing(cdef)) {