Bugfix in the SQL SELECT parser

This fixes a bug that led some SPARQL queries to be created without
any variable in the SELECT clause. We now use * in case the planner
cannot identify which nodes should be retrieved, which can be a bit
inefficent if we're dealing with many columns, but it is better than
an error message.

rdf_fdw.c

@@ -2714,7 +2714,7 @@ static void CreateSPARQL(RDFfdwState *state, PlannerInfo *root)
 		elog(DEBUG1, "  %s: SPARQL is valid and contains a DISTINCT modifier > pushing down DISTINCT", __func__);
 		appendStringInfo(&sparql,"%s\nSELECT DISTINCT %s\n%s%s",
 			state->sparql_prefixes, 
-			state->sparql_select,
+			strlen(state->sparql_select) == 0 ? " * " : state->sparql_select,
 			state->sparql_from,
 			where_graph.data);		
 	} 
@@ -2728,7 +2728,7 @@ static void CreateSPARQL(RDFfdwState *state, PlannerInfo *root)
 		elog(DEBUG1, "  %s: SPARQL is valid and contains a REDUCED modifier > pushing down REDUCED", __func__);
 		appendStringInfo(&sparql,"%s\nSELECT REDUCED %s\n%s%s",
 			state->sparql_prefixes, 
-			state->sparql_select, 
+			strlen(state->sparql_select) == 0 ? " * " : state->sparql_select,
 			state->sparql_from, 
 			where_graph.data);		
 	}
@@ -2744,15 +2744,15 @@ static void CreateSPARQL(RDFfdwState *state, PlannerInfo *root)
 	{
 		appendStringInfo(&sparql,"%s\nSELECT DISTINCT %s\n%s%s",
 			state->sparql_prefixes, 
-			state->sparql_select,
+			strlen(state->sparql_select) == 0 ? " * " : state->sparql_select,
 			state->sparql_from,
 			where_graph.data);
 	}
 	else
 	{	
 		appendStringInfo(&sparql,"%s\nSELECT %s\n%s%s",
 			state->sparql_prefixes, 
-			state->sparql_select, 
+			strlen(state->sparql_select) == 0 ? " * " : state->sparql_select,
 			state->sparql_from, 
 			where_graph.data);
 	}