Make SELECT DISTINCT handle non-var targetlists

The current SELECT DISTINCT pushdown code assumed that the targetlist
will always contain references to column attributes of the target
table.

So, even a simple "SELECT DISTINCT 1 from table;" causes a segmentation
fault because the "varno" field is not assigned. Fix this oversight.

Issue reported by @svenklemm

Fixes timescale/timescaledb-private#920

tsl/src/fdw/deparse.c

@@ -860,7 +860,7 @@ deparseDistinctClause(StringInfo buf, deparse_expr_cxt *context)
 	PlannerInfo *root = context->root;
 	Query *query = root->parse;
 	ListCell *l, *dc_l;
-	bool first = true;
+	bool first = true, varno_assigned = false;
 	Index varno;
 	RangeTblEntry *dc_rte;
 	RangeTblEntry *rte;
@@ -889,6 +889,7 @@ deparseDistinctClause(StringInfo buf, deparse_expr_cxt *context)
 			{
 				varno = var->varno;
 				first = false;
+				varno_assigned = true;
 			}
 
 			if (varno != var->varno)
@@ -899,6 +900,10 @@ deparseDistinctClause(StringInfo buf, deparse_expr_cxt *context)
 			return;
 	}
 
+	/* If there are no varno entries in the distinctClause, we are done */
+	if (!varno_assigned)
+		return;
+
 	/*
 	 * If all distinctClause entries point to our rte->relid then it's
 	 * safe to push down to the datanode

tsl/test/shared/expected/dist_distinct.out

@@ -888,6 +888,34 @@ LIMIT 10;
                            Remote SQL: SELECT "time" FROM public.metrics_dist WHERE _timescaledb_internal.chunks_in(public.metrics_dist.*, ARRAY[1, 2, 3]) ORDER BY public.time_bucket('01:00:00'::interval, "time") ASC NULLS LAST
 (23 rows)
 
+SELECT DISTINCT without any var references is handled correctly
+EXPLAIN (verbose, costs off)
+SELECT DISTINCT 1, 'constx'
+FROM metrics_dist;
+                                                                    QUERY PLAN                                                                     
+---------------------------------------------------------------------------------------------------------------------------------------------------
+ Unique
+   Output: 1, 'constx'::text
+   ->  Custom Scan (AsyncAppend)
+         Output: 1, 'constx'::text
+         ->  Append
+               ->  Custom Scan (DataNodeScan) on public.metrics_dist metrics_dist_1
+                     Output: 1, 'constx'::text
+                     Data node: data_node_1
+                     Chunks: _dist_hyper_7_37_chunk, _dist_hyper_7_40_chunk, _dist_hyper_7_43_chunk
+                     Remote SQL: SELECT NULL FROM public.metrics_dist WHERE _timescaledb_internal.chunks_in(public.metrics_dist.*, ARRAY[1, 2, 3])
+               ->  Custom Scan (DataNodeScan) on public.metrics_dist metrics_dist_2
+                     Output: 1, 'constx'::text
+                     Data node: data_node_2
+                     Chunks: _dist_hyper_7_38_chunk, _dist_hyper_7_41_chunk, _dist_hyper_7_44_chunk
+                     Remote SQL: SELECT NULL FROM public.metrics_dist WHERE _timescaledb_internal.chunks_in(public.metrics_dist.*, ARRAY[1, 2, 3])
+               ->  Custom Scan (DataNodeScan) on public.metrics_dist metrics_dist_3
+                     Output: 1, 'constx'::text
+                     Data node: data_node_3
+                     Chunks: _dist_hyper_7_39_chunk, _dist_hyper_7_42_chunk, _dist_hyper_7_45_chunk
+                     Remote SQL: SELECT NULL FROM public.metrics_dist WHERE _timescaledb_internal.chunks_in(public.metrics_dist.*, ARRAY[1, 2, 3])
+(20 rows)
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %%% RUNNING TESTS on table: metrics
 %%% PREFIX: 

tsl/test/shared/sql/include/dist_distinct_run.sql

@@ -133,3 +133,9 @@ SELECT DISTINCT time_bucket('1h',time) col1
 FROM :TABLE_NAME
 ORDER BY col1
 LIMIT 10;
+
+-- SELECT DISTINCT without any var references is handled correctly
+\qecho SELECT DISTINCT without any var references is handled correctly
+:PREFIX
+SELECT DISTINCT 1, 'constx'
+FROM :TABLE_NAME;