Cherry-pick from stable15 of two bonded commits:

1.
Load neighbours with the fss hash except duplicated neighours.

Rewrite test for look-a-like functional. Current tests contain
correlation columns and queries have more nodes and description
features.

Add aqo_k as custom parameter to define few number of features
for prediction. Its default value is 3. Queries can contain
a larger number of features than 3 especially generic queries.

Also add predict_a_few_neibours parameter for switch avalable
to predict a few neibors than 3. It is done for not to change
the previous logic of the code

2.
Add disabled nestloop and mergejoin parameters to stabilize look-a-like test,
besides add two additional cases where look-a-like should not be applied.

Author: Alena Rybakina

Diff for: aqo.c

@@ -38,6 +38,7 @@ void _PG_init(void);
 /* Strategy of determining feature space for new queries. */
 int		aqo_mode = AQO_MODE_CONTROLLED;
 bool	force_collect_stat;
+bool	aqo_predict_with_few_neighbors;
 
 /*
  * Show special info in EXPLAIN mode.
@@ -75,7 +76,7 @@ int			auto_tuning_infinite_loop = 8;
 /* Machine learning parameters */
 
 /* The number of nearest neighbors which will be chosen for ML-operations */
-int			aqo_k = 3;
+int			aqo_k;
 double		log_selectivity_lower_bound = -30;
 
 bool		cleanup_bgworker = false;
@@ -415,6 +416,29 @@ _PG_init(void)
 							 NULL
 	);
 
+	DefineCustomIntVariable("aqo.k_neighbors_threshold",
+							"Set the threshold of number of neighbors for predicting.",
+							NULL,
+							&aqo_k,
+							3,
+							1, INT_MAX / 1000,
+							PGC_USERSET,
+							0,
+							NULL,
+							NULL,
+							NULL);
+
+	DefineCustomBoolVariable("aqo.predict_with_few_neighbors",
+							"Make prediction with less neighbors than we should have.",
+							 NULL,
+							 &aqo_predict_with_few_neighbors,
+							 true,
+							 PGC_USERSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+
 	prev_shmem_startup_hook						= shmem_startup_hook;
 	shmem_startup_hook							= aqo_init_shmem;
 	prev_planner_hook							= planner_hook;

Diff for: aqo.h

@@ -217,6 +217,7 @@ extern double auto_tuning_convergence_error;
 /* Machine learning parameters */
 
 extern int	aqo_k;
+extern bool aqo_predict_with_few_neighbors;
 extern double log_selectivity_lower_bound;
 
 /* Parameters for current query */

Diff for: cardinality_estimation.c

@@ -93,7 +93,7 @@ predict_for_relation(List *clauses, List *selectivities, List *relsigns,
 		 */
 
 		/* Try to search in surrounding feature spaces for the same node */
-		if (!load_aqo_data(query_context.fspace_hash, *fss, data, NULL, use_wide_search))
+		if (!load_aqo_data(query_context.fspace_hash, *fss, data, NULL, use_wide_search, features))
 			result = -1;
 		else
 		{