skewedparam-v2.patch

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 8b76e98529..14b8bec6ff 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -154,6 +154,8 @@ bool		enable_partition_pruning = true;
 bool		enable_presorted_aggregate = true;
 bool		enable_async_append = true;
 
+double		skewed_param_factor = DEFAULT_SKEWED_PARAM_FACTOR;
+
 typedef struct
 {
 	PlannerInfo *root;
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index edc25d712e..8b922c0c95 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -2493,6 +2493,8 @@ eval_const_expressions_mutator(Node *node,
 											pval,
 											prm->isnull,
 											typByVal);
+							if (paramLI->paramFetch == NULL)
+								con->paramid = param->paramid;
 							con->location = param->location;
 							return (Node *) con;
 						}
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index cea777e9d4..9fa6548d23 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -273,10 +273,32 @@ eqsel_internal(PG_FUNCTION_ARGS, bool negate)
 	 * in the query.)
 	 */
 	if (IsA(other, Const))
-		selec = var_eq_const(&vardata, operator, collation,
-							 ((Const *) other)->constvalue,
-							 ((Const *) other)->constisnull,
-							 varonleft, negate);
+	{
+		Const * con = (Const *) other;
+
+		if (con->paramid > 0)
+		{
+			bool	skewed_stat = false;
+
+			selec = var_eq_const_ext(&vardata, operator, collation,
+								((Const *) other)->constvalue,
+								((Const *) other)->constisnull,
+								varonleft, negate, &skewed_stat);
+
+			if (skewed_stat)
+			{
+				ParamExternData *prm;
+
+				prm = &root->glob->boundParams->params[con->paramid - 1];
+				prm->pflags |= PARAM_FLAG_SKEWEDSTAT;
+			}
+		}
+		else
+			selec = var_eq_const(&vardata, operator, collation,
+								((Const *) other)->constvalue,
+								((Const *) other)->constisnull,
+								varonleft, negate);
+	}
 	else
 		selec = var_eq_non_const(&vardata, operator, collation, other,
 								 varonleft, negate);
@@ -295,6 +317,16 @@ double
 var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 			 Datum constval, bool constisnull,
 			 bool varonleft, bool negate)
+{
+	return var_eq_const_ext(vardata, oproid, collation,
+			 constval, constisnull,
+			 varonleft, negate, NULL);
+}
+
+double
+var_eq_const_ext(VariableStatData *vardata, Oid oproid, Oid collation,
+			 Datum constval, bool constisnull,
+			 bool varonleft, bool negate, bool *skewed_stat)
 {
 	double		selec;
 	double		nullfrac = 0.0;
@@ -387,6 +419,39 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 					break;
 				}
 			}
+
+			Assert(sslot.nvalues > 0);
+			if (skewed_stat)
+			{
+				/* Check whether the values of the field is skewed according to the MCVs. */
+				if (sslot.nvalues > 1 &&
+						sslot.numbers[0] > sslot.numbers[sslot.nvalues - 1] * skewed_param_factor)
+					/* Compare the maximum and minimum quantities in the MCVs. */
+					*skewed_stat = true;
+				else
+				{
+					/*
+					 * Compare the maximum number in the MCVs with the average number of
+					 * other values(not in the MCVs).
+					 */
+					int			j;
+					double		sumcommon = 0.0;
+					double		otherdistinct, avgselec;
+
+					for (j = 0; j < sslot.nnumbers; j++)
+						sumcommon += sslot.numbers[j];
+
+					otherdistinct = get_variable_numdistinct(vardata, &isdefault) -
+						sslot.nnumbers;
+
+					if (otherdistinct > 0)
+					{
+						avgselec = (1.0 - sumcommon - nullfrac) / otherdistinct;
+						if (sslot.numbers[0] > (avgselec * skewed_param_factor))
+							*skewed_stat = true;
+					}
+				}
+			}
 		}
 		else
 		{
diff --git a/src/backend/utils/cache/plancache.c b/src/backend/utils/cache/plancache.c
index 5194cbf2cc..d1ee00139e 100644
--- a/src/backend/utils/cache/plancache.c
+++ b/src/backend/utils/cache/plancache.c
@@ -991,6 +991,20 @@ BuildCachedPlan(CachedPlanSource *plansource, List *qlist,
 	else
 		plan_context = CurrentMemoryContext;
 
+	if (boundParams)
+	{
+		int i;
+
+		for (i =0; i < boundParams->numParams; i++)
+		{
+			if (boundParams->params[i].pflags & PARAM_FLAG_SKEWEDSTAT)
+			{
+				plansource->hasSkewedparam = true;
+				break;
+			}
+		}
+	}
+
 	/*
 	 * Create and fill the CachedPlan struct within the new context.
 	 */
@@ -1066,6 +1080,12 @@ choose_custom_plan(CachedPlanSource *plansource, ParamListInfo boundParams)
 	if (plan_cache_mode == PLAN_CACHE_MODE_FORCE_CUSTOM_PLAN)
 		return true;
 
+	if (plan_cache_mode == PLAN_CACHE_MODE_STAT_ADAPTIVE_PLAN)
+	{
+		if (plansource->hasSkewedparam)
+			return true;
+	}
+
 	/* See if caller wants to force the decision */
 	if (plansource->cursor_options & CURSOR_OPT_GENERIC_PLAN)
 		return false;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 70652f0a3f..93f85baedb 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -403,6 +403,7 @@ static const struct config_enum_entry debug_parallel_query_options[] = {
 
 static const struct config_enum_entry plan_cache_mode_options[] = {
 	{"auto", PLAN_CACHE_MODE_AUTO, false},
+	{"stat_adaptive_plan", PLAN_CACHE_MODE_STAT_ADAPTIVE_PLAN, false},
 	{"force_generic_plan", PLAN_CACHE_MODE_FORCE_GENERIC_PLAN, false},
 	{"force_custom_plan", PLAN_CACHE_MODE_FORCE_CUSTOM_PLAN, false},
 	{NULL, 0, false}
@@ -3866,6 +3867,17 @@ struct config_real ConfigureNamesReal[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"skewed_param_factor", PGC_SUSET, LOGGING_WHEN,
+			gettext_noop("Sets the skewed factor for parameters of the prepare statement."),
+			gettext_noop("Use a thredhold to determine whether a skewed parameter exists."
+						 "If true, use the custom plan.")
+		},
+		&skewed_param_factor,
+		DEFAULT_SKEWED_PARAM_FACTOR, 2.0, 10000.0,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0.0, 0.0, 0.0, NULL, NULL, NULL
diff --git a/src/include/nodes/params.h b/src/include/nodes/params.h
index 10c9fc5413..051a12bb1d 100644
--- a/src/include/nodes/params.h
+++ b/src/include/nodes/params.h
@@ -85,7 +85,13 @@ struct ParseState;
  *	  and paramCompileArg is rather arbitrary.
  */
 
-#define PARAM_FLAG_CONST	0x0001	/* parameter is constant */
+/* parameter is constant */
+#define PARAM_FLAG_CONST			0x0001
+/*
+ * The var side of 'var eq param' or 'param eq var' expression
+ * corresponding to the parameter contains skewed data.
+ */
+#define PARAM_FLAG_SKEWEDSTAT		0x0002
 
 typedef struct ParamExternData
 {
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index 4a154606d2..7e6762de47 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -314,6 +314,12 @@ typedef struct Const
 	 */
 	bool		constbyval pg_node_attr(query_jumble_ignore);
 
+	/*
+	 * >0 indicates that the constant is extracted from Param.
+	 * Its value is paramid of the Param struct.
+	 */
+	int			paramid pg_node_attr(query_jumble_ignore);
+
 	/*
 	 * token location, or -1 if unknown.  All constants are tracked as
 	 * locations in query jumbling, to be marked as parameters.
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index b1c51a4e70..3606e47681 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -28,6 +28,7 @@
 #define DEFAULT_CPU_OPERATOR_COST  0.0025
 #define DEFAULT_PARALLEL_TUPLE_COST 0.1
 #define DEFAULT_PARALLEL_SETUP_COST  1000.0
+#define DEFAULT_SKEWED_PARAM_FACTOR  10.0
 
 /* defaults for non-Cost parameters */
 #define DEFAULT_RECURSIVE_WORKTABLE_FACTOR  10.0
@@ -71,6 +72,7 @@ extern PGDLLIMPORT bool enable_partition_pruning;
 extern PGDLLIMPORT bool enable_presorted_aggregate;
 extern PGDLLIMPORT bool enable_async_append;
 extern PGDLLIMPORT int constraint_exclusion;
+extern PGDLLIMPORT double skewed_param_factor;
 
 extern double index_pages_fetched(double tuples_fetched, BlockNumber pages,
 								  double index_pages, PlannerInfo *root);
diff --git a/src/include/utils/plancache.h b/src/include/utils/plancache.h
index a90dfdf906..c742c6449c 100644
--- a/src/include/utils/plancache.h
+++ b/src/include/utils/plancache.h
@@ -30,6 +30,7 @@ struct RawStmt;
 typedef enum
 {
 	PLAN_CACHE_MODE_AUTO,
+	PLAN_CACHE_MODE_STAT_ADAPTIVE_PLAN,
 	PLAN_CACHE_MODE_FORCE_GENERIC_PLAN,
 	PLAN_CACHE_MODE_FORCE_CUSTOM_PLAN,
 }			PlanCacheMode;
@@ -124,6 +125,7 @@ typedef struct CachedPlanSource
 	bool		is_complete;	/* has CompleteCachedPlan been done? */
 	bool		is_saved;		/* has CachedPlanSource been "saved"? */
 	bool		is_valid;		/* is the query_list currently valid? */
+	bool		hasSkewedparam; /* see the definition of PARAM FLAG SKEWEDSTAT */
 	int			generation;		/* increments each time we create a plan */
 	/* If CachedPlanSource has been saved, it is a member of a global list */
 	dlist_node	node;			/* list link, if is_saved */
diff --git a/src/include/utils/selfuncs.h b/src/include/utils/selfuncs.h
index 2fa4c4fc1b..a7532b8d7b 100644
--- a/src/include/utils/selfuncs.h
+++ b/src/include/utils/selfuncs.h
@@ -184,6 +184,10 @@ extern double var_eq_const(VariableStatData *vardata,
 						   Oid oproid, Oid collation,
 						   Datum constval, bool constisnull,
 						   bool varonleft, bool negate);
+extern double var_eq_const_ext(VariableStatData *vardata,
+						   Oid oproid, Oid collation,
+						   Datum constval, bool constisnull,
+						   bool varonleft, bool negate, bool *skewed_stat);
 extern double var_eq_non_const(VariableStatData *vardata,
 							   Oid oproid, Oid collation,
 							   Node *other,