Add plan_cache_mode setting

This allows overriding the choice of custom or generic plan.

Author: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAFj8pRAGLaiEm8ur5DWEBo7qHRWTk9HxkuUAz00CZZtJj-LkCA%40mail.gmail.com

doc/src/sgml/config.sgml

@@ -4616,6 +4616,36 @@ SELECT * FROM parent WHERE key = 2400;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-plan-cache_mode" xreflabel="plan_cache_mode">
+      <term><varname>plan_cache_mode</varname> (<type>enum</type>)
+      <indexterm>
+       <primary><varname>plan_cache_mode</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Prepared statements (either explicitly prepared or implicitly
+        generated, for example in PL/pgSQL) can be executed using custom or
+        generic plans.  A custom plan is replanned for a new parameter value,
+        a generic plan is reused for repeated executions of the prepared
+        statement.  The choice between them is normally made automatically.
+        This setting overrides the default behavior and forces either a custom
+        or a generic plan.  This can be used to work around performance
+        problems in specific cases.  Note, however, that the plan cache
+        behavior is subject to change, so this setting, like all settings that
+        force the planner's hand, should be reevaluated regularly.
+       </para>
+
+       <para>
+        The allowed values are <literal>auto</literal>,
+        <literal>force_custom_plan</literal> and
+        <literal>force_generic_plan</literal>.  The default value is
+        <literal>auto</literal>.  The setting is applied when a cached plan is
+        to be executed, not when it is prepared.
+       </para>
+      </listitem>
+     </varlistentry>
+
      </variablelist>
     </sect2>
    </sect1>

src/backend/utils/cache/plancache.c

@@ -106,6 +106,8 @@ static void PlanCacheRelCallback(Datum arg, Oid relid);
 static void PlanCacheFuncCallback(Datum arg, int cacheid, uint32 hashvalue);
 static void PlanCacheSysCallback(Datum arg, int cacheid, uint32 hashvalue);
 
+/* GUC parameter */
+int	plan_cache_mode;
 
 /*
  * InitPlanCache: initialize module during InitPostgres.
@@ -1033,6 +1035,12 @@ choose_custom_plan(CachedPlanSource *plansource, ParamListInfo boundParams)
 	if (IsTransactionStmtPlan(plansource))
 		return false;
 
+	/* Let settings force the decision */
+	if (plan_cache_mode == PLAN_CACHE_MODE_FORCE_GENERIC_PLAN)
+		return false;
+	if (plan_cache_mode == PLAN_CACHE_MODE_FORCE_CUSTOM_PLAN)
+		return true;
+
 	/* See if caller wants to force the decision */
 	if (plansource->cursor_options & CURSOR_OPT_GENERIC_PLAN)
 		return false;

src/backend/utils/misc/guc.c

@@ -405,6 +405,13 @@ static const struct config_enum_entry force_parallel_mode_options[] = {
 	{NULL, 0, false}
 };
 
+static const struct config_enum_entry plan_cache_mode_options[] = {
+	{"auto", PLAN_CACHE_MODE_AUTO, false},
+	{"force_generic_plan", PLAN_CACHE_MODE_FORCE_GENERIC_PLAN, false},
+	{"force_custom_plan", PLAN_CACHE_MODE_FORCE_CUSTOM_PLAN, false},
+	{NULL, 0, false}
+};
+
 /*
  * password_encryption used to be a boolean, so accept all the likely
  * variants of "on", too. "off" used to store passwords in plaintext,
@@ -4150,6 +4157,18 @@ static struct config_enum ConfigureNamesEnum[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"plan_cache_mode", PGC_USERSET, QUERY_TUNING_OTHER,
+			gettext_noop("Controls the planner's selection of custom or generic plan."),
+			gettext_noop("Prepared statements can have custom and generic plans, and the planner "
+						 "will attempt to choose which is better.  This can be set to override "
+						 "the default behavior.")
+		},
+		&plan_cache_mode,
+		PLAN_CACHE_MODE_AUTO, plan_cache_mode_options,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0, NULL, NULL, NULL, NULL

src/backend/utils/misc/postgresql.conf.sample

@@ -350,6 +350,7 @@
 #join_collapse_limit = 8		# 1 disables collapsing of explicit
 					# JOIN clauses
 #force_parallel_mode = off
+#plan_cache_mode = auto
 
 
 #------------------------------------------------------------------------------

src/include/utils/plancache.h

@@ -182,4 +182,15 @@ extern CachedPlan *GetCachedPlan(CachedPlanSource *plansource,
 			  QueryEnvironment *queryEnv);
 extern void ReleaseCachedPlan(CachedPlan *plan, bool useResOwner);
 
+/* possible values for plan_cache_mode */
+typedef enum
+{
+	PLAN_CACHE_MODE_AUTO,
+	PLAN_CACHE_MODE_FORCE_GENERIC_PLAN,
+	PLAN_CACHE_MODE_FORCE_CUSTOM_PLAN
+}			PlanCacheMode;
+
+/* GUC parameter */
+extern int plan_cache_mode;
+
 #endif							/* PLANCACHE_H */

src/test/regress/expected/plancache.out

@@ -278,3 +278,80 @@ drop table pc_list_part_1;
 execute pstmt_def_insert(1);
 drop table pc_list_parted, pc_list_part_null;
 deallocate pstmt_def_insert;
+-- Test plan_cache_mode
+create table test_mode (a int);
+insert into test_mode select 1 from generate_series(1,1000) union all select 2;
+create index on test_mode (a);
+analyze test_mode;
+prepare test_mode_pp (int) as select count(*) from test_mode where a = $1;
+-- up to 5 executions, custom plan is used
+explain (costs off) execute test_mode_pp(2);
+                        QUERY PLAN                        
+----------------------------------------------------------
+ Aggregate
+   ->  Index Only Scan using test_mode_a_idx on test_mode
+         Index Cond: (a = 2)
+(3 rows)
+
+-- force generic plan
+set plan_cache_mode to force_generic_plan;
+explain (costs off) execute test_mode_pp(2);
+         QUERY PLAN          
+-----------------------------
+ Aggregate
+   ->  Seq Scan on test_mode
+         Filter: (a = $1)
+(3 rows)
+
+-- get to generic plan by 5 executions
+set plan_cache_mode to auto;
+execute test_mode_pp(1); -- 1x
+ count 
+-------
+  1000
+(1 row)
+
+execute test_mode_pp(1); -- 2x
+ count 
+-------
+  1000
+(1 row)
+
+execute test_mode_pp(1); -- 3x
+ count 
+-------
+  1000
+(1 row)
+
+execute test_mode_pp(1); -- 4x
+ count 
+-------
+  1000
+(1 row)
+
+execute test_mode_pp(1); -- 5x
+ count 
+-------
+  1000
+(1 row)
+
+-- we should now get a really bad plan
+explain (costs off) execute test_mode_pp(2);
+         QUERY PLAN          
+-----------------------------
+ Aggregate
+   ->  Seq Scan on test_mode
+         Filter: (a = $1)
+(3 rows)
+
+-- but we can force a custom plan
+set plan_cache_mode to force_custom_plan;
+explain (costs off) execute test_mode_pp(2);
+                        QUERY PLAN                        
+----------------------------------------------------------
+ Aggregate
+   ->  Index Only Scan using test_mode_a_idx on test_mode
+         Index Cond: (a = 2)
+(3 rows)
+
+drop table test_mode;

src/test/regress/sql/plancache.sql

@@ -177,3 +177,36 @@ drop table pc_list_part_1;
 execute pstmt_def_insert(1);
 drop table pc_list_parted, pc_list_part_null;
 deallocate pstmt_def_insert;
+
+-- Test plan_cache_mode
+
+create table test_mode (a int);
+insert into test_mode select 1 from generate_series(1,1000) union all select 2;
+create index on test_mode (a);
+analyze test_mode;
+
+prepare test_mode_pp (int) as select count(*) from test_mode where a = $1;
+
+-- up to 5 executions, custom plan is used
+explain (costs off) execute test_mode_pp(2);
+
+-- force generic plan
+set plan_cache_mode to force_generic_plan;
+explain (costs off) execute test_mode_pp(2);
+
+-- get to generic plan by 5 executions
+set plan_cache_mode to auto;
+execute test_mode_pp(1); -- 1x
+execute test_mode_pp(1); -- 2x
+execute test_mode_pp(1); -- 3x
+execute test_mode_pp(1); -- 4x
+execute test_mode_pp(1); -- 5x
+
+-- we should now get a really bad plan
+explain (costs off) execute test_mode_pp(2);
+
+-- but we can force a custom plan
+set plan_cache_mode to force_custom_plan;
+explain (costs off) execute test_mode_pp(2);
+
+drop table test_mode;