Set join_collapse_limit default value to 13 (#1525)

The default value of join_collapse_limit was 20. When this value is set and
the query contains about 20 joins (see added test), Postgres query optimizer
cannot build a plan during hours and consumes a lot of memory, because the
planner checks a lot of possible ways to join the tables.
When join_collapse_limit is 8, the query plan is built in reasonable time.

Author: andr-sokolov

src/backend/utils/misc/guc.c

@@ -2290,7 +2290,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_EXPLAIN
 		},
 		&join_collapse_limit,
-		20, 1, INT_MAX,
+		13, 1, INT_MAX,
 		NULL, NULL, NULL
 	},
 	{

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

@@ -434,7 +434,7 @@ max_prepared_transactions = 250		# can be 0 or more
 #cursor_tuple_fraction = 0.1		# range 0.0-1.0
 
 #from_collapse_limit = 20
-#join_collapse_limit = 20		# 1 disables collapsing of explicit
+#join_collapse_limit = 13		# 1 disables collapsing of explicit
 					# JOIN clauses
 #force_parallel_mode = off
 #jit = on				# allow JIT compilation

src/test/regress/expected/bfv_joins.out

@@ -4189,6 +4189,48 @@ INSERT INTO ext_stats_tbl VALUES('tC', true);
 ANALYZE ext_stats_tbl;
 explain SELECT 1 FROM ext_stats_tbl t11 FULL JOIN ext_stats_tbl t12 ON t12.c2;
 ERROR:  FULL JOIN is only supported with merge-joinable or hash-joinable join conditions
+-- Check that Postgres Planner can build a plan with 20 joins in reasonable time
+do $$
+begin 
+  for i in 1..20 loop
+    execute 'create table tj' ||  i ||  '(id int)';
+  end loop; 
+end
+$$;
+set optimizer to off;
+select trunc(extract(epoch from now())) unix_time1 \gset
+select *
+from tj1 
+  join tj2 on tj1.id = tj2.id
+  join tj3 on tj2.id = tj3.id
+  join tj4 on tj3.id = tj4.id
+  join tj5 on tj4.id = tj5.id
+  join tj6 on tj5.id = tj6.id
+  join tj7 on tj6.id = tj7.id
+  join tj8 on tj7.id = tj8.id
+  join tj9 on tj8.id = tj9.id
+  join tj10 on tj9.id = tj10.id
+  join tj11 on tj10.id = tj11.id
+  join tj12 on tj11.id = tj12.id
+  join tj13 on tj12.id = tj13.id
+  join tj14 on tj13.id = tj14.id
+  join tj15 on tj14.id = tj15.id
+  join tj16 on tj15.id = tj16.id
+  join tj17 on tj16.id = tj17.id
+  join tj18 on tj17.id = tj18.id
+  join tj19 on tj18.id = tj19.id
+  join tj20 on tj19.id = tj20.id;
+ id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id 
+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----
+(0 rows)
+
+select (trunc(extract(epoch from now())) - :unix_time1) < 100 is_ok;
+ is_ok 
+-------
+ t
+(1 row)
+
+reset optimizer;
 -- Clean up. None of the objects we create are very interesting to keep around.
 reset search_path;
 set client_min_messages='warning';

src/test/regress/expected/bfv_joins_optimizer.out

@@ -4206,6 +4206,48 @@ INSERT INTO ext_stats_tbl VALUES('tC', true);
 ANALYZE ext_stats_tbl;
 explain SELECT 1 FROM ext_stats_tbl t11 FULL JOIN ext_stats_tbl t12 ON t12.c2;
 ERROR:  FULL JOIN is only supported with merge-joinable or hash-joinable join conditions
+-- Check that Postgres Planner can build a plan with 20 joins in reasonable time
+do $$
+begin 
+  for i in 1..20 loop
+    execute 'create table tj' ||  i ||  '(id int)';
+  end loop; 
+end
+$$;
+set optimizer to off;
+select trunc(extract(epoch from now())) unix_time1 \gset
+select *
+from tj1 
+  join tj2 on tj1.id = tj2.id
+  join tj3 on tj2.id = tj3.id
+  join tj4 on tj3.id = tj4.id
+  join tj5 on tj4.id = tj5.id
+  join tj6 on tj5.id = tj6.id
+  join tj7 on tj6.id = tj7.id
+  join tj8 on tj7.id = tj8.id
+  join tj9 on tj8.id = tj9.id
+  join tj10 on tj9.id = tj10.id
+  join tj11 on tj10.id = tj11.id
+  join tj12 on tj11.id = tj12.id
+  join tj13 on tj12.id = tj13.id
+  join tj14 on tj13.id = tj14.id
+  join tj15 on tj14.id = tj15.id
+  join tj16 on tj15.id = tj16.id
+  join tj17 on tj16.id = tj17.id
+  join tj18 on tj17.id = tj18.id
+  join tj19 on tj18.id = tj19.id
+  join tj20 on tj19.id = tj20.id;
+ id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id | id 
+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----
+(0 rows)
+
+select (trunc(extract(epoch from now())) - :unix_time1) < 100 is_ok;
+ is_ok 
+-------
+ t
+(1 row)
+
+reset optimizer;
 -- Clean up. None of the objects we create are very interesting to keep around.
 reset search_path;
 set client_min_messages='warning';

src/test/regress/sql/bfv_joins.sql

@@ -604,6 +604,45 @@ ANALYZE ext_stats_tbl;
 
 explain SELECT 1 FROM ext_stats_tbl t11 FULL JOIN ext_stats_tbl t12 ON t12.c2;
 
+-- Check that Postgres Planner can build a plan with 20 joins in reasonable time
+do $$
+begin 
+  for i in 1..20 loop
+    execute 'create table tj' ||  i ||  '(id int)';
+  end loop; 
+end
+$$;
+
+set optimizer to off;
+
+select trunc(extract(epoch from now())) unix_time1 \gset
+
+select *
+from tj1 
+  join tj2 on tj1.id = tj2.id
+  join tj3 on tj2.id = tj3.id
+  join tj4 on tj3.id = tj4.id
+  join tj5 on tj4.id = tj5.id
+  join tj6 on tj5.id = tj6.id
+  join tj7 on tj6.id = tj7.id
+  join tj8 on tj7.id = tj8.id
+  join tj9 on tj8.id = tj9.id
+  join tj10 on tj9.id = tj10.id
+  join tj11 on tj10.id = tj11.id
+  join tj12 on tj11.id = tj12.id
+  join tj13 on tj12.id = tj13.id
+  join tj14 on tj13.id = tj14.id
+  join tj15 on tj14.id = tj15.id
+  join tj16 on tj15.id = tj16.id
+  join tj17 on tj16.id = tj17.id
+  join tj18 on tj17.id = tj18.id
+  join tj19 on tj18.id = tj19.id
+  join tj20 on tj19.id = tj20.id;
+
+select (trunc(extract(epoch from now())) - :unix_time1) < 100 is_ok;
+
+reset optimizer;
+
 -- Clean up. None of the objects we create are very interesting to keep around.
 reset search_path;
 set client_min_messages='warning';