MDEV-32351 Significant slowdown for query with many outer joins

This patch fixes a performance regression introduced in the patch for the bug MDEV-21104. The performance regression could affect queries for which join buffer was used for an outer join such that its on expression from which a conjunctive condition depended only on outer tables can be extracted. If the number of records in the join buffer for which this condition was false greatly exceeded the number of other records the slowdown could be significant. If there is a conjunctive condition extracted from the ON expression depending only on outer tables this condition is evaluated when interesting fields of each survived record of outer tables are put into the join buffer. Each such set of fields for any join operation is supplied with a match flag field used to generate null complemented rows. If the result of the evaluation of the condition is false the flag is set to MATCH_IMPOSSIBLE. When looking in the join buffer for records matching a record of the right operand of the outer join operation the records with such flags are not needed to be unpacked into record buffers for evaluation of on expressions. The patch for MDEV-21104 fixing some problem of wrong results when 'not exists' optimization by mistake broke the code that allowed to ignore records with the match flag set to MATCH_IMPOSSIBLE when looking for matching records. As a result such records were unpacked for each record of the right operand of the outer join operation. This caused significant execution penalty in some cases. One of the test cases added in the patch can be used only for demonstration of the restored performance for the reported query. The second test case is needed to demonstrate the validity of the fix.
MariaDB · Oct 27, 2023 · 954a6de · 954a6de
1 parent 11abc21
commit 954a6de
Show file tree

Hide file tree

Showing 4 changed files with 202 additions and 12 deletions.
diff --git a/mysql-test/main/join_cache.result b/mysql-test/main/join_cache.result
@@ -6270,5 +6270,131 @@ ERROR HY001: Could not create a join buffer. Please check and adjust the value o
 SET JOIN_buffer_size=16384;
 SELECT * FROM information_schema.statistics JOIN information_schema.COLUMNS USING (table_name,column_name);
 #
+# MDEV-32351: Join buffer used for outer join with ON condition
+#             depending only on outer tables
+#
+CREATE TABLE t1 (b int NOT NULL, PRIMARY KEY (b)) ENGINE=MYISAM;
+INSERT INTO t1 select seq from seq_1_to_10000;
+CREATE TABLE t2 (b int NOT NULL, d varchar(255), KEY (b)) ENGINE=MYISAM ;
+INSERT INTO t2 VALUES (1,1),(2,2),(3,3);
+CREATE TABLE t3 (c int NOT NULL, PRIMARY KEY (c)) ENGINE=MYISAM ;
+INSERT INTO t3 select seq from seq_1_to_3000;
+CREATE TABLE t4 (c int NOT NULL, PRIMARY KEY (c)) ENGINE=MYISAM;
+INSERT INTO t4 select seq from seq_1_to_3000;
+ANALYZE TABLE t1,t2,t3,t4;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+test.t2	analyze	status	Engine-independent statistics collected
+test.t2	analyze	status	OK
+test.t3	analyze	status	Engine-independent statistics collected
+test.t3	analyze	status	OK
+test.t4	analyze	status	Engine-independent statistics collected
+test.t4	analyze	status	OK
+set join_cache_level=0;
+EXPLAIN SELECT COUNT(*)
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	10000	Using index
+1	SIMPLE	t2	ref	b	b	4	test.t1.b	1	
+1	SIMPLE	t3	eq_ref	PRIMARY	PRIMARY	4	test.t2.d	1	Using where; Using index
+1	SIMPLE	t4	index	NULL	PRIMARY	4	NULL	3000	Using where; Using index
+SELECT COUNT(*)
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+COUNT(*)
+12999
+set join_cache_level=default;
+EXPLAIN SELECT COUNT(*)
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	10000	Using index
+1	SIMPLE	t2	ref	b	b	4	test.t1.b	1	
+1	SIMPLE	t3	eq_ref	PRIMARY	PRIMARY	4	test.t2.d	1	Using where; Using index
+1	SIMPLE	t4	index	NULL	PRIMARY	4	NULL	3000	Using where; Using index; Using join buffer (flat, BNL join)
+SELECT COUNT(*)
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+COUNT(*)
+12999
+DROP TABLE t1,t2,t3,t4;
+CREATE TABLE t1 (b int NOT NULL, PRIMARY KEY (b));
+INSERT INTO t1 select seq from seq_1_to_10;
+CREATE TABLE t2 (b int NOT NULL, d varchar(255), KEY (b)) ;
+INSERT INTO t2 VALUES (1,1),(2,2),(3,3);
+CREATE TABLE t3 (c int NOT NULL, PRIMARY KEY (c)) ;
+INSERT INTO t3 select seq from seq_1_to_3;
+CREATE TABLE t4 (c int NOT NULL, PRIMARY KEY (c)) ;
+INSERT INTO t4 select seq from seq_1_to_3;
+set join_cache_level=0;
+EXPLAIN SELECT *
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	10	Using index
+1	SIMPLE	t2	ALL	b	NULL	NULL	NULL	3	Using where
+1	SIMPLE	t3	eq_ref	PRIMARY	PRIMARY	4	test.t2.d	1	Using where; Using index
+1	SIMPLE	t4	index	NULL	PRIMARY	4	NULL	3	Using where; Using index
+SELECT *
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+b	b	d	c	c
+1	1	1	1	1
+1	1	1	1	2
+1	1	1	1	3
+2	2	2	2	NULL
+3	3	3	3	NULL
+4	NULL	NULL	NULL	NULL
+5	NULL	NULL	NULL	NULL
+6	NULL	NULL	NULL	NULL
+7	NULL	NULL	NULL	NULL
+8	NULL	NULL	NULL	NULL
+9	NULL	NULL	NULL	NULL
+10	NULL	NULL	NULL	NULL
+set join_cache_level=default;
+EXPLAIN SELECT *
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	10	Using index
+1	SIMPLE	t2	ALL	b	NULL	NULL	NULL	3	Using where; Using join buffer (flat, BNL join)
+1	SIMPLE	t3	eq_ref	PRIMARY	PRIMARY	4	test.t2.d	1	Using where; Using index
+1	SIMPLE	t4	index	NULL	PRIMARY	4	NULL	3	Using where; Using index; Using join buffer (flat, BNL join)
+SELECT *
+FROM t1
+LEFT JOIN t2 ON t1.b = t2.b
+LEFT JOIN t3 ON t2.d = t3.c
+LEFT JOIN t4 ON t3.c=1;
+b	b	d	c	c
+1	1	1	1	1
+1	1	1	1	2
+1	1	1	1	3
+2	2	2	2	NULL
+3	3	3	3	NULL
+4	NULL	NULL	NULL	NULL
+5	NULL	NULL	NULL	NULL
+6	NULL	NULL	NULL	NULL
+7	NULL	NULL	NULL	NULL
+8	NULL	NULL	NULL	NULL
+9	NULL	NULL	NULL	NULL
+10	NULL	NULL	NULL	NULL
+DROP TABLE t1,t2,t3,t4;
+#
 # End of 10.4 tests
 #
diff --git a/mysql-test/main/join_cache.test b/mysql-test/main/join_cache.test
@@ -4247,6 +4247,64 @@ SET JOIN_buffer_size=16384;
 SELECT * FROM information_schema.statistics JOIN information_schema.COLUMNS USING (table_name,column_name);
 --enable_result_log
 
+--echo #
+--echo # MDEV-32351: Join buffer used for outer join with ON condition
+--echo #             depending only on outer tables
+--echo #
+
+--source include/have_sequence.inc
+
+CREATE TABLE t1 (b int NOT NULL, PRIMARY KEY (b)) ENGINE=MYISAM;
+INSERT INTO t1 select seq from seq_1_to_10000;
+CREATE TABLE t2 (b int NOT NULL, d varchar(255), KEY (b)) ENGINE=MYISAM ;
+INSERT INTO t2 VALUES (1,1),(2,2),(3,3);
+CREATE TABLE t3 (c int NOT NULL, PRIMARY KEY (c)) ENGINE=MYISAM ;
+INSERT INTO t3 select seq from seq_1_to_3000;
+CREATE TABLE t4 (c int NOT NULL, PRIMARY KEY (c)) ENGINE=MYISAM;
+INSERT INTO t4 select seq from seq_1_to_3000;
+ANALYZE TABLE t1,t2,t3,t4;
+
+let $q1=
+SELECT COUNT(*)
+FROM t1
+     LEFT JOIN t2 ON t1.b = t2.b
+     LEFT JOIN t3 ON t2.d = t3.c
+     LEFT JOIN t4 ON t3.c=1;
+
+set join_cache_level=0;
+eval EXPLAIN $q1;
+eval $q1;
+set join_cache_level=default;
+eval EXPLAIN $q1;
+eval $q1;
+
+DROP TABLE t1,t2,t3,t4;
+
+CREATE TABLE t1 (b int NOT NULL, PRIMARY KEY (b));
+INSERT INTO t1 select seq from seq_1_to_10;
+CREATE TABLE t2 (b int NOT NULL, d varchar(255), KEY (b)) ;
+INSERT INTO t2 VALUES (1,1),(2,2),(3,3);
+CREATE TABLE t3 (c int NOT NULL, PRIMARY KEY (c)) ;
+INSERT INTO t3 select seq from seq_1_to_3;
+CREATE TABLE t4 (c int NOT NULL, PRIMARY KEY (c)) ;
+INSERT INTO t4 select seq from seq_1_to_3;
+
+let $q2=
+SELECT *
+FROM t1
+     LEFT JOIN t2 ON t1.b = t2.b
+     LEFT JOIN t3 ON t2.d = t3.c
+     LEFT JOIN t4 ON t3.c=1;
+
+set join_cache_level=0;
+eval EXPLAIN $q2;
+eval $q2;
+set join_cache_level=default;
+eval EXPLAIN $q2;
+eval $q2;
+
+DROP TABLE t1,t2,t3,t4;
+
 --echo #
 --echo # End of 10.4 tests
 --echo #
diff --git a/sql/sql_join_cache.cc b/sql/sql_join_cache.cc
@@ -2054,10 +2054,11 @@ bool JOIN_CACHE::skip_if_matched()
       - In the case of a semi-nest the match flag may be in two states
         {MATCH_NOT_FOUND, MATCH_FOUND}. The record is skipped if the flag is set
         to MATCH_FOUND.
-      - In the case of a outer join nest when not_exists optimization is applied
-        the match may be in three states {MATCH_NOT_FOUND, MATCH_IMPOSSIBLE,
-        MATCH_FOUND. The record is skipped if the flag is set to MATCH_FOUND or
-        to MATCH_IMPOSSIBLE.
+      - In the case of an outer join the match may be in three states
+        {MATCH_NOT_FOUND, MATCH_IMPOSSIBLE, MATCH_FOUND}.
+        If not_exists optimization is applied the record is skipped when
+        the flag is set to MATCH_FOUND or to MATCH_IMPOSSIBLE. Otherwise
+        the record is skipped only when the flag is set to MATCH_IMPOSSIBLE.
 
     If the record is skipped the value of 'pos' is set to point to the position
     right after the record.
@@ -2080,13 +2081,13 @@ bool JOIN_CACHE::skip_if_not_needed_match()
   if (prev_cache)
     offset+= prev_cache->get_size_of_rec_offset();
 
-  if (!join_tab->check_only_first_match())
-    return FALSE;
-
   match_fl= get_match_flag_by_pos(pos+offset);
   skip= join_tab->first_sj_inner_tab ?
-        match_fl == MATCH_FOUND :           // the case of semi-join
-        match_fl != MATCH_NOT_FOUND;        // the case of outer-join
+          match_fl == MATCH_FOUND :           // the case of semi-join
+          not_exists_opt_is_applicable &&
+          join_tab->table->reginfo.not_exists_optimize ?
+            match_fl != MATCH_NOT_FOUND :     // the case of not exist opt
+            match_fl == MATCH_IMPOSSIBLE;
 
   if (skip)
   {
@@ -2383,7 +2384,7 @@ enum_nested_loop_state JOIN_CACHE::join_matching_records(bool skip_last)
         as candidates for matches.
       */
 
-      bool not_exists_opt_is_applicable= true;
+      not_exists_opt_is_applicable= true;
       if (check_only_first_match && join_tab->first_inner)
       {
         /*
@@ -2408,8 +2409,9 @@ enum_nested_loop_state JOIN_CACHE::join_matching_records(bool skip_last)
         }
       }
 
-      if (!check_only_first_match ||
-	  (join_tab->first_inner && !not_exists_opt_is_applicable) ||
+      if ((!join_tab->on_precond &&
+           (!check_only_first_match ||
+            (join_tab->first_inner && !not_exists_opt_is_applicable))) ||
           !skip_next_candidate_for_match(rec_ptr))
       {
 	read_next_candidate_for_match(rec_ptr);

diff --git a/sql/sql_join_cache.h b/sql/sql_join_cache.h
@@ -340,6 +340,9 @@ class JOIN_CACHE :public Sql_alloc
   /* The length of an embedded key value */ 
   uint emb_key_length;
 
+  /* This flag is used only when 'not exists' optimization can be applied */
+  bool not_exists_opt_is_applicable;
+
   /*
     This object provides the methods to iterate over records of
     the joined table join_tab when looking for join matches between
@@ -536,6 +539,7 @@ class JOIN_CACHE :public Sql_alloc
     prev_cache= next_cache= 0;
     buff= 0;
     min_buff_size= max_buff_size= 0;            // Caches
+    not_exists_opt_is_applicable= false;
   }
 
   /*