Bug#30302: Tables that were optimized away are printed in the

EXPLAIN EXTENDED warning.

Query optimizer searches for the constant tables and optimizes them away. This
means that fields of such tables are substituted for their values and on later
phases they are treated as constants. After this constant tables are removed
from the query execution plan. Nevertheless constant tables were shown in 
the EXPLAIN EXTENDED warning thus producing query that might be not an
equivalent of the original query.
        
Now the print_join function skips all tables that were optimized away from
printing to the EXPLAIN EXTENDED warning. If all tables were optimized away it
produces the 'FROM dual' clause.

mysql-test/r/explain.result

@@ -64,7 +64,7 @@ explain extended select * from v1 where f2=1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select '1' AS `f1`,'1' AS `f2` from `test`.`t1` where 1
+Note	1003	select '1' AS `f1`,'1' AS `f2` from dual where 1
 explain extended select * from t1 where 0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
@@ -74,7 +74,7 @@ explain extended select * from t1 where 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select '1' AS `f1`,'1' AS `f2` from `test`.`t1` where 1
+Note	1003	select '1' AS `f1`,'1' AS `f2` from dual where 1
 explain extended select * from t1 having 0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible HAVING
@@ -84,7 +84,7 @@ explain extended select * from t1 having 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select '1' AS `f1`,'1' AS `f2` from `test`.`t1` having 1
+Note	1003	select '1' AS `f1`,'1' AS `f2` from dual having 1
 drop view v1;
 drop table t1;
 CREATE TABLE t1(c INT);
@@ -194,4 +194,24 @@ dt
 2001-01-01 01:01:01
 2001-01-01 01:01:01
 drop tables t1, t2;
+#
+# Bug#30302: Tables that were optimized away are printed in the
+#            EXPLAIN EXTENDED warning.
+#
+create table t1(f1 int);
+create table t2(f2 int);
+insert into t1 values(1);
+insert into t2 values(1),(2);
+explain extended select * from t1 where f1=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
+Warnings:
+Note	1003	select '1' AS `f1` from dual where 1
+explain extended select * from t1 join t2 on f1=f2 where f1=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+Warnings:
+Note	1003	select '1' AS `f1`,`test`.`t2`.`f2` AS `f2` from `test`.`t2` where (`test`.`t2`.`f2` = 1)
+drop table t1,t2;
 End of 5.1 tests.

mysql-test/r/func_default.result

@@ -8,7 +8,7 @@ explain extended select default(str), default(strnull), default(intg), default(r
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select default('') AS `default(str)`,default('') AS `default(strnull)`,default('0') AS `default(intg)`,default('0') AS `default(rel)` from `test`.`t1`
+Note	1003	select default('') AS `default(str)`,default('') AS `default(strnull)`,default('0') AS `default(intg)`,default('0') AS `default(rel)` from dual
 select * from t1 where str <> default(str);
 str	strnull	intg	rel
 		0	0

mysql-test/r/func_regexp.result

@@ -52,7 +52,7 @@ explain extended select * from t1 where xxx regexp('is a test of some long text
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select 'this is a test of some long text to see what happens' AS `xxx` from `test`.`t1` where ('this is a test of some long text to see what happens' regexp 'is a test of some long text to')
+Note	1003	select 'this is a test of some long text to see what happens' AS `xxx` from dual where ('this is a test of some long text to see what happens' regexp 'is a test of some long text to')
 select * from t1 where xxx regexp('is a test of some long text to ');
 xxx
 this is a test of some long text to see what happens

mysql-test/r/func_test.result

@@ -87,7 +87,7 @@ explain extended select - a from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select -('1') AS `- a` from `test`.`t1`
+Note	1003	select -('1') AS `- a` from dual
 drop table t1;
 select 5 between 0 and 10 between 0 and 1,(5 between 0 and 10) between 0 and 1;
 5 between 0 and 10 between 0 and 1	(5 between 0 and 10) between 0 and 1

mysql-test/r/having.result

@@ -12,7 +12,7 @@ explain extended select count(a) as b from t1 where a=0 having b >=0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 Warnings:
-Note	1003	select count('0') AS `b` from `test`.`t1` where 0 having (`b` >= 0)
+Note	1003	select count('0') AS `b` from dual where 0 having (`b` >= 0)
 drop table t1;
 CREATE TABLE t1 (
 raw_id int(10) NOT NULL default '0',

mysql-test/r/select.result

@@ -4435,7 +4435,7 @@ EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a=a AND a=a) OR b > 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	select '1' AS `a`,'1' AS `b` from `test`.`t1` where 1
+Note	1003	select '1' AS `a`,'1' AS `b` from dual where 1
 SELECT * FROM t1 WHERE (a=a AND a=a) OR b > 2;
 a	b
 1	1

mysql-test/r/subselect.result

@@ -50,7 +50,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 Warnings:
 Note	1276	Field or reference 'b.a' of SELECT #3 was resolved in SELECT #1
 Note	1276	Field or reference 'b.a' of SELECT #3 was resolved in SELECT #1
-Note	1003	select 1 AS `1` from (select 1 AS `a`) `b` having ((select '1' AS `a`) = 1)
+Note	1003	select 1 AS `1` from dual having ((select '1' AS `a`) = 1)
 SELECT 1 FROM (SELECT 1 as a) as b HAVING (SELECT a)=1;
 1
 1
@@ -203,7 +203,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 3	DERIVED	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 2	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using filesort
 Warnings:
-Note	1003	select (select `test`.`t3`.`a` AS `a` from `test`.`t3` where (`test`.`t3`.`a` < 8) order by 1 desc limit 1) AS `(select t3.a from t3 where a<8 order by 1 desc limit 1)`,'2' AS `a` from (select `test`.`t2`.`a` AS `a`,`test`.`t2`.`b` AS `b` from `test`.`t2` where (`test`.`t2`.`a` > 1)) `tt`
+Note	1003	select (select `test`.`t3`.`a` AS `a` from `test`.`t3` where (`test`.`t3`.`a` < 8) order by 1 desc limit 1) AS `(select t3.a from t3 where a<8 order by 1 desc limit 1)`,'2' AS `a` from dual
 select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3) order by 1 desc limit 1);
 a
 2
@@ -314,7 +314,7 @@ NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	NULL
 Warnings:
 Note	1276	Field or reference 'test.t2.a' of SELECT #2 was resolved in SELECT #1
 Note	1276	Field or reference 'test.t2.a' of SELECT #3 was resolved in SELECT #1
-Note	1003	select (select '2' AS `a` from `test`.`t1` where ('2' = `test`.`t2`.`a`) union select `test`.`t5`.`a` AS `a` from `test`.`t5` where (`test`.`t5`.`a` = `test`.`t2`.`a`)) AS `(select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a)`,`test`.`t2`.`a` AS `a` from `test`.`t2`
+Note	1003	select (select '2' AS `a` from dual where ('2' = `test`.`t2`.`a`) union select `test`.`t5`.`a` AS `a` from `test`.`t5` where (`test`.`t5`.`a` = `test`.`t2`.`a`)) AS `(select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a)`,`test`.`t2`.`a` AS `a` from `test`.`t2`
 select (select a from t1 where t1.a=t2.a union all select a from t5 where t5.a=t2.a), a from t2;
 ERROR 21000: Subquery returns more than 1 row
 create table t6 (patient_uq int, clinic_uq int, index i1 (clinic_uq));
@@ -1429,7 +1429,7 @@ explain extended (select * from t1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 Warnings:
-Note	1003	(select 'tttt' AS `s1` from `test`.`t1`)
+Note	1003	(select 'tttt' AS `s1` from dual)
 (select * from t1);
 s1
 tttt
@@ -1617,7 +1617,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 3	UNION	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	
 Warnings:
-Note	1003	select 'e' AS `s1` from `test`.`t1` where 1
+Note	1003	select 'e' AS `s1` from dual where 1
 drop table t1;
 CREATE TABLE t1 (number char(11) NOT NULL default '') ENGINE=MyISAM CHARSET=latin1;
 INSERT INTO t1 VALUES ('69294728265'),('18621828126'),('89356874041'),('95895001874');

mysql-test/r/subselect3.result

@@ -864,7 +864,7 @@ Level	Code	Message
 Note	1276	Field or reference 'test.t1.a' of SELECT #3 was resolved in SELECT #2
 Note	1276	Field or reference 'test.t1.c' of SELECT #3 was resolved in SELECT #2
 Error	1054	Unknown column 'c' in 'field list'
-Note	1003	select `c` AS `c` from (select (select count(`test`.`t1`.`a`) AS `COUNT(a)` from (select count(`test`.`t1`.`b`) AS `COUNT(b)` from `test`.`t1`) `x` group by `c`) AS `(SELECT COUNT(a) FROM 
+Note	1003	select `c` AS `c` from (select (select count(`test`.`t1`.`a`) AS `COUNT(a)` from dual group by `c`) AS `(SELECT COUNT(a) FROM 
 (SELECT COUNT(b) FROM t1) AS x GROUP BY c
 )` from `test`.`t1` group by `test`.`t1`.`b`) `y`
 DROP TABLE t1;

mysql-test/r/type_datetime.result

@@ -517,7 +517,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 Warnings:
 Note	1276	Field or reference 'test.t1.cur_date' of SELECT #2 was resolved in SELECT #1
-Note	1003	select '1' AS `id`,'2007-04-25 18:30:22' AS `cur_date` from `test`.`t1` where <in_optimizer>('1',<exists>(select 1 AS `Not_used` from `test`.`t1` `x1` where 0))
+Note	1003	select '1' AS `id`,'2007-04-25 18:30:22' AS `cur_date` from dual where <in_optimizer>('1',<exists>(select 1 AS `Not_used` from `test`.`t1` `x1` where 0))
 select * from t1
 where id in (select id from t1 as x1 where (t1.cur_date is null));
 id	cur_date
@@ -529,7 +529,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 Warnings:
 Note	1276	Field or reference 'test.t2.cur_date' of SELECT #2 was resolved in SELECT #1
-Note	1003	select '1' AS `id`,'2007-04-25' AS `cur_date` from `test`.`t2` where <in_optimizer>('1',<exists>(select 1 AS `Not_used` from `test`.`t2` `x1` where 0))
+Note	1003	select '1' AS `id`,'2007-04-25' AS `cur_date` from dual where <in_optimizer>('1',<exists>(select 1 AS `Not_used` from `test`.`t2` `x1` where 0))
 select * from t2
 where id in (select id from t2 as x1 where (t2.cur_date is null));
 id	cur_date

mysql-test/t/explain.test

@@ -167,4 +167,16 @@ flush tables;
 SELECT OUTR.dt FROM t1 AS OUTR WHERE OUTR.dt IN ( SELECT INNR.dt FROM t2 AS INNR WHERE OUTR.t < '2005-11-13 7:41:31' );
 drop tables t1, t2;
 
+
+--echo #
+--echo # Bug#30302: Tables that were optimized away are printed in the
+--echo #            EXPLAIN EXTENDED warning.
+--echo #
+create table t1(f1 int);
+create table t2(f2 int);
+insert into t1 values(1);
+insert into t2 values(1),(2);
+explain extended select * from t1 where f1=1;
+explain extended select * from t1 join t2 on f1=f2 where f1=1;
+drop table t1,t2;
 --echo End of 5.1 tests.

sql/sql_select.cc

@@ -2668,7 +2668,10 @@ make_join_statistics(JOIN *join, TABLE_LIST *tables_arg, COND *conds,
 	goto error;		// Fatal error
     }
     else
+    {
       found_const_table_map|= s->table->map;
+      s->table->pos_in_table_list->optimized_away= TRUE;
+    }
   }
 
   /* loop until no more const tables are found */
@@ -16564,18 +16567,35 @@ static void print_join(THD *thd,
 {
   /* List is reversed => we should reverse it before using */
   List_iterator_fast<TABLE_LIST> ti(*tables);
-  TABLE_LIST **table= (TABLE_LIST **)thd->alloc(sizeof(TABLE_LIST*) *
-                                                tables->elements);
-  if (table == 0)
+  TABLE_LIST **table;
+  uint non_const_tables= 0;
+
+  for (TABLE_LIST *t= ti++; t ; t= ti++)
+    if (!t->optimized_away)
+      non_const_tables++;
+  if (!non_const_tables)
+  {
+    str->append(STRING_WITH_LEN("dual"));
+    return; // all tables were optimized away
+  }
+  ti.rewind();
+
+  if (!(table= (TABLE_LIST **)thd->alloc(sizeof(TABLE_LIST*) *
+                                                non_const_tables)))
     return;  // out of memory
 
-  for (TABLE_LIST **t= table + (tables->elements - 1); t >= table; t--)
-    *t= ti++;
+  TABLE_LIST *tmp, **t= table + (non_const_tables - 1);
+  while ((tmp= ti++))
+  {
+    if (tmp->optimized_away)
+      continue;
+    *t--= tmp;
+  }
 
   DBUG_ASSERT(tables->elements >= 1);
   (*table)->print(thd, str, query_type);
 
-  TABLE_LIST **end= table + tables->elements;
+  TABLE_LIST **end= table + non_const_tables;
   for (TABLE_LIST **tbl= table + 1; tbl < end; tbl++)
   {
     TABLE_LIST *curr= *tbl;

sql/table.h

@@ -1356,6 +1356,8 @@ struct TABLE_LIST
     the parsed tree is created.
   */
   uint8 trg_event_map;
+  /* TRUE <=> this table is a const one and was optimized away. */
+  bool optimized_away;
 
   uint i_s_requested_object;
   bool has_db_lookup_value;