MWL#17: Table elimination

- First code. Elimination works for simple cases, passes the testsuite.
- Known issues:
  = No elimination is done for aggregate functions.
  = EXPLAIN EXTENDED shows eliminated tables (I think it better not)
  = No benchmark yet
  = The code needs some polishing.

mysql-test/r/table_elim.result:
  MWL#17: Table elimination
  - Testcases
mysql-test/t/table_elim.test:
  MWL#17: Table elimination
  - Testcases
sql/sql_select.cc:
  MWL#17: Table elimination
sql/sql_select.h:
  MWL#17: Table elimination
  - Added JOIN_TAB::eliminated (is JOIN_TAB the best place to store this flag?)
sql/table.h:
  MWL#17: Table elimination
  - ADded NESTED_JOIN::n_tables. We need to have the number of real tables remaining in an outer join nest.

mysql-test/r/table_elim.result

+drop table if exists t0, t1, t2, t3;
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3);
+create table t0 as select * from t1;
+create table t2 (a int primary key, b int) 
+as select a, a as b from t1 where a in (1,2);
+create table t3 (a int primary key, b int) 
+as select a, a as b from t1 where a in (1,3);
+# This will be  eliminated:
+explain select t1.a from t1 left join t2 on t2.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+select t1.a from t1 left join t2 on t2.a=t1.a;
+a
+0
+1
+2
+3
+# This will not be eliminated as t2.b is in in select list:
+explain select * from t1 left join t2 on t2.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+1	SIMPLE	t2	eq_ref	PRIMARY	PRIMARY	4	test.t1.a	1	
+# This will not be eliminated as t2.b is in in order list:
+explain select t1.a from t1 left join t2 on t2.a=t1.a order by t2.b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	Using temporary; Using filesort
+1	SIMPLE	t2	eq_ref	PRIMARY	PRIMARY	4	test.t1.a	1	
+# This will not be eliminated as t2.b is in group list:
+explain select t1.a from t1 left join t2 on t2.a=t1.a group by t2.b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	Using temporary; Using filesort
+1	SIMPLE	t2	eq_ref	PRIMARY	PRIMARY	4	test.t1.a	1	
+# This will not be eliminated as t2.b is in the WHERE
+explain select t1.a from t1 left join t2 on t2.a=t1.a where t2.b < 3 or t2.b is null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+1	SIMPLE	t2	eq_ref	PRIMARY	PRIMARY	4	test.t1.a	1	Using where
+# Elimination of multiple tables:
+explain select t1.a from t1 left join (t2 join t3) on t2.a=t1.a and t3.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+# Elimination of multiple tables (2):
+explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and t3.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+# Elimination when done within an outer join nest:
+explain
+select t0.*
+from
+t0 left join (t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
+t3.a=t1.a) on t0.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	4	
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+drop table t0, t1, t2, t3;

mysql-test/t/table_elim.test

+#
+# Table elimination (MWL#17) tests
+#
+--disable_warnings
+drop table if exists t0, t1, t2, t3;
+--enable_warnings
+
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3);
+create table t0 as select * from t1;
+
+create table t2 (a int primary key, b int) 
+  as select a, a as b from t1 where a in (1,2);
+
+create table t3 (a int primary key, b int) 
+  as select a, a as b from t1 where a in (1,3);
+
+--echo # This will be  eliminated:
+explain select t1.a from t1 left join t2 on t2.a=t1.a;
+
+select t1.a from t1 left join t2 on t2.a=t1.a;
+
+--echo # This will not be eliminated as t2.b is in in select list:
+explain select * from t1 left join t2 on t2.a=t1.a;
+
+--echo # This will not be eliminated as t2.b is in in order list:
+explain select t1.a from t1 left join t2 on t2.a=t1.a order by t2.b;
+
+--echo # This will not be eliminated as t2.b is in group list:
+explain select t1.a from t1 left join t2 on t2.a=t1.a group by t2.b;
+
+## TODO: Aggregate functions prevent table elimination ATM.
+
+--echo # This will not be eliminated as t2.b is in the WHERE
+explain select t1.a from t1 left join t2 on t2.a=t1.a where t2.b < 3 or t2.b is null;
+
+--echo # Elimination of multiple tables:
+explain select t1.a from t1 left join (t2 join t3) on t2.a=t1.a and t3.a=t1.a;
+
+--echo # Elimination of multiple tables (2):
+explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and t3.a=t1.a;
+
+--echo # Elimination when done within an outer join nest:
+explain
+select t0.*
+from
+  t0 left join (t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
+  t3.a=t1.a) on t0.a=t1.a;
+
+
+drop table t0, t1, t2, t3;
+

sql/sql_select.h

@@ -210,6 +210,9 @@ typedef struct st_join_table {
   JOIN		*join;
   /** Bitmap of nested joins this table is part of */
   nested_join_map embedding_map;
+  
+  //psergey-todo: more justified place
+  bool eliminated;
 
   void cleanup();
   inline bool is_using_loose_index_scan()

sql/table.h

@@ -1626,6 +1626,8 @@ typedef struct st_nested_join
     Before each use the counters are zeroed by reset_nj_counters.
   */
   uint              counter;
+  /* Tables left after elimination */
+  uint              n_tables;
   nested_join_map   nj_map;          /* Bit used to identify this nested join*/
 } NESTED_JOIN;