Make EXPLAIN better at displaying MRR/BKA:

- "Using MRR" is no longer shown with range access.
- Instead, both range and BKA accesses will show one of the following:
  = "Rowid-ordered scan"
  = "Key-ordered scan"
  = "Key-ordered Rowid-ordered scan"
depending on whether DS-MRR implementation will do scan keys in order, rowids in order,
or both.
- The patch also introduces a way for other storage engines/MRR implementations to
  pass information to EXPLAIN output about the properties of employed MRR scans.

1 files changed, 27 insertions, 27 deletions

diff --git a/mysql-test/r/join_nested_jcl6.result b/mysql-test/r/join_nested_jcl6.result
index a7923fc5b7c..3345540af96 100644
--- a/mysql-test/r/join_nested_jcl6.result
+++ b/mysql-test/r/join_nested_jcl6.result
@@ -858,7 +858,7 @@ ON t3.a=1 AND t3.b=t2.b AND t2.b=t4.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	
 1	SIMPLE	t4	ALL	NULL	NULL	NULL	NULL	2	100.00	Using join buffer (flat, BNL join)
-1	SIMPLE	t2	ref	idx_b	idx_b	5	test.t3.b	2	100.00	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t2	ref	idx_b	idx_b	5	test.t3.b	2	100.00	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	Using join buffer (incremental, BNL join)
 Warnings:
 Note	1003	select `test`.`t2`.`a` AS `a`,`test`.`t2`.`b` AS `b`,`test`.`t3`.`a` AS `a`,`test`.`t3`.`b` AS `b`,`test`.`t4`.`a` AS `a`,`test`.`t4`.`b` AS `b` from `test`.`t3` join `test`.`t4` left join (`test`.`t1` join `test`.`t2`) on((((`test`.`t3`.`a` = 1) and ((`test`.`t4`.`b` = `test`.`t3`.`b`) and (`test`.`t2`.`b` = `test`.`t3`.`b`))) and (`test`.`t3`.`b` is not null))) where 1
@@ -966,7 +966,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	hash_ALL	NULL	#hash#$hj	5	test.t0.b	3	100.00	Using where; Using join buffer (flat, BNLH join)
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where; Using join buffer (incremental, BNL join)
-1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	100.00	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	100.00	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t5	ALL	idx_b	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 1	SIMPLE	t7	hash_ALL	NULL	#hash#$hj	5	test.t5.b	2	100.00	Using where; Using join buffer (incremental, BNLH join)
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
@@ -1015,11 +1015,11 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	hash_ALL	NULL	#hash#$hj	5	test.t0.b	3	100.00	Using where; Using join buffer (flat, BNLH join)
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where; Using join buffer (incremental, BNL join)
-1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	100.00	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	100.00	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t5	ALL	idx_b	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 1	SIMPLE	t7	hash_ALL	NULL	#hash#$hj	5	test.t5.b	2	100.00	Using where; Using join buffer (incremental, BNLH join)
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
-1	SIMPLE	t8	ref	idx_b	idx_b	5	test.t5.b	2	100.00	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t8	ref	idx_b	idx_b	5	test.t5.b	2	100.00	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t9	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 Warnings:
 Note	1003	select `test`.`t0`.`a` AS `a`,`test`.`t0`.`b` AS `b`,`test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t2`.`a` AS `a`,`test`.`t2`.`b` AS `b`,`test`.`t3`.`a` AS `a`,`test`.`t3`.`b` AS `b`,`test`.`t4`.`a` AS `a`,`test`.`t4`.`b` AS `b`,`test`.`t5`.`a` AS `a`,`test`.`t5`.`b` AS `b`,`test`.`t6`.`a` AS `a`,`test`.`t6`.`b` AS `b`,`test`.`t7`.`a` AS `a`,`test`.`t7`.`b` AS `b`,`test`.`t8`.`a` AS `a`,`test`.`t8`.`b` AS `b`,`test`.`t9`.`a` AS `a`,`test`.`t9`.`b` AS `b` from `test`.`t0` join `test`.`t1` left join (`test`.`t2` left join (`test`.`t3` join `test`.`t4`) on((((`test`.`t3`.`a` = 1) and (`test`.`t4`.`b` = `test`.`t2`.`b`)) and (`test`.`t2`.`b` is not null))) join `test`.`t5` left join (`test`.`t6` join `test`.`t7` left join `test`.`t8` on((((`test`.`t6`.`b` < 10) and ((`test`.`t7`.`b` = `test`.`t5`.`b`) and (`test`.`t8`.`b` = `test`.`t5`.`b`))) and (`test`.`t5`.`b` is not null)))) on((((`test`.`t6`.`b` >= 2) and (`test`.`t7`.`b` = `test`.`t5`.`b`)) and (`test`.`t5`.`b` is not null)))) on((((`test`.`t3`.`b` = 2) or isnull(`test`.`t3`.`c`)) and ((`test`.`t6`.`b` = 2) or isnull(`test`.`t6`.`c`)) and (((`test`.`t1`.`b` = `test`.`t0`.`b`) and (`test`.`t5`.`b` = `test`.`t0`.`b`)) or isnull(`test`.`t3`.`c`) or isnull(`test`.`t6`.`c`) or isnull(`test`.`t8`.`c`)) and (`test`.`t1`.`a` <> 2))) join `test`.`t9` where ((`test`.`t9`.`a` = 1) and (`test`.`t1`.`b` = `test`.`t0`.`b`) and (`test`.`t0`.`a` = 1) and ((`test`.`t2`.`a` >= 4) or isnull(`test`.`t2`.`c`)) and ((`test`.`t3`.`a` < 5) or isnull(`test`.`t3`.`c`)) and ((`test`.`t4`.`b` = `test`.`t3`.`b`) or isnull(`test`.`t3`.`c`) or isnull(`test`.`t4`.`c`)) and ((`test`.`t5`.`a` >= 2) or isnull(`test`.`t5`.`c`)) and ((`test`.`t6`.`a` >= 4) or isnull(`test`.`t6`.`c`)) and ((`test`.`t7`.`a` <= 2) or isnull(`test`.`t7`.`c`)) and ((`test`.`t8`.`a` < 1) or isnull(`test`.`t8`.`c`)) and ((`test`.`t9`.`b` = `test`.`t8`.`b`) or isnull(`test`.`t8`.`c`)))
@@ -1062,14 +1062,14 @@ t0.b=t1.b AND
 (t9.a=1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t0	ref	idx_a	idx_a	5	const	1	100.00	Using where
-1	SIMPLE	t1	ref	idx_b	idx_b	5	test.t0.b	2	100.00	Using join buffer (flat, BKA join)
+1	SIMPLE	t1	ref	idx_b	idx_b	5	test.t0.b	2	100.00	Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where; Using join buffer (incremental, BNL join)
-1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	100.00	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	100.00	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t5	ALL	idx_b	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 1	SIMPLE	t7	hash_ALL	NULL	#hash#$hj	5	test.t5.b	2	100.00	Using where; Using join buffer (incremental, BNLH join)
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
-1	SIMPLE	t8	ref	idx_b	idx_b	5	test.t7.b	2	100.00	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t8	ref	idx_b	idx_b	5	test.t7.b	2	100.00	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t9	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (incremental, BNL join)
 Warnings:
 Note	1003	select `test`.`t0`.`a` AS `a`,`test`.`t0`.`b` AS `b`,`test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t2`.`a` AS `a`,`test`.`t2`.`b` AS `b`,`test`.`t3`.`a` AS `a`,`test`.`t3`.`b` AS `b`,`test`.`t4`.`a` AS `a`,`test`.`t4`.`b` AS `b`,`test`.`t5`.`a` AS `a`,`test`.`t5`.`b` AS `b`,`test`.`t6`.`a` AS `a`,`test`.`t6`.`b` AS `b`,`test`.`t7`.`a` AS `a`,`test`.`t7`.`b` AS `b`,`test`.`t8`.`a` AS `a`,`test`.`t8`.`b` AS `b`,`test`.`t9`.`a` AS `a`,`test`.`t9`.`b` AS `b` from `test`.`t0` join `test`.`t1` left join (`test`.`t2` left join (`test`.`t3` join `test`.`t4`) on((((`test`.`t3`.`a` = 1) and (`test`.`t4`.`b` = `test`.`t2`.`b`)) and (`test`.`t2`.`b` is not null))) join `test`.`t5` left join (`test`.`t6` join `test`.`t7` left join `test`.`t8` on((((`test`.`t6`.`b` < 10) and ((`test`.`t7`.`b` = `test`.`t5`.`b`) and (`test`.`t8`.`b` = `test`.`t5`.`b`))) and (`test`.`t7`.`b` is not null)))) on((((`test`.`t6`.`b` >= 2) and (`test`.`t7`.`b` = `test`.`t5`.`b`)) and (`test`.`t5`.`b` is not null)))) on((((`test`.`t3`.`b` = 2) or isnull(`test`.`t3`.`c`)) and ((`test`.`t6`.`b` = 2) or isnull(`test`.`t6`.`c`)) and (((`test`.`t1`.`b` = `test`.`t0`.`b`) and (`test`.`t5`.`b` = `test`.`t0`.`b`)) or isnull(`test`.`t3`.`c`) or isnull(`test`.`t6`.`c`) or isnull(`test`.`t8`.`c`)) and (`test`.`t1`.`a` <> 2))) join `test`.`t9` where ((`test`.`t9`.`a` = 1) and (`test`.`t1`.`b` = `test`.`t0`.`b`) and (`test`.`t0`.`a` = 1) and ((`test`.`t2`.`a` >= 4) or isnull(`test`.`t2`.`c`)) and ((`test`.`t3`.`a` < 5) or isnull(`test`.`t3`.`c`)) and ((`test`.`t4`.`b` = `test`.`t3`.`b`) or isnull(`test`.`t3`.`c`) or isnull(`test`.`t4`.`c`)) and ((`test`.`t5`.`a` >= 2) or isnull(`test`.`t5`.`c`)) and ((`test`.`t6`.`a` >= 4) or isnull(`test`.`t6`.`c`)) and ((`test`.`t7`.`a` <= 2) or isnull(`test`.`t7`.`c`)) and ((`test`.`t8`.`a` < 1) or isnull(`test`.`t8`.`c`)) and ((`test`.`t9`.`b` = `test`.`t8`.`b`) or isnull(`test`.`t8`.`c`)))
@@ -1452,27 +1452,27 @@ explain select * from t4 join
 t2 left join (t3 join t5 on t5.a=t3.b) on t3.a=t2.b where t4.a<=>t3.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	X	
-1	SIMPLE	t3	ref	a	a	5	test.t2.b	X	Using where; Using join buffer (flat, BKA join)
-1	SIMPLE	t5	ref	a	a	5	test.t3.b	X	Using join buffer (incremental, BKA join)
-1	SIMPLE	t4	ref	a	a	5	test.t3.b	X	Using index condition(BKA); Using join buffer (incremental, BKA join)
+1	SIMPLE	t3	ref	a	a	5	test.t2.b	X	Using where; Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t5	ref	a	a	5	test.t3.b	X	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t4	ref	a	a	5	test.t3.b	X	Using index condition(BKA); Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 explain select * from (t4 join t6 on t6.a=t4.b) right join t3 on t4.a=t3.b
 join t2 left join (t5 join t7 on t7.a=t5.b) on t5.a=t2.b where t3.a<=>t2.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	X	
-1	SIMPLE	t3	ref	a	a	5	test.t2.b	X	Using index condition(BKA); Using join buffer (flat, BKA join)
-1	SIMPLE	t4	ref	a	a	5	test.t3.b	X	Using where; Using join buffer (incremental, BKA join)
-1	SIMPLE	t6	ref	a	a	5	test.t4.b	X	Using join buffer (incremental, BKA join)
-1	SIMPLE	t5	ref	a	a	5	test.t2.b	X	Using where; Using join buffer (incremental, BKA join)
-1	SIMPLE	t7	ref	a	a	5	test.t5.b	X	Using join buffer (incremental, BKA join)
+1	SIMPLE	t3	ref	a	a	5	test.t2.b	X	Using index condition(BKA); Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t4	ref	a	a	5	test.t3.b	X	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t6	ref	a	a	5	test.t4.b	X	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t5	ref	a	a	5	test.t2.b	X	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t7	ref	a	a	5	test.t5.b	X	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 explain select * from t2 left join
 (t3 left join (t4 join t6 on t6.a=t4.b) on t4.a=t3.b 
 join t5 on t5.a=t3.b) on t3.a=t2.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	X	
-1	SIMPLE	t3	ref	a	a	5	test.t2.b	X	Using where; Using join buffer (flat, BKA join)
-1	SIMPLE	t5	ref	a	a	5	test.t3.b	X	Using join buffer (incremental, BKA join)
-1	SIMPLE	t4	ref	a	a	5	test.t5.a	X	Using where; Using join buffer (incremental, BKA join)
-1	SIMPLE	t6	ref	a	a	5	test.t4.b	X	Using join buffer (incremental, BKA join)
+1	SIMPLE	t3	ref	a	a	5	test.t2.b	X	Using where; Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t5	ref	a	a	5	test.t3.b	X	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t4	ref	a	a	5	test.t5.a	X	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t6	ref	a	a	5	test.t4.b	X	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 drop table t0, t1, t2, t3, t4, t5, t6, t7;
 create table t1 (a int);
 insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
@@ -1485,8 +1485,8 @@ explain select * from t1 left join
 on (t1.a = t2.a);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	
-1	SIMPLE	t2	ref	a	a	5	test.t1.a	1	Using where; Using join buffer (flat, BKA join)
-1	SIMPLE	t3	ref	a	a	5	test.t1.a	1	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t2	ref	a	a	5	test.t1.a	1	Using where; Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
+1	SIMPLE	t3	ref	a	a	5	test.t1.a	1	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 drop table t1, t2, t3;
 CREATE TABLE t1 (id int NOT NULL PRIMARY KEY, type varchar(10));
 CREATE TABLE t2 (pid int NOT NULL PRIMARY KEY, type varchar(10));
@@ -1737,7 +1737,7 @@ LEFT JOIN
 ON t4.carrier = t1.carrier;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	index	package_id	package_id	5	NULL	45	Using where; Using index
-1	SIMPLE	t1	eq_ref	PRIMARY	PRIMARY	4	test.t2.package_id	1	Using join buffer (flat, BKA join)
+1	SIMPLE	t1	eq_ref	PRIMARY	PRIMARY	4	test.t2.package_id	1	Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t4	eq_ref	PRIMARY,id	PRIMARY	2	test.t1.carrier	1	Using where
 1	SIMPLE	t5	ref	carrier_id	carrier_id	5	test.t4.id	22	Using index
 1	SIMPLE	t3	ref	package_id	package_id	5	test.t2.package_id	1	Using index
@@ -1831,9 +1831,9 @@ ON t6.b >= 2 AND t5.b=t7.b AND
 (t8.a > 0 OR t8.c IS NULL);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	3	
-1	SIMPLE	t7	ref	b_i	b_i	5	test.t5.b	2	Using where; Using join buffer (flat, BKA join)
+1	SIMPLE	t7	ref	b_i	b_i	5	test.t5.b	2	Using where; Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t6	ALL	b_i	NULL	NULL	NULL	3	Using where; Using join buffer (incremental, BNL join)
-1	SIMPLE	t8	ref	b_i	b_i	5	test.t5.b	2	Using where; Using join buffer (incremental, BKA join)
+1	SIMPLE	t8	ref	b_i	b_i	5	test.t5.b	2	Using where; Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 SELECT t5.a,t5.b,t6.a,t6.b,t7.a,t7.b,t8.a,t8.b
 FROM t5 
 LEFT JOIN 
@@ -1867,7 +1867,7 @@ ON (t5.b=t8.b);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	2	
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	1	Using join buffer (flat, BNL join)
-1	SIMPLE	t7	const	PRIMARY	PRIMARY	4	const	1	Using join buffer (incremental, BKA join)
+1	SIMPLE	t7	const	PRIMARY	PRIMARY	4	const	1	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t8	ALL	b_i	NULL	NULL	NULL	1	Using where; Using join buffer (incremental, BNL join)
 SELECT t5.a,t5.b,t6.a,t6.b,t7.a,t7.b,t8.a,t8.b
 FROM t5 LEFT JOIN                
@@ -1884,7 +1884,7 @@ ON (t5.b=t8.b);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	2	
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	1	Using join buffer (flat, BNL join)
-1	SIMPLE	t7	ref	b_i	b_i	5	const	0	Using join buffer (incremental, BKA join)
+1	SIMPLE	t7	ref	b_i	b_i	5	const	0	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 1	SIMPLE	t8	ALL	b_i	NULL	NULL	NULL	1	Using where; Using join buffer (incremental, BNL join)
 SELECT t5.a,t5.b,t6.a,t6.b,t7.a,t7.b,t8.a,t8.b
 FROM t5 LEFT JOIN                
@@ -1902,7 +1902,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	2	
 1	SIMPLE	t8	ALL	b_i	NULL	NULL	NULL	1	Using where; Using join buffer (flat, BNL join)
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	1	Using join buffer (incremental, BNL join)
-1	SIMPLE	t7	const	PRIMARY	PRIMARY	4	const	1	Using join buffer (incremental, BKA join)
+1	SIMPLE	t7	const	PRIMARY	PRIMARY	4	const	1	Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan
 SELECT t5.a,t5.b,t6.a,t6.b,t7.a,t7.b,t8.a,t8.b
 FROM t5 LEFT JOIN                
 (t8, t6 LEFT JOIN t7 ON t7.a=1)