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Commit 5dee4a99 authored by Igor Babaev's avatar Igor Babaev
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MDEV-19324 Wrong results from query, using brackets with ORDER BY ..LIMIT 

If a select query was of the form (SELECT ... ORDER BY ...) LIMIT ...
then in most cases it returned incorrect result. It happened because
SELECT ... ORDER BY ... was wrapped into a select with materialized
derived table:
  SELECT ... ORDER BY ... =>
  SELECT * FROM (SELECT ... ORDER BY ...) dt.
Yet for any materialized derived table ORDER BY without LIMIT is ignored.
This patch resolves the problem by the conversion
  (SELECT ... ORDER BY ...) LIMIT ... =>
  SELECT ... ORDER BY ... LIMIT ...
at the parser stage.

Similarly
  ((SELECT ... UNION ...) ORDER BY ...) LIMIT ...
is converted to
  (SELECT ... UNION ...) ORDER BY ... LIMIT ...
This conversion optimizes execution of the query because the result of
(SELECT ... UNION ...) ORDER BY ... is not materialized into a temporary
table anymore.
parent d1a43973
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Showing with 194 additions and 2 deletions
mysql-test/main/brackets.result
View file @ 5dee4a99
......@@ -243,4 +243,154 @@ a
a
1
DROP TABLE t1,t2;
#
# MDEV-19324: ((SELECT ...) ORDER BY col ) LIMIT n
#
create table t1 (a int);
insert into t1 values (10),(20),(30);
select a from t1 order by a desc limit 1;
a
30
explain extended select a from t1 order by a desc limit 1;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 100.00 Using filesort
Warnings:
Note 1003 select `test`.`t1`.`a` AS `a` from `test`.`t1` order by `test`.`t1`.`a` desc limit 1
explain format=json select a from t1 order by a desc limit 1;
EXPLAIN
{
"query_block": {
"select_id": 1,
"read_sorted_file": {
"filesort": {
"sort_key": "t1.a desc",
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100
}
}
}
}
}
(select a from t1 order by a desc) limit 1;
a
30
explain extended (select a from t1 order by a desc) limit 1;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 100.00 Using filesort
Warnings:
Note 1003 (select `test`.`t1`.`a` AS `a` from `test`.`t1` order by `test`.`t1`.`a` desc limit 1)
explain format=json (select a from t1 order by a desc) limit 1;
EXPLAIN
{
"query_block": {
"select_id": 1,
"read_sorted_file": {
"filesort": {
"sort_key": "t1.a desc",
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100
}
}
}
}
}
(select a from t1 where a=20 union select a from t1) order by a desc limit 1;
a
30
explain extended (select a from t1 where a=20 union select a from t1) order by a desc limit 1;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 3 100.00 Using where
2 UNION t1 ALL NULL NULL NULL NULL 3 100.00
NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL NULL NULL Using filesort
Warnings:
Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` = 20 union /* select#2 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` order by `a` desc limit 1
explain format=json (select a from t1 where a=20 union select a from t1) order by a desc limit 1;
EXPLAIN
{
"query_block": {
"union_result": {
"table_name": "<union1,2>",
"access_type": "ALL",
"query_specifications": [
{
"query_block": {
"select_id": 1,
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100,
"attached_condition": "t1.a = 20"
}
}
},
{
"query_block": {
"select_id": 2,
"operation": "UNION",
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100
}
}
}
]
}
}
}
((select a from t1 where a=20 union select a from t1) order by a desc) limit 1;
a
30
explain extended ((select a from t1 where a=20 union select a from t1) order by a desc) limit 1;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 3 100.00 Using where
2 UNION t1 ALL NULL NULL NULL NULL 3 100.00
NULL UNION RESULT <union1,2> ALL NULL NULL NULL NULL NULL NULL Using filesort
Warnings:
Note 1003 (/* select#1 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` where `test`.`t1`.`a` = 20) union /* select#2 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` order by `a` desc limit 1
explain format=json ((select a from t1 where a=20 union select a from t1) order by a desc) limit 1;
EXPLAIN
{
"query_block": {
"union_result": {
"table_name": "<union1,2>",
"access_type": "ALL",
"query_specifications": [
{
"query_block": {
"select_id": 1,
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100,
"attached_condition": "t1.a = 20"
}
}
},
{
"query_block": {
"select_id": 2,
"operation": "UNION",
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100
}
}
}
]
}
}
}
drop table t1;
# End of 10.4 tests
mysql-test/main/brackets.test
View file @ 5dee4a99
......@@ -106,5 +106,38 @@ INSERT INTO t2 VALUES (4),(5),(6),(7);
DROP TABLE t1,t2;
--echo #
--echo # MDEV-19324: ((SELECT ...) ORDER BY col ) LIMIT n
--echo #
create table t1 (a int);
insert into t1 values (10),(20),(30);
let $q1=
select a from t1 order by a desc limit 1;
eval $q1;
eval explain extended $q1;
eval explain format=json $q1;
let $q2=
(select a from t1 order by a desc) limit 1;
eval $q2;
eval explain extended $q2;
eval explain format=json $q2;
let $q1=
(select a from t1 where a=20 union select a from t1) order by a desc limit 1;
eval $q1;
eval explain extended $q1;
eval explain format=json $q1;
let $q2=
((select a from t1 where a=20 union select a from t1) order by a desc) limit 1;
eval $q2;
eval explain extended $q2;
eval explain format=json $q2;
drop table t1;
--echo # End of 10.4 tests
sql/sql_lex.cc
View file @ 5dee4a99
......@@ -9129,6 +9129,12 @@ SELECT_LEX *LEX::parsed_select(SELECT_LEX *sel, Lex_order_limit_lock * l)
l->set_to(unit->fake_select_lex);
else
{
if (!l->order_list && !unit->fake_select_lex->explicit_limit)
{
sel= unit->fake_select_lex;
l->order_list= &sel->order_list;
}
else
sel= wrap_unit_into_derived(unit);
if (!sel)
return NULL;
......@@ -9144,6 +9150,9 @@ SELECT_LEX *LEX::parsed_select(SELECT_LEX *sel, Lex_order_limit_lock * l)
SELECT_LEX_UNIT *unit= create_unit(sel);
if (!unit)
return NULL;
if (!l->order_list && !sel->explicit_limit)
l->order_list= &sel->order_list;
else
sel= wrap_unit_into_derived(unit);
if (!sel)
return NULL;
......
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