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MDEV-25969: Condition pushdown into derived table doesn't work if sel… 
…ect list uses SP

Consider a query of the form:

  select ... from (select item2 as COL1) as T where COL1=123

Condition pushdown into derived table will try to push "COL1=123" condition
down into table T.
The process of pushdown involves "substituting" the item, that is,
replacing Item_field("T.COL1") with its "producing item" item2.
In order to use item2, one needs to clone it (call Item::build_clone).

If the item is not cloneable (e.g. Item_func_sp is not), the pushdown
process will fail and nothing at all will be pushed.

Fixed by introducing transform_condition_or_part() which will try to apply
the transformation for as many parts of condition as possible. The parts of
condition that couldn't be transformed are dropped.
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spetrunia committed Jun 30, 2021
1 parent 768c518 commit eb20c91
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146 mysql-test/r/derived_cond_pushdown.result
View file
@@ -10673,4 +10673,150 @@ Warnings:
Note 1003 select `v2`.`a` AS `a`,`v2`.`f` AS `f`,`v2`.`g` AS `g` from `test`.`v2` where `v2`.`f` = `v2`.`a` and `v2`.`g` = `v2`.`a`
drop view v1,v2;
drop table t1;
#
# MDEV-25969: Condition pushdown into derived table doesn't work if select list uses SP
#
create function f1(a int) returns int DETERMINISTIC return (a+1);
create table t1 (
pk int primary key,
a int,
b int,
key(a)
);
create table t2(a int);
insert into t2 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
create table t3(a int);
insert into t3 select A.a + B.a* 10 + C.a * 100 from t2 A, t2 B, t2 C;
insert into t1 select a,a,a from t3;
create view v1 as
select
t1.a as col1,
f1(t1.b) as col2
from
t1;
create view v2 as
select
t1.a as col1,
f1(t1.b) as col2
from
t1;
create view v3 as
select col2, col1 from v1
union all
select col2, col1 from v2;
explain select * from v3 where col1=123;
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 2 Using where
2 DERIVED t1 ref a a 5 const 1
3 UNION t1 ref a a 5 const 1
# This must use ref accesses for reading table t1, not full scans:
explain format=json
select * from v3 where col1=123 and col2=321;
EXPLAIN
{
"query_block": {
"select_id": 1,
"table": {
"table_name": "<derived2>",
"access_type": "ALL",
"rows": 2,
"filtered": 100,
"attached_condition": "v3.col1 = 123 and v3.col2 = 321",
"materialized": {
"query_block": {
"union_result": {
"table_name": "<union2,3>",
"access_type": "ALL",
"query_specifications": [
{
"query_block": {
"select_id": 2,
"table": {
"table_name": "t1",
"access_type": "ref",
"possible_keys": ["a"],
"key": "a",
"key_length": "5",
"used_key_parts": ["a"],
"ref": ["const"],
"rows": 1,
"filtered": 100
}
}
},
{
"query_block": {
"select_id": 3,
"table": {
"table_name": "t1",
"access_type": "ref",
"possible_keys": ["a"],
"key": "a",
"key_length": "5",
"used_key_parts": ["a"],
"ref": ["const"],
"rows": 1,
"filtered": 100
}
}
}
]
}
}
}
}
}
}
drop function f1;
drop view v1,v2,v3;
drop table t1, t2,t3;
#
# Another testcase, with pushdown through GROUP BY
#
create table t1 (a int, b int);
insert into t1 values (1,1),(2,2),(3,3);
create function f1(a int) returns int DETERMINISTIC return (a+1);
create view v2(a, a2, s) as
select a, f1(a), sum(b) from t1 group by a, f1(a);
# Here,
# "(s+1) > 10" will be pushed into HAVING
# "a > 1" will be pushed all the way to the table scan on t1
# "a2>123" will be pushed into HAVING (as it refers to an SP call which
# prevents pushing it to the WHERE)
explain format=json
select * from v2 where (s+1) > 10 AND a > 1 and a2>123;
EXPLAIN
{
"query_block": {
"select_id": 1,
"table": {
"table_name": "<derived2>",
"access_type": "ALL",
"rows": 3,
"filtered": 100,
"attached_condition": "v2.s + 1 > 10 and v2.a > 1 and v2.a2 > 123",
"materialized": {
"query_block": {
"select_id": 2,
"having_condition": "s + 1 > 10 and a2 > 123",
"filesort": {
"sort_key": "t1.a, f1(t1.a)",
"temporary_table": {
"table": {
"table_name": "t1",
"access_type": "ALL",
"rows": 3,
"filtered": 100,
"attached_condition": "t1.a > 1"
}
}
}
}
}
}
}
}
drop view v2;
drop function f1;
drop table t1;
# End of 10.2 tests
70 mysql-test/t/derived_cond_pushdown.test
View file
@@ -2237,4 +2237,74 @@ eval explain extended $q2;
drop view v1,v2;
drop table t1;

--echo #
--echo # MDEV-25969: Condition pushdown into derived table doesn't work if select list uses SP
--echo #
create function f1(a int) returns int DETERMINISTIC return (a+1);

create table t1 (
pk int primary key,
a int,
b int,
key(a)
);

create table t2(a int);
insert into t2 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table t3(a int);
insert into t3 select A.a + B.a* 10 + C.a * 100 from t2 A, t2 B, t2 C;

insert into t1 select a,a,a from t3;

create view v1 as
select
t1.a as col1,
f1(t1.b) as col2
from
t1;

create view v2 as
select
t1.a as col1,
f1(t1.b) as col2
from
t1;
create view v3 as
select col2, col1 from v1
union all
select col2, col1 from v2;

explain select * from v3 where col1=123;

--echo # This must use ref accesses for reading table t1, not full scans:
explain format=json
select * from v3 where col1=123 and col2=321;

drop function f1;
drop view v1,v2,v3;
drop table t1, t2,t3;

--echo #
--echo # Another testcase, with pushdown through GROUP BY
--echo #
create table t1 (a int, b int);
insert into t1 values (1,1),(2,2),(3,3);

create function f1(a int) returns int DETERMINISTIC return (a+1);

create view v2(a, a2, s) as
select a, f1(a), sum(b) from t1 group by a, f1(a);

--echo # Here,
--echo # "(s+1) > 10" will be pushed into HAVING
--echo # "a > 1" will be pushed all the way to the table scan on t1
--echo # "a2>123" will be pushed into HAVING (as it refers to an SP call which
--echo # prevents pushing it to the WHERE)
explain format=json
select * from v2 where (s+1) > 10 AND a > 1 and a2>123;

drop view v2;
drop function f1;
drop table t1;
--echo # End of 10.2 tests
79 sql/sql_derived.cc
View file
@@ -1192,6 +1192,68 @@ bool mysql_derived_reinit(THD *thd, LEX *lex, TABLE_LIST *derived)
}


/*
@brief
Given condition cond and transformer+argument, try transforming as many
conjuncts as possible.
@detail
The motivation of this function is to convert the condition that's being
pushed into a WHERE clause with derived_field_transformer_for_where or
with derived_grouping_field_transformer_for_where.
The transformer may fail for some sub-condition, in this case we want to
convert the most restrictive part of the condition that can be pushed.
This function only does it for top-level AND: conjuncts that could not be
converted are dropped.
@return
Converted condition, or NULL if nothing could be converted
*/

static
Item *transform_condition_or_part(THD *thd,
Item *cond,
Item_transformer transformer,
uchar *arg)
{
if (cond->type() != Item::COND_ITEM ||
((Item_cond*) cond)->functype() != Item_func::COND_AND_FUNC)
{
Item *new_item= cond->transform(thd, transformer, arg);
// Indicate that the condition is not pushable
if (!new_item)
cond->clear_extraction_flag();
return new_item;
}

List_iterator<Item> li(*((Item_cond*) cond)->argument_list());
Item *item;
while ((item=li++))
{
Item *new_item= item->transform(thd, transformer, arg);
if (!new_item)
{
// Indicate that the condition is not pushable
item->clear_extraction_flag();
li.remove();
}
else
li.replace(new_item);
}

switch (((Item_cond*) cond)->argument_list()->elements)
{
case 0:
return NULL;
case 1:
return ((Item_cond*) cond)->argument_list()->head();
default:
return cond;
}
}


/**
@brief
Extract the condition depended on derived table/view and pushed it there
@@ -1287,9 +1349,11 @@ bool pushdown_cond_for_derived(THD *thd, Item *cond, TABLE_LIST *derived)
if (!sl->join->group_list && !sl->with_sum_func)
{
/* extracted_cond_copy is pushed into where of sl */
extracted_cond_copy= extracted_cond_copy->transform(thd,
&Item::derived_field_transformer_for_where,
(uchar*) sl);
extracted_cond_copy=
transform_condition_or_part(thd,
extracted_cond_copy,
&Item::derived_field_transformer_for_where,
(uchar*)sl);
if (extracted_cond_copy)
{
extracted_cond_copy->walk(
@@ -1316,9 +1380,12 @@ bool pushdown_cond_for_derived(THD *thd, Item *cond, TABLE_LIST *derived)
pushed into the where clause of sl to make them usable in the new context
*/
if (cond_over_grouping_fields)
cond_over_grouping_fields= cond_over_grouping_fields->transform(thd,
&Item::derived_grouping_field_transformer_for_where,
(uchar*) sl);
{
cond_over_grouping_fields=
transform_condition_or_part(thd, cond_over_grouping_fields,
&Item::derived_grouping_field_transformer_for_where,
(uchar*) sl);
}

if (cond_over_grouping_fields)
{

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