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mariadb/sql/opt_split.cc
Igor Babaev b1c8ea83a5 MDEV-32064 Crash when searching for the best split of derived table 
This bug could affect queries with IN subqueries in WHERE clause and using
derived tables to which split optimization potentially could be applied.

When looking for the best split of a splittable derived table T any key
access from a semi-join materialized table used for lookups S to table T
must be excluded from consideration because in the current implementation
of such tables as S the values from its records cannot be used to access
other tables.

Approved by Oleksandr Byelkin <sanja@mariadb.com>
2023-10-16 11:15:57 -07:00

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/*
Copyright (c) 2017, 2020, MariaDB
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
/*
This file contains functions to support the splitting technique.
This optimization technique can be applied to equi-joins involving
materialized tables such as materialized views, materialized derived tables
and materialized CTEs. The technique also could be applied to materialized
semi-joins though the code below does not support this usage yet.
Here are the main ideas behind this technique that we'll call SM optimization
(SplitMaterialization).
Consider the query
SELECT t1.a, t.min
FROM t1, (SELECT t2.a, MIN(t2.b) as min FROM t2 GROUP BY t2.a) t
WHERE t1.a = t.a and t1.b < const
Re-write the query into
SELECT t1.a, t.min
FROM t1, LATERAL (SELECT t2.a, MIN(t2.b) as min
FROM t2 WHERE t2.a = t1.a GROUP BY t2.a) t
WHERE t1.b < const
The execution of the original query (Q1) does the following:
1. Executes the query in the specification of the derived table
and puts the result set into a temporary table with an index
on the first column.
2. Joins t1 with the temporary table using the its index.
The execution of the transformed query (Q1R) follows these steps:
1. For each row of t1 where t1.b < const a temporary table
containing all rows of of t2 with t2.a = t1.a is created
2. If there are any rows in the temporary table aggregation
is performed for them
3. The result of the aggregation is joined with t1.
The second execution can win if:
a) There is an efficient way to select rows of t2 for which t2.a = t1.a
(For example if there is an index on t2.a)
and
b) The number of temporary tables created for partitions
is much smaller that the total number of partitions
It should be noted that for the transformed query aggregation
for a partition may be performed several times.
As we can see the optimization basically splits table t2 into
partitions and performs aggregation over each of them
independently.
If we have only one equi-join condition then we either push it as
for Q1R or we don't. In a general case we may have much more options.
Consider the query (Q3)
SELECT *
FROM t1,t2 (SELECT t3.a, t3.b, MIN(t3.c) as min
FROM t3 GROUP BY a,b) t
WHERE t.a = t1.a AND t.b = t2.b
AND t1.c < c1 and t2.c < c2
AND P(t1,t2);
(P(t1,t2) designates some additional conditions over columns of t1,t2).
Assuming that there indexes on t3(a,b) and t3(b) here we have several
reasonable options to push equi-join conditions into the derived.
All these options should be taken into account when the optimizer
evaluates different join orders. When the join order (t1,t,t2) is
evaluated there is only one way of splitting : to push the condition
t.a = t1.a into t. With the join order (t2,t,t1) only the condition
t.b = t2.b can be pushed. When the join orders (t1,t2,t) and (t2,t1,t)
are evaluated then the optimizer should consider pushing t.a = t1.a,
t.b = t2.b and (t.a = t1.a AND t.b = t2.b) to choose the best condition
for splitting. Apparently here last condition is the best one because
it provides the miximum possible number of partitions.
If we dropped the index on t3(a,b) and created the index on t3(a) instead
then we would have two options for splitting: to push t.a = t1.a or to
push t.b = t2.b. If the selectivity of the index t3(a) is better than
the selectivity of t3(b) then the first option is preferred.
Although the condition (t.a = t1.a AND t.b = t2.b) provides a better
splitting than the condition t.a = t1.a the latter will be used for
splitting if the execution plan with the join order (t1,t,t2) turns out
to be the cheapest one. It's quite possible when the join condition
P(t1,t2) has a bad selectivity.
Whenever the optimizer evaluates the cost of using a splitting it
compares it with the cost of materialization without splitting.
If we just drop the index on t3(a,b) the chances that the splitting
will be used becomes much lower but they still exists providing that
the fanout of the partial join of t1 and t2 is small enough.
The lateral derived table LT formed as a result of SM optimization applied
to a materialized derived table DT must be joined after all parameters
of splitting has been evaluated, i.e. after all expressions used in the
equalities pushed into DT that make the employed splitting effective
could be evaluated. With the chosen join order all the parameters can be
evaluated after the last table LPT that contains any columns referenced in
the parameters has been joined and the table APT following LPT in the chosen
join order is accessed.
Usually the formed lateral derived table LT is accessed right after the table
LPT. As in such cases table LT must be refilled for each combination of
splitting parameters this table must be populated before each access to LT
and the estimate of the expected number of refills that could be suggested in
such cases is the number of rows in the partial join ending with table LPT.
However in other cases the chosen join order may contain tables between LPT
and LT.
Consider the query (Q4)
SELECT *
FROM t1 JOIN t2 ON t1.b = t2.b
LEFT JOIN (SELECT t3.a, t3.b, MIN(t3.c) as min
FROM t3 GROUP BY a,b) t
ON t.a = t1.a AND t.c > 0
[WHERE P(t1,t2)];
Let's assume that the join order t1,t2,t was chosen for this query and
SP optimization was applied to t with splitting over t3.a using the index
on column t3.a. Here the table t1 serves as LPT, t2 as APT while t with
pushed condition t.a = t1.a serves as LT. Note that here LT is accessed
after t2, not right after t1. Here the number of refills of the lateral
derived is not more that the number of key values of t1.a that might be
less than the cardinality of the partial join (t1,t2). That's why it makes
sense to signal that t3 has to be refilled just before t2 is accessed.
However if the cardinality of the partial join (t1,t2) happens to be less
than the cardinality of the partial join (t1) due to additional selective
condition P(t1,t2) then the flag informing about necessity of a new refill
can be set either when accessing t2 or right after it has been joined.
The current code sets such flag right after generating a record of the
partial join with minimal cardinality for all those partial joins that
end between APT and LT. It allows sometimes to push extra conditions
into the lateral derived without any increase of the number of refills.
However this flag can be set only after the last join table between
APT and LT using join buffer has been joined.
*/
/*
Splitting can be applied to a materialized table specified by the query
with post-join operations that require partitioning of the result set produced
by the join expression used in the FROM clause the query such as GROUP BY
operation and window function operation. In any of these cases the post-join
operation can be executed independently for any partition only over the rows
of this partition. Also if the set of all partitions is divided into disjoint
subsets the operation can applied to each subset independently. In this case
all rows are first partitioned into the groups each of which contains all the
rows from the partitions belonging the same subset and then each group
is subpartitioned into groups in the the post join operation.
The set of all rows belonging to the union of several partitions is called
here superpartition. If a grouping operation is defined by the list
e_1,...,e_n then any set S = {e_i1,...,e_ik} can be used to devide all rows
into superpartions such that for any two rows r1, r2 the following holds:
e_ij(r1) = e_ij(r2) for each e_ij from S. We use the splitting technique
only if S consists of references to colums of the joined tables.
For example if the GROUP BY list looks like this a, g(b), c we can consider
applying the splitting technique to the superpartitions defined by {a,c},
{a}, {c} (a and c here may be the references to the columns from different
tables).
*/
/*
The following describes when and how the optimizer decides whether it
makes sense to employ the splitting technique.
1. For each instance of a materialized table (derived/view/CTE) it is
checked that it is potentially splittable. Now it is done right after the
execution plan for the select specifying this table has been chosen.
2. Any potentially splittable materialized table T is subject to two-phase
optimization. It means that the optimizer first builds the best execution
plan for join that specifies T. Then the control is passed back to the
optimization process of the embedding select Q. After the execution plan
for Q has been chosen the optimizer finishes the optimization of the join
specifying T.
3. When the optimizer builds the container with the KEYUSE structures
for the join of embedding select it detects the equi-join conditions
PC that potentially could be pushed into a potentially splittable
materialized table T. The collected information about such conditions
is stored together with other facts on potential splittings for table T.
4. When the optimizer starts looking for the best execution plan for the
embedding select Q for each potentially splittable materialized table T
it creates special KEYUSE structures for pushable equi-join conditions
PC. These structures are used to add new elements to the container
of KEYUSE structures built for T. The specifics of these elements is
that they can be ebabled and disabled during the process of choosing
the best plan for Q.
5. When the optimizer extends a partial join order with a potentially
splittable materialized table T (in function best_access_path) it
first evaluates a new execution plan for the modified specification
of T that adds all equi-join conditions that can be pushed with
current join prefix to the WHERE conditions of the original
specification of T. If the cost of the new plan is better than the
the cost of the original materialized table then the optimizer
prefers to use splitting for the current join prefix. As the cost
of the plan depends only on the pushed conditions it makes sense
to cache this plan for other prefixes.
6. The optimizer takes into account the cost of splitting / materialization
of a potentially splittable materialized table T as a startup cost
to access table T.
7. When the optimizer finally chooses the best execution plan for
the embedding select Q and this plan prefers using splitting
for table T with pushed equi-join conditions PC then the execution
plan for the underlying join with these conditions is chosen for T.
*/
/*
The implementation of the splitting technique below allows to apply
the technique only to a materialized derived table / view / CTE whose
specification is either a select with GROUP BY or a non-grouping select
with window functions that share the same PARTITION BY list.
*/
#include "mariadb.h"
#include "sql_select.h"
#include "opt_trace.h"
/* Info on a splitting field */
struct SplM_field_info
{
/* Splitting field in the materialized table T */
Field *mat_field;
/* The item from the select list of the specification of T */
Item *producing_item;
/* The corresponding splitting field from the specification of T */
Field *underlying_field;
};
/* Info on the splitting execution plan saved in SplM_opt_info::cache */
struct SplM_plan_info
{
/* The cached splitting execution plan P */
POSITION *best_positions;
/* The cost of the above plan */
double cost;
/* Selectivity of splitting used in P */
double split_sel;
/* For fast search of KEYUSE_EXT elements used for splitting in P */
struct KEYUSE_EXT *keyuse_ext_start;
/* The tables that contains the fields used for splitting in P */
TABLE *table;
/* The number of the key from 'table' used for splitting in P */
uint key;
/* Number of the components of 'key' used for splitting in P */
uint parts;
};
/*
The structure contains the information that is used by the optimizer
for potentially splittable materialization of T that is a materialized
derived_table / view / CTE
*/
class SplM_opt_info : public Sql_alloc
{
public:
/* The join for the select specifying T */
JOIN *join;
/* The map of tables from 'join' whose columns can be used for partitioning */
table_map tables_usable_for_splitting;
/* Info about the fields of the joined tables usable for splitting */
SplM_field_info *spl_fields;
/* The number of elements in the above list */
uint spl_field_cnt;
/* The list of equalities injected into WHERE for split optimization */
List<Item> inj_cond_list;
/* Contains the structures to generate all KEYUSEs for pushable equalities */
List<KEY_FIELD> added_key_fields;
/* The cache of evaluated execution plans for 'join' with pushed equalities */
List<SplM_plan_info> plan_cache;
/* Cost of best execution plan for join when nothing is pushed */
double unsplit_cost;
/* Cardinality of T when nothing is pushed */
double unsplit_card;
/* Lastly evaluated execution plan for 'join' with pushed equalities */
SplM_plan_info *last_plan;
double last_refills;
SplM_plan_info *find_plan(TABLE *table, uint key, uint parts);
};
void TABLE::set_spl_opt_info(SplM_opt_info *spl_info)
{
if (spl_info)
spl_info->join->spl_opt_info= spl_info;
spl_opt_info= spl_info;
}
void TABLE::deny_splitting()
{
DBUG_ASSERT(spl_opt_info != NULL);
spl_opt_info->join->spl_opt_info= NULL;
spl_opt_info= NULL;
}
double TABLE::get_materialization_cost()
{
DBUG_ASSERT(spl_opt_info != NULL);
return spl_opt_info->unsplit_cost;
}
/* This structure is auxiliary and used only in the function that follows it */
struct SplM_field_ext_info: public SplM_field_info
{
uint item_no;
bool is_usable_for_ref_access;
};
/**
@brief
Check whether this join is one for potentially splittable materialized table
@details
The function checks whether this join is for select that specifies
a potentially splittable materialized table T. If so, the collected
info on potential splittability of T is attached to the field spl_opt_info
of the TABLE structure for T.
The function returns a positive answer if the following holds:
1. the optimizer switch 'split_materialized' is set 'on'
2. the select owning this join specifies a materialized derived/view/cte T
3. this is the only select in the specification of T
4. condition pushdown is not prohibited into T
5. T is not recursive
6. not all of this join are constant or optimized away
7. T is either
7.1. a grouping table with GROUP BY list P
or
7.2. a non-grouping table with window functions over the same non-empty
partition specified by the PARTITION BY list P
8. P contains some references on the columns of the joined tables C
occurred also in the select list of this join
9. There are defined some keys usable for ref access of fields from C
with available statistics.
10. The select doesn't use WITH ROLLUP (This limitation can probably be
lifted)
@retval
true if the answer is positive
false otherwise
*/
bool JOIN::check_for_splittable_materialized()
{
ORDER *partition_list= 0;
st_select_lex_unit *unit= select_lex->master_unit();
TABLE_LIST *derived= unit->derived;
if (!(optimizer_flag(thd, OPTIMIZER_SWITCH_SPLIT_MATERIALIZED)) || // !(1)
!(derived && derived->is_materialized_derived()) || // !(2)
(unit->first_select()->next_select()) || // !(3)
(derived->prohibit_cond_pushdown) || // !(4)
(derived->is_recursive_with_table()) || // !(5)
(table_count == 0 || const_tables == top_join_tab_count) || // !(6)
rollup.state != ROLLUP::STATE_NONE) // (10)
return false;
if (group_list) // (7.1)
{
if (!select_lex->have_window_funcs())
partition_list= group_list;
}
else if (select_lex->have_window_funcs() &&
select_lex->window_specs.elements == 1) // (7.2)
{
partition_list=
select_lex->window_specs.head()->partition_list->first;
}
if (!partition_list)
return false;
Json_writer_object trace_wrapper(thd);
Json_writer_object trace_split(thd, "check_split_materialized");
ORDER *ord;
Dynamic_array<SplM_field_ext_info> candidates;
/*
Select from partition_list all candidates for splitting.
A candidate must be
- field item or refer to such (8.1)
- item mentioned in the select list (8.2)
Put info about such candidates into the array candidates
*/
table_map usable_tables= 0; // tables that contains the candidate
for (ord= partition_list; ord; ord= ord->next)
{
Item *ord_item= *ord->item;
if (ord_item->real_item()->type() != Item::FIELD_ITEM) // !(8.1)
continue;
Field *ord_field= ((Item_field *) (ord_item->real_item()))->field;
/* Ignore fields from of inner tables of outer joins */
TABLE_LIST *tbl= ord_field->table->pos_in_table_list;
if (tbl->is_inner_table_of_outer_join())
continue;
List_iterator<Item> li(fields_list);
Item *item;
uint item_no= 0;
while ((item= li++))
{
if ((*ord->item)->eq(item, 0)) // (8.2)
{
SplM_field_ext_info new_elem;
new_elem.producing_item= item;
new_elem.item_no= item_no;
new_elem.mat_field= derived->table->field[item_no];
new_elem.underlying_field= ord_field;
new_elem.is_usable_for_ref_access= false;
candidates.push(new_elem);
usable_tables|= ord_field->table->map;
break;
}
item_no++;
}
}
if (candidates.elements() == 0) // no candidates satisfying (8.1) && (8.2)
{
trace_split.add("not_applicable", "group list has no candidates");
return false;
}
/*
For each table from this join find the keys that can be used for ref access
of the fields mentioned in the 'array candidates'
*/
SplM_field_ext_info *cand;
SplM_field_ext_info *cand_start= &candidates.at(0);
SplM_field_ext_info *cand_end= cand_start + candidates.elements();
for (JOIN_TAB *tab= join_tab;
tab < join_tab + top_join_tab_count; tab++)
{
TABLE *table= tab->table;
if (!(table->map & usable_tables))
continue;
table->keys_usable_for_splitting.clear_all();
uint i;
for (i= 0; i < table->s->keys; i++)
{
if (!table->keys_in_use_for_query.is_set(i))
continue;
KEY *key_info= table->key_info + i;
uint key_parts= table->actual_n_key_parts(key_info);
uint usable_kp_cnt= 0;
for ( ; usable_kp_cnt < key_parts; usable_kp_cnt++)
{
if (key_info->actual_rec_per_key(usable_kp_cnt) == 0)
break;
int fldnr= key_info->key_part[usable_kp_cnt].fieldnr;
for (cand= cand_start; cand < cand_end; cand++)
{
if (cand->underlying_field->table == table &&
cand->underlying_field->field_index + 1 == fldnr)
{
cand->is_usable_for_ref_access= true;
break;
}
}
if (cand == cand_end)
break;
}
if (usable_kp_cnt)
table->keys_usable_for_splitting.set_bit(i);
}
}
/* Count the candidate fields that can be accessed by ref */
uint spl_field_cnt= (uint)candidates.elements();
for (cand= cand_start; cand < cand_end; cand++)
{
if (!cand->is_usable_for_ref_access)
spl_field_cnt--;
}
if (!spl_field_cnt) // No candidate field can be accessed by ref => !(9)
{
trace_split.add("not_applicable",
"no candidate field can be accessed through ref");
return false;
}
/*
Create a structure of the type SplM_opt_info and fill it with
the collected info on potential splittability of T
*/
SplM_opt_info *spl_opt_info= new (thd->mem_root) SplM_opt_info();
SplM_field_info *spl_field=
(SplM_field_info *) (thd->calloc(sizeof(SplM_field_info) *
spl_field_cnt));
if (!(spl_opt_info && spl_field)) // consider T as not good for splitting
return false;
spl_opt_info->join= this;
spl_opt_info->tables_usable_for_splitting= 0;
spl_opt_info->spl_field_cnt= spl_field_cnt;
spl_opt_info->spl_fields= spl_field;
{
Json_writer_array trace_range(thd, "split_candidates");
for (cand= cand_start; cand < cand_end; cand++)
{
if (!cand->is_usable_for_ref_access)
continue;
trace_range.add(cand->producing_item);
spl_field->producing_item= cand->producing_item;
spl_field->underlying_field= cand->underlying_field;
spl_field->mat_field= cand->mat_field;
spl_opt_info->tables_usable_for_splitting|=
cand->underlying_field->table->map;
spl_field++;
}
}
/* Attach this info to the table T */
derived->table->set_spl_opt_info(spl_opt_info);
/*
If this is specification of a materialized derived table T that is
potentially splittable and is used in the from list of the right operand
of an IN predicand transformed to a semi-join then the embedding semi-join
nest is not allowed to be materialized.
*/
if (derived && derived->is_materialized_derived() &&
derived->embedding && derived->embedding->sj_subq_pred)
derived->embedding->sj_subq_pred->types_allow_materialization= FALSE;
return true;
}
/**
@brief
Collect info on KEY_FIELD usable for splitting
@param
key_field KEY_FIELD to collect info on
@details
The function assumes that this table is potentially splittable.
The function checks whether the KEY_FIELD structure key_field built for
this table was created for a splitting field f. If so, the function does
the following using info from key_field:
1. Builds an equality of the form f = key_field->val that could be
pushed into this table.
2. Creates a new KEY_FIELD structure for this equality and stores
a reference to this structure in this->spl_opt_info.
*/
void TABLE::add_splitting_info_for_key_field(KEY_FIELD *key_field)
{
DBUG_ASSERT(spl_opt_info != NULL);
JOIN *join= spl_opt_info->join;
Field *field= key_field->field;
SplM_field_info *spl_field= spl_opt_info->spl_fields;
uint i= spl_opt_info->spl_field_cnt;
for ( ; i; i--, spl_field++)
{
if (spl_field->mat_field == field)
break;
}
if (!i) // field is not usable for splitting
return;
/*
Any equality condition that can be potentially pushed into the
materialized derived table is constructed now though later it may turn out
that it is not needed, because it is not used for splitting.
The reason for this is that the failure to construct it when it has to be
injected causes denial for further processing of the query.
Formally this equality is needed in the KEY_FIELD structure constructed
here that will be used to generate additional keyuses usable for splitting.
However key_field.cond could be used for this purpose (see implementations
of virtual function can_optimize_keypart_ref()).
The condition is built in such a form that it can be added to the WHERE
condition of the select that specifies this table.
*/
THD *thd= in_use;
Item *left_item= spl_field->producing_item->build_clone(thd);
Item *right_item= key_field->val->build_clone(thd);
Item_func_eq *eq_item= 0;
if (left_item && right_item)
{
right_item->walk(&Item::set_fields_as_dependent_processor,
false, join->select_lex);
right_item->update_used_tables();
eq_item= new (thd->mem_root) Item_func_eq(thd, left_item, right_item);
}
if (!eq_item)
return;
KEY_FIELD *added_key_field=
(KEY_FIELD *) thd->alloc(sizeof(KEY_FIELD));
if (!added_key_field ||
spl_opt_info->added_key_fields.push_back(added_key_field,thd->mem_root))
return;
added_key_field->field= spl_field->underlying_field;
added_key_field->cond= eq_item;
added_key_field->val= key_field->val;
added_key_field->level= 0;
added_key_field->optimize= KEY_OPTIMIZE_EQ;
added_key_field->eq_func= true;
Item *real= key_field->val->real_item();
if ((real->type() == Item::FIELD_ITEM) &&
((Item_field*)real)->field->maybe_null())
added_key_field->null_rejecting= true;
else
added_key_field->null_rejecting= false;
added_key_field->cond_guard= NULL;
added_key_field->sj_pred_no= UINT_MAX;
return;
}
static bool
add_ext_keyuse_for_splitting(Dynamic_array<KEYUSE_EXT> *ext_keyuses,
KEY_FIELD *added_key_field, uint key, uint part)
{
KEYUSE_EXT keyuse_ext;
Field *field= added_key_field->field;
JOIN_TAB *tab=field->table->reginfo.join_tab;
key_map possible_keys=field->get_possible_keys();
possible_keys.intersect(field->table->keys_usable_for_splitting);
tab->keys.merge(possible_keys);
Item_func_eq *eq_item= (Item_func_eq *) (added_key_field->cond);
keyuse_ext.table= field->table;
keyuse_ext.val= eq_item->arguments()[1];
keyuse_ext.key= key;
keyuse_ext.keypart=part;
keyuse_ext.keypart_map= (key_part_map) 1 << part;
keyuse_ext.used_tables= keyuse_ext.val->used_tables();
keyuse_ext.optimize= added_key_field->optimize & KEY_OPTIMIZE_REF_OR_NULL;
keyuse_ext.ref_table_rows= 0;
keyuse_ext.null_rejecting= added_key_field->null_rejecting;
keyuse_ext.cond_guard= added_key_field->cond_guard;
keyuse_ext.sj_pred_no= added_key_field->sj_pred_no;
keyuse_ext.validity_ref= 0;
keyuse_ext.needed_in_prefix= added_key_field->val->used_tables() &
~(OUTER_REF_TABLE_BIT | RAND_TABLE_BIT);
keyuse_ext.validity_var= false;
return ext_keyuses->push(keyuse_ext);
}
static int
sort_ext_keyuse(KEYUSE_EXT *a, KEYUSE_EXT *b)
{
if (a->table->tablenr != b->table->tablenr)
return (int) (a->table->tablenr - b->table->tablenr);
if (a->key != b->key)
return (int) (a->key - b->key);
return (int) (a->keypart - b->keypart);
}
static void
sort_ext_keyuses(Dynamic_array<KEYUSE_EXT> *keyuses)
{
KEYUSE_EXT *first_keyuse= &keyuses->at(0);
my_qsort(first_keyuse, keyuses->elements(), sizeof(KEYUSE_EXT),
(qsort_cmp) sort_ext_keyuse);
}
/**
@brief
Add info on keyuses usable for splitting into an array
*/
static bool
add_ext_keyuses_for_splitting_field(Dynamic_array<KEYUSE_EXT> *ext_keyuses,
KEY_FIELD *added_key_field)
{
Field *field= added_key_field->field;
TABLE *table= field->table;
for (uint key= 0; key < table->s->keys; key++)
{
if (!(table->keys_usable_for_splitting.is_set(key)))
continue;
KEY *key_info= table->key_info + key;
uint key_parts= table->actual_n_key_parts(key_info);
KEY_PART_INFO *key_part_info= key_info->key_part;
for (uint part=0; part < key_parts; part++, key_part_info++)
{
if (!field->eq(key_part_info->field))
continue;
if (add_ext_keyuse_for_splitting(ext_keyuses, added_key_field, key, part))
return true;
}
}
return false;
}
/*
@brief
Cost of the post join operation used in specification of splittable table
*/
static
double spl_postjoin_oper_cost(THD *thd, double join_record_count, uint rec_len)
{
double cost;
cost= get_tmp_table_write_cost(thd, join_record_count,rec_len) *
join_record_count; // cost to fill tmp table
cost+= get_tmp_table_lookup_cost(thd, join_record_count,rec_len) *
join_record_count; // cost to perform post join operation used here
cost+= get_tmp_table_lookup_cost(thd, join_record_count, rec_len) +
(join_record_count == 0 ? 0 :
join_record_count * log2 (join_record_count)) *
SORT_INDEX_CMP_COST; // cost to perform sorting
return cost;
}
/**
@brief
Add KEYUSE structures that can be usable for splitting
@details
This function is called only for joins created for potentially
splittable materialized tables. The function does the following:
1. Creates the dynamic array ext_keyuses_for_splitting of KEYUSE_EXT
structures and fills is with info about all keyuses that
could be used for splitting.
2. Sort the array ext_keyuses_for_splitting for fast access by key
on certain columns.
3. Collects and stores cost and cardinality info on the best execution
plan that does not use splitting and save this plan together with
corresponding array of keyuses.
4. Expand this array with KEYUSE elements built from the info stored
in ext_keyuses_for_splitting that could be produced by pushed
equalities employed for splitting.
5. Prepare the extended array of keyuses to be used in the function
best_access_plan()
*/
void JOIN::add_keyuses_for_splitting()
{
uint i;
uint idx;
KEYUSE_EXT *keyuse_ext;
KEYUSE_EXT keyuse_ext_end;
double oper_cost;
uint rec_len;
uint added_keyuse_count;
TABLE *table= select_lex->master_unit()->derived->table;
List_iterator_fast<KEY_FIELD> li(spl_opt_info->added_key_fields);
KEY_FIELD *added_key_field;
if (!spl_opt_info->added_key_fields.elements)
goto err;
if (!(ext_keyuses_for_splitting= new Dynamic_array<KEYUSE_EXT>))
goto err;
while ((added_key_field= li++))
{
(void) add_ext_keyuses_for_splitting_field(ext_keyuses_for_splitting,
added_key_field);
}
added_keyuse_count= (uint)ext_keyuses_for_splitting->elements();
if (!added_keyuse_count)
goto err;
sort_ext_keyuses(ext_keyuses_for_splitting);
bzero((char*) &keyuse_ext_end, sizeof(keyuse_ext_end));
if (ext_keyuses_for_splitting->push(keyuse_ext_end))
goto err;
// psergey-todo: trace anything here?
spl_opt_info->unsplit_card= join_record_count;
rec_len= table->s->rec_buff_length;
oper_cost= spl_postjoin_oper_cost(thd, join_record_count, rec_len);
spl_opt_info->unsplit_cost= best_positions[table_count-1].read_time +
oper_cost;
if (!(save_qep= new Join_plan_state(table_count + 1)))
goto err;
save_query_plan(save_qep);
if (!keyuse.buffer &&
my_init_dynamic_array(&keyuse, sizeof(KEYUSE), 20, 64,
MYF(MY_THREAD_SPECIFIC)))
goto err;
if (allocate_dynamic(&keyuse,
save_qep->keyuse.elements +
added_keyuse_count))
goto err;
idx= keyuse.elements= save_qep->keyuse.elements;
if (keyuse.elements)
memcpy(keyuse.buffer,
save_qep->keyuse.buffer,
(size_t) keyuse.elements * keyuse.size_of_element);
keyuse_ext= &ext_keyuses_for_splitting->at(0);
for (i=0; i < added_keyuse_count; i++, keyuse_ext++, idx++)
{
set_dynamic(&keyuse, (KEYUSE *) keyuse_ext, idx);
KEYUSE *added_keyuse= ((KEYUSE *) (keyuse.buffer)) + idx;
added_keyuse->validity_ref= &keyuse_ext->validity_var;
}
if (sort_and_filter_keyuse(thd, &keyuse, true))
goto err;
optimize_keyuse(this, &keyuse);
for (uint i= 0; i < table_count; i++)
{
JOIN_TAB *tab= join_tab + i;
map2table[tab->table->tablenr]= tab;
}
return;
err:
if (save_qep)
restore_query_plan(save_qep);
table->deny_splitting();
return;
}
/**
@brief
Add KEYUSE structures that can be usable for splitting of this joined table
*/
void JOIN_TAB::add_keyuses_for_splitting()
{
DBUG_ASSERT(table->spl_opt_info != NULL);
SplM_opt_info *spl_opt_info= table->spl_opt_info;
spl_opt_info->join->add_keyuses_for_splitting();
}
/*
@brief
Find info on the splitting plan by the splitting key
*/
SplM_plan_info *SplM_opt_info::find_plan(TABLE *table, uint key, uint parts)
{
List_iterator_fast<SplM_plan_info> li(plan_cache);
SplM_plan_info *spl_plan;
while ((spl_plan= li++))
{
if (spl_plan->table == table &&
spl_plan->key == key &&
spl_plan->parts == parts)
break;
}
return spl_plan;
}
/*
@breaf
Enable/Disable a keyuses that can be used for splitting
*/
static
void reset_validity_vars_for_keyuses(KEYUSE_EXT *key_keyuse_ext_start,
TABLE *table, uint key,
table_map excluded_tables,
bool validity_val)
{
KEYUSE_EXT *keyuse_ext= key_keyuse_ext_start;
do
{
if (!(keyuse_ext->needed_in_prefix & excluded_tables))
{
/*
The enabling/disabling flags are set just in KEYUSE_EXT structures.
Yet keyuses that are used by best_access_path() have pointers
to these flags.
*/
keyuse_ext->validity_var= validity_val;
}
keyuse_ext++;
}
while (keyuse_ext->key == key && keyuse_ext->table == table);
}
/**
@brief
Choose the best splitting to extend the evaluated partial join
@param
idx index for joined table T in current partial join P
remaining_tables tables not joined yet
spl_pd_boundary OUT bitmap of the table from P extended by T that
starts the sub-sequence of tables S from which
no conditions are allowed to be pushed into T.
@details
This function is called during the search for the best execution
plan of the join that contains this table T. The function is called
every time when the optimizer tries to extend a partial join by
joining it with table T. Depending on what tables are already in the
partial join different equalities usable for splitting can be pushed
into T. The function evaluates different variants and chooses the
best one. Then the function finds the plan for the materializing join
with the chosen equality conditions pushed into it. If the cost of the
plan turns out to be less than the cost of the best plan without
splitting the function set it as the true plan of materialization
of the table T.
The function caches the found plans for materialization of table T
together with the info what key was used for splitting. Next time when
the optimizer prefers to use the same key the plan is taken from
the cache of plans
@retval
Pointer to the info on the found plan that employs the pushed equalities
if the plan has been chosen, NULL - otherwise.
If the function returns NULL the value of spl_param_tables is set to 0.
*/
SplM_plan_info * JOIN_TAB::choose_best_splitting(uint idx,
table_map remaining_tables,
const POSITION *join_positions,
table_map *spl_pd_boundary)
{
SplM_opt_info *spl_opt_info= table->spl_opt_info;
DBUG_ASSERT(spl_opt_info != NULL);
JOIN *join= spl_opt_info->join;
THD *thd= join->thd;
table_map tables_usable_for_splitting=
spl_opt_info->tables_usable_for_splitting;
KEYUSE_EXT *keyuse_ext= &join->ext_keyuses_for_splitting->at(0);
KEYUSE_EXT *UNINIT_VAR(best_key_keyuse_ext_start);
TABLE *best_table= 0;
double best_rec_per_key= DBL_MAX;
SplM_plan_info *spl_plan= 0;
uint best_key= 0;
uint best_key_parts= 0;
table_map best_param_tables= 0L;
Json_writer_object trace_obj(thd, "choose_best_splitting");
Json_writer_array trace_arr(thd, "considered_keys");
/*
Check whether there are keys that can be used to join T employing splitting
and if so, select the best out of such keys
*/
for (uint tablenr= 0; tablenr < join->table_count; tablenr++)
{
if (!((1ULL << tablenr) & tables_usable_for_splitting))
continue;
JOIN_TAB *tab= join->map2table[tablenr];
TABLE *table= tab->table;
if (keyuse_ext->table != table)
continue;
do
{
uint key= keyuse_ext->key;
KEYUSE_EXT *key_keyuse_ext_start= keyuse_ext;
key_part_map found_parts= 0;
table_map needed_in_prefix= 0;
do
{
if (keyuse_ext->needed_in_prefix &
(remaining_tables | this->join->sjm_lookup_tables))
{
keyuse_ext++;
continue;
}
if (!(keyuse_ext->keypart_map & found_parts))
{
if ((!found_parts && !keyuse_ext->keypart) ||
(found_parts && ((keyuse_ext->keypart_map >> 1) & found_parts)))
found_parts|= keyuse_ext->keypart_map;
else
{
do
{
keyuse_ext++;
}
while (keyuse_ext->key == key && keyuse_ext->table == table);
break;
}
}
KEY *key_info= table->key_info + key;
double rec_per_key=
key_info->actual_rec_per_key(keyuse_ext->keypart);
needed_in_prefix|= keyuse_ext->needed_in_prefix;
if (rec_per_key < best_rec_per_key)
{
best_table= keyuse_ext->table;
best_key= keyuse_ext->key;
best_key_parts= keyuse_ext->keypart + 1;
best_rec_per_key= rec_per_key;
best_key_keyuse_ext_start= key_keyuse_ext_start;
best_param_tables= needed_in_prefix;
// trace table, key_name, parts, needed_tables.
Json_writer_object cur_index(thd);
cur_index.
add("table_name", best_table->alias.ptr()).
add("index", best_table->key_info[best_key].name).
add("rec_per_key", best_rec_per_key).
add("param_tables", best_param_tables);
}
keyuse_ext++;
}
while (keyuse_ext->key == key && keyuse_ext->table == table);
}
while (keyuse_ext->table == table);
}
trace_arr.end();
spl_opt_info->last_plan= 0;
double refills= DBL_MAX;
table_map excluded_tables= remaining_tables | this->join->sjm_lookup_tables;
if (best_table)
{
*spl_pd_boundary= this->table->map;
if (!best_param_tables)
refills= 1;
else
{
table_map last_found= this->table->map;
for (const POSITION *pos= &join_positions[idx - 1]; ; pos--)
{
if (pos->table->table->map & excluded_tables)
continue;
if (pos->partial_join_cardinality < refills)
{
*spl_pd_boundary= last_found;
refills= pos->partial_join_cardinality;
}
last_found= pos->table->table->map;
if ((last_found & best_param_tables) || pos->use_join_buffer)
break;
}
}
trace_obj.add("refills", refills).
add("spl_pd_boundary", *spl_pd_boundary);
/*
The key for splitting was chosen, look for the plan for this key
in the cache
*/
spl_plan= spl_opt_info->find_plan(best_table, best_key, best_key_parts);
if (!spl_plan)
{
/*
The plan for the chosen key has not been found in the cache.
Build a new plan and save info on it in the cache
*/
Json_writer_array wrapper(thd, "split_plan_search");
table_map all_table_map= (((table_map) 1) << join->table_count) - 1;
reset_validity_vars_for_keyuses(best_key_keyuse_ext_start, best_table,
best_key, excluded_tables, true);
choose_plan(join, all_table_map & ~join->const_table_map);
wrapper.end();
/*
Check that the chosen plan is really a splitting plan.
If not or if there is not enough memory to save the plan in the cache
then just return with no splitting plan.
*/
POSITION *first_non_const_pos= join->best_positions + join->const_tables;
TABLE *table= first_non_const_pos->table->table;
key_map spl_keys= table->keys_usable_for_splitting;
if (!(first_non_const_pos->key &&
spl_keys.is_set(first_non_const_pos->key->key)) ||
!(spl_plan= (SplM_plan_info *) thd->alloc(sizeof(SplM_plan_info))) ||
!(spl_plan->best_positions=
(POSITION *) thd->alloc(sizeof(POSITION) * join->table_count)) ||
spl_opt_info->plan_cache.push_back(spl_plan))
{
reset_validity_vars_for_keyuses(best_key_keyuse_ext_start, best_table,
best_key, excluded_tables, false);
trace_obj.add("split_plan_discarded", "constructed unapplicable query plan");
return 0;
}
spl_plan->keyuse_ext_start= best_key_keyuse_ext_start;
spl_plan->table= best_table;
spl_plan->key= best_key;
spl_plan->parts= best_key_parts;
spl_plan->split_sel= best_rec_per_key /
(spl_opt_info->unsplit_card ?
spl_opt_info->unsplit_card : 1);
uint rec_len= table->s->rec_buff_length;
double split_card= spl_opt_info->unsplit_card * spl_plan->split_sel;
double oper_cost= split_card *
spl_postjoin_oper_cost(thd, split_card, rec_len);
spl_plan->cost= join->best_positions[join->table_count-1].read_time +
+ oper_cost;
memcpy((char *) spl_plan->best_positions,
(char *) join->best_positions,
sizeof(POSITION) * join->table_count);
reset_validity_vars_for_keyuses(best_key_keyuse_ext_start, best_table,
best_key, excluded_tables, false);
}
else
trace_obj.add("cached_plan_found", 1);
if (spl_plan)
{
trace_obj.
add("lead_table", spl_plan->table->alias.ptr()).
add("index", spl_plan->table->key_info[spl_plan->key].name).
add("parts", spl_plan->parts).
add("split_sel", spl_plan->split_sel).
add("cost", spl_plan->cost).
add("records", (ha_rows) (records * spl_plan->split_sel)).
add("refills", refills);
if (refills * spl_plan->cost < spl_opt_info->unsplit_cost)
{
/*
The best plan that employs splitting is cheaper than
the plan without splitting
*/
spl_opt_info->last_plan= spl_plan;
spl_opt_info->last_refills= refills;
trace_obj.add("chosen", true);
}
else
trace_obj.add("chosen", false);
}
}
/* Set the cost of the preferred materialization for this partial join */
records= (ha_rows)spl_opt_info->unsplit_card;
spl_plan= spl_opt_info->last_plan;
if (spl_plan)
{
startup_cost= spl_opt_info->last_refills * spl_plan->cost;
records= (ha_rows) (records * spl_plan->split_sel);
}
else
{
startup_cost= spl_opt_info->unsplit_cost;
*spl_pd_boundary= 0;
}
return spl_plan;
}
/**
@brief
Inject equalities for splitting used by the materialization join
@param
excluded_tables used to filter out the equalities that are not
to be pushed.
@details
This function injects equalities pushed into a derived table T for which
the split optimization has been chosen by the optimizer. The function
is called by JOIN::inject_splitting_cond_for_all_tables_with_split_opt().
All equalities usable for splitting T whose right parts do not depend on
any of the 'excluded_tables' can be pushed into the where clause of the
derived table T.
The function also marks the select that specifies T as
UNCACHEABLE_DEPENDENT_INJECTED.
@retval
false on success
true on failure
*/
bool JOIN::inject_best_splitting_cond(table_map excluded_tables)
{
Item *inj_cond= 0;
List<Item> *inj_cond_list= &spl_opt_info->inj_cond_list;
List_iterator<KEY_FIELD> li(spl_opt_info->added_key_fields);
KEY_FIELD *added_key_field;
while ((added_key_field= li++))
{
if (excluded_tables & added_key_field->val->used_tables())
continue;
if (inj_cond_list->push_back(added_key_field->cond, thd->mem_root))
return true;
}
DBUG_ASSERT(inj_cond_list->elements);
switch (inj_cond_list->elements) {
case 1:
inj_cond= inj_cond_list->head(); break;
default:
inj_cond= new (thd->mem_root) Item_cond_and(thd, *inj_cond_list);
if (!inj_cond)
return true;
}
if (inj_cond)
inj_cond->fix_fields(thd,0);
if (inject_cond_into_where(inj_cond->copy_andor_structure(thd)))
return true;
select_lex->uncacheable|= UNCACHEABLE_DEPENDENT_INJECTED;
st_select_lex_unit *unit= select_lex->master_unit();
unit->uncacheable|= UNCACHEABLE_DEPENDENT_INJECTED;
return false;
}
/**
@brief
Test if equality is injected for split optimization
@param
eq_item equality to to test
@retval
true eq_item is equality injected for split optimization
false otherwise
*/
bool is_eq_cond_injected_for_split_opt(Item_func_eq *eq_item)
{
Item *left_item= eq_item->arguments()[0]->real_item();
if (left_item->type() != Item::FIELD_ITEM)
return false;
Field *field= ((Item_field *) left_item)->field;
if (!field->table->reginfo.join_tab)
return false;
JOIN *join= field->table->reginfo.join_tab->join;
if (!join->spl_opt_info)
return false;
List_iterator_fast<Item> li(join->spl_opt_info->inj_cond_list);
Item *item;
while ((item= li++))
{
if (item == eq_item)
return true;
}
return false;
}
/**
@brief
Fix the splitting chosen for a splittable table in the final query plan
@param
spl_plan info on the splitting plan chosen for the splittable table T
excluded_tables tables that cannot be used in equalities pushed into T
is_const_table the table T is a constant table
@details
If in the final query plan the optimizer has chosen a splitting plan
then the function sets this plan as the final execution plan to
materialized the table T. Otherwise the plan that does not use
splitting is set for the materialization.
@retval
false on success
true on failure
*/
bool JOIN_TAB::fix_splitting(SplM_plan_info *spl_plan,
table_map excluded_tables,
bool is_const_table)
{
SplM_opt_info *spl_opt_info= table->spl_opt_info;
DBUG_ASSERT(table->spl_opt_info != 0);
JOIN *md_join= spl_opt_info->join;
if (spl_plan && !is_const_table)
{
is_split_derived= true;
memcpy((char *) md_join->best_positions,
(char *) spl_plan->best_positions,
sizeof(POSITION) * md_join->table_count);
/*
This is called for a proper work of JOIN::get_best_combination()
called for the join that materializes T
*/
reset_validity_vars_for_keyuses(spl_plan->keyuse_ext_start,
spl_plan->table,
spl_plan->key,
excluded_tables,
true);
}
else if (md_join->save_qep)
{
md_join->restore_query_plan(md_join->save_qep);
}
return false;
}
/**
@brief
Fix the splittings chosen splittable tables in the final query plan
@details
The function calls JOIN_TAB::fix_splittins for all potentially
splittable tables in this join to set all final materialization
plans chosen for these tables.
@retval
false on success
true on failure
*/
bool JOIN::fix_all_splittings_in_plan()
{
table_map prev_tables= 0;
table_map all_tables= (table_map(1) << table_count) - 1;
for (uint tablenr= 0; tablenr < table_count; tablenr++)
{
POSITION *cur_pos= &best_positions[tablenr];
JOIN_TAB *tab= cur_pos->table;
if (tab->table->is_splittable())
{
SplM_plan_info *spl_plan= cur_pos->spl_plan;
table_map excluded_tables= (all_tables & ~prev_tables) |
sjm_lookup_tables;
;
if (spl_plan)
{
POSITION *pos= cur_pos;
table_map spl_pd_boundary= pos->spl_pd_boundary;
do
{
excluded_tables|= pos->table->table->map;
}
while (!((pos--)->table->table->map & spl_pd_boundary));
}
if (tab->fix_splitting(spl_plan,
excluded_tables,
tablenr < const_tables ))
return true;
}
prev_tables|= tab->table->map;
}
return false;
}
/**
@brief
Inject splitting conditions into WHERE of split derived
@details
The function calls JOIN_TAB::inject_best_splitting_cond() for each
materialized derived table T used in this join for which the split
optimization has been chosen by the optimizer. It is done in order to
inject equalities pushed into the where clause of the specification
of T that would be helpful to employ the splitting technique.
@retval
false on success
true on failure
*/
bool JOIN::inject_splitting_cond_for_all_tables_with_split_opt()
{
table_map prev_tables= 0;
table_map all_tables= (table_map(1) << table_count) - 1;
for (uint tablenr= 0; tablenr < table_count; tablenr++)
{
POSITION *cur_pos= &best_positions[tablenr];
JOIN_TAB *tab= cur_pos->table;
prev_tables|= tab->table->map;
if (!(tab->table->is_splittable() && cur_pos->spl_plan))
continue;
SplM_opt_info *spl_opt_info= tab->table->spl_opt_info;
JOIN *join= spl_opt_info->join;
table_map excluded_tables= (all_tables & ~prev_tables) | sjm_lookup_tables;
table_map spl_pd_boundary= cur_pos->spl_pd_boundary;
for (POSITION *pos= cur_pos; ; pos--)
{
excluded_tables|= pos->table->table->map;
pos->table->no_forced_join_cache= true;
if (pos->table->table->map & spl_pd_boundary)
{
pos->table->split_derived_to_update|= tab->table->map;
break;
}
}
if (join->inject_best_splitting_cond(excluded_tables))
return true;
}
return false;
}
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