Details
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New Feature
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Status: Stalled (View Workflow)
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Major
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Resolution: Unresolved
Description
A long standing (and informally known) issue:
Join optimizer makes its choices [almost] without regard for ORDER BY ... LIMIT clause. ORDER BY ... LIMIT optimizer is invoked when the join order is already fixed. If the picked join order doesn't allow to resolve ORDER BY ... LIMIT efficiently... then we end up with a very poor query plan.
Example:
select * from
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t_fact
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join dim1 on t_fact.dim1_id= dim1.dim1_id
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join dim2 on t_fact.dim2_id= dim2.dim2_id
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order by
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t_fact.col1
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limit 1000;
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+------+-------------+--------+--------+-----------------+---------+---------+-------------------+------+---------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+--------+--------+-----------------+---------+---------+-------------------+------+---------------------------------+
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| 1 | SIMPLE | dim1 | ALL | PRIMARY | NULL | NULL | NULL | 500 | Using temporary; Using filesort |
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| 1 | SIMPLE | t_fact | ref | dim1_id,dim2_id | dim1_id | 4 | j3.dim1.dim1_id | 1 | |
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| 1 | SIMPLE | dim2 | eq_ref | PRIMARY | PRIMARY | 4 | j3.t_fact.dim2_id | 1 | |
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+------+-------------+--------+--------+-----------------+---------+---------+-------------------+------+---------------------------------+
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This uses filesort and takes ~8 sec.
Now, let's force the right join order:
select * from
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t_fact
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straight_join dim1 on t_fact.dim1_id= dim1.dim1_id
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straight_join dim2 on t_fact.dim2_id= dim2.dim2_id
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order by
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t_fact.col1
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limit 1000;
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+------+-------------+--------+--------+-----------------+---------+---------+-------------------+------+-------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+--------+--------+-----------------+---------+---------+-------------------+------+-------+
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| 1 | SIMPLE | t_fact | index | dim1_id,dim2_id | col1 | 4 | NULL | 1000 | |
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| 1 | SIMPLE | dim1 | eq_ref | PRIMARY | PRIMARY | 4 | j3.t_fact.dim1_id | 1 | |
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| 1 | SIMPLE | dim2 | eq_ref | PRIMARY | PRIMARY | 4 | j3.t_fact.dim2_id | 1 | |
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+------+-------------+--------+--------+-----------------+---------+---------+-------------------+------+-------+
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This uses index to resolve the ORDER BY ... LIMIT and the select takes 0.01 sec to execute.
Dataset:
create table ten(a int);
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insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
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create table one_k(a int);
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insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;
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create table t_fact
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(
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fact_id int not null,
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dim1_id int not null,
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dim2_id int not null,
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col1 int not null,
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primary key(fact_id),
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key(dim1_id),
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key(dim2_id),
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key(col1)
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);
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insert into t_fact
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select
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A.a+1000*B.a+1000*1000*C.a,
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A.a,
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B.a,
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A.a+1000*B.a+1000*1000*C.a
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from
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one_k A ,
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one_k B,
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ten C
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where
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A.a<500 and B.a<500
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;
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create table dim1
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(
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dim1_id int not null primary key,
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col1 int
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);
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insert into dim1
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select a,a from one_k where a<500;
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create table dim2
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(
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dim2_id int not null primary key,
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col1 int
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);
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insert into dim2
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select a,a from one_k where a<500;
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Attachments
Issue Links
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- Open
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MDEV-8002 Ability to use index for order-by and covering index changes with type of join to small table
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- Open
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MDEV-21643 test correctness of cost_based_order_by_limit
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Activity
The entire snippet is
else |
{
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/* |
'join' is either the best partial QEP with 'search_depth' relations,
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or the best complete QEP so far, whichever is smaller.
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*/
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if (!nest_created && |
((join->sort_by_table &&
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join->sort_by_table !=
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join->positions[join->const_tables].table->table) ||
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join->sort_nest_possible))
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{
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double cost; |
if (join->sort_nest_possible) |
{
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ulong rec_len= cache_record_length_for_nest(join, idx);
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cost= join->sort_nest_oper_cost(partial_join_cardinality, idx,
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rec_len);
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trace_one_table.add("cost_of_sorting", cost); |
}
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else |
{
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/* |
We may have to make a temp table, note that this is only a
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heuristic since we cannot know for sure at this point.
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Hence it may be wrong.
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*/
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cost= current_record_count;
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}
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current_read_time= COST_ADD(current_read_time, cost);
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}
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So when the ORDER BY LIMIT optimization is not enabled we enter the branch
else
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{
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/* |
We may have to make a temp table, note that this is only a
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heuristic since we cannot know for sure at this point.
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Hence it may be wrong.
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*/
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cost= current_record_count;
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}
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And then this cost of sorting is added to the cost of the current plan
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current_read_time= COST_ADD(current_read_time, cost);
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So we still provide the same old behaviour when the ORDER BY LIMIT optimization is not enabled.
The rebased code on top of 10.5 is in the branch 10.5-mdev8306-2
A related bug in MySQL to look at : https://bugs.mysql.com/bug.php?id=97001
The updated branch is on 10.6-order_by_limt
Also a push to buildbot has been made
http://buildbot.askmonty.org/buildbot/grid?category=main&branch=bb-10.6-varun
Before MDEV-8306, best_extension_by_limited_search had this code:
join->sort_by_table !=join->positions[join->const_tables].table->table)We may have to make a temp table, note that this is only aheuristic since we cannot know for sure at this point.Hence it may be wrong.*/current_read_time= COST_ADD(current_read_time, current_record_count);Now, I see only the comment is left:
We may have to make a temp table, note that this is only aheuristic since we cannot know for sure at this point.Hence it may be wrong.*/cost= current_record_count;Does this mean we can't provide the old behavior? (Yes I know it is ugly and incorrect) Still, being able to provide old behavior has value and we should discuss if we could do it, here.