MySQL Bugs: #35674: Range optimizer ignores conditions on inner tables in semi-join IN subqueries

Bug #35674	Range optimizer ignores conditions on inner tables in semi-join IN subqueries
Submitted:	29 Mar 2008 16:16	Modified:	30 May 2008 20:31
Reporter:	Sergey Petrunya
Status:	Closed
Category:	Server: Optimizer	Severity:	S3 (Non-critical)
Version:	6.0-bk	OS:	Any
Assigned to:	Bugs System	Target Version:
Triage:	D3 (Medium)

Description:
Range optimizer ignores conditions on inner tables in semi-join IN subqueries. This causes
the optimizer to miss good QEPs, WL#2980 had been no intent to have this property. In
fact, the reverse was assumed.

How to repeat:
# Create the test tables

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

create table t1(a int, b int, filler char(100), key(a));
insert into t1 
select
  A.a + 10*(B.a + 10* C.a),
  A.a + 10*(B.a + 10* C.a),
  'filler'
from
  ten A, ten B, ten C;

create table t2 like t1;
insert into t2 select * from t1;

# This produces a poor 1000 x 1000 execution plan:

explain select * from t1
where 
  t1.a < 200 and
  t1.b in (select t2.b from t2 where t2.a<10)\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
         type: ALL
possible_keys: a
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1000
        Extra: Using where; Start temporary
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: t2
         type: ALL
possible_keys: a
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1000
        Extra: Using where; End temporary; Using join buffer
2 rows in set (0.01 sec)

#
# The optimizer could use a join order of range(t2.a < 20), ALL(t1).
# 
# If we try a corresponding inner join, we see that it is indeed used:
#

explain select * from t1, t2 where t1.a < 200 and t1.b=t2.b and t2.a<20\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t2
         type: range
possible_keys: a
          key: a
      key_len: 5
          ref: NULL
         rows: 20
        Extra: Using index condition; Using MRR
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
         type: ALL
possible_keys: a
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1000
        Extra: Using where; Using join buffer
2 rows in set (0.00 sec)

Suggested fix:
Make the range optimizer make use of subquery's WHERE.

Thank you for the bug report. Verified as described.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/44954

ChangeSet@1.2623, 2008-04-06 01:24:29+04:00, sergefp@mysql.com +3 -0
  BUG#35674: Range optimizer ignores conditions on inner tables in semi-join IN subqueries
  - Let range optimizer use such conditions.

Pushed into 6.0.6-alpha

Noted in 6.0.6 changelog.

The range optimizer ignored conditions on inner tables in semi-join
IN subqueries, causing the the optimizer to miss good query execution
plans.