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Fix bug lp:833777

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Analysis:
The optimizer distinguishes two kinds of 'constant' conditions:
expensive ones, and non-expensive ones. The non-expensive conditions
are evaluated inside make_join_select(), and if false, already the
optimizer detects empty query results.

In order to avoid arbitrarily expensive optimization, the evaluation of
expensive constant conditions is delayed until execution. These conditions
are attached to JOIN::exec_const_cond and evaluated in the beginning of
JOIN::exec. The relevant execution logic is:

JOIN::exec()
{
  if (! join->exec_const_cond->val_int())
  {
    produce an empty result;
    stop execution
  }
  continue execution
  execute the original WHERE clause (that contains exec_const_cond)
 ...
}

As a result, when an expensive constant condition is
TRUE, it is evaluated twice - once through
JOIN::exec_const_cond, and once through JOIN::cond.
When the expensive constant condition is a subquery,
predicate, the subquery is evaluated twice. If we have
many levels of subqueries, this logic results in a chain
of recursive subquery executions that walk a perfect
binary tree. The result is that for subquries with depth N,
JOIN::exec is executed O(2^N) times.

Solution:
Notice that the second execution of the constant conditions
happens inside do_select(), in the branch:
if (join->table_count == join->const_tables) { ... }
In this case exec_const_cond is equivalent to the whole WHERE
clause, therefore the WHERE clause has already been checked in
the beginnig of JOIN::exec, and has been found to be true.
The bug is addressed by not evaluating the WHERE clause if there
was exec_const_conds, and it was TRUE.
This commit is contained in:
unknown
2011-11-21 18:00:55 +02:00
parent 0693f4d916
commit f8dbbc010f
8 changed files with 68 additions and 3 deletions
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1
mysql-test/r/func_compress.result
View File

@ -103,7 +103,6 @@ foo
Warnings:
Error 1259 ZLIB: Input data corrupted
Error 1259 ZLIB: Input data corrupted
Error 1259 ZLIB: Input data corrupted
explain select *, uncompress(a) from t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 system NULL NULL NULL NULL 1

10
mysql-test/r/subselect.result
View File

@ -5665,4 +5665,14 @@ ERROR 21000: Subquery returns more than 1 row
SET SESSION sql_mode=@old_sql_mode;
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
#
# LP BUG#833777 Performance regression with deeply nested subqueries
#
create table t1 (a int not null, b char(10) not null);
insert into t1 values (1, 'a');
set @@optimizer_switch='in_to_exists=on,semijoin=off,materialization=off,subquery_cache=off';
select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1))))))))))))))))))))))))))))))));
a
1
drop table t1;
set optimizer_switch=@subselect_tmp;

10
mysql-test/r/subselect_no_mat.result
View File

@ -5670,6 +5670,16 @@ ERROR 21000: Subquery returns more than 1 row
SET SESSION sql_mode=@old_sql_mode;
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
#
# LP BUG#833777 Performance regression with deeply nested subqueries
#
create table t1 (a int not null, b char(10) not null);
insert into t1 values (1, 'a');
set @@optimizer_switch='in_to_exists=on,semijoin=off,materialization=off,subquery_cache=off';
select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1))))))))))))))))))))))))))))))));
a
1
drop table t1;
set optimizer_switch=@subselect_tmp;
set optimizer_switch=default;
select @@optimizer_switch like '%materialization=on%';

10
mysql-test/r/subselect_no_opts.result
View File

@ -5666,5 +5666,15 @@ ERROR 21000: Subquery returns more than 1 row
SET SESSION sql_mode=@old_sql_mode;
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
#
# LP BUG#833777 Performance regression with deeply nested subqueries
#
create table t1 (a int not null, b char(10) not null);
insert into t1 values (1, 'a');
set @@optimizer_switch='in_to_exists=on,semijoin=off,materialization=off,subquery_cache=off';
select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1))))))))))))))))))))))))))))))));
a
1
drop table t1;
set optimizer_switch=@subselect_tmp;
set @optimizer_switch_for_subselect_test=null;

10
mysql-test/r/subselect_no_scache.result
View File

@ -5669,6 +5669,16 @@ ERROR 21000: Subquery returns more than 1 row
SET SESSION sql_mode=@old_sql_mode;
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
#
# LP BUG#833777 Performance regression with deeply nested subqueries
#
create table t1 (a int not null, b char(10) not null);
insert into t1 values (1, 'a');
set @@optimizer_switch='in_to_exists=on,semijoin=off,materialization=off,subquery_cache=off';
select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1))))))))))))))))))))))))))))))));
a
1
drop table t1;
set optimizer_switch=@subselect_tmp;
set optimizer_switch=default;
select @@optimizer_switch like '%subquery_cache=on%';

10
mysql-test/r/subselect_no_semijoin.result
View File

@ -5666,5 +5666,15 @@ ERROR 21000: Subquery returns more than 1 row
SET SESSION sql_mode=@old_sql_mode;
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
#
# LP BUG#833777 Performance regression with deeply nested subqueries
#
create table t1 (a int not null, b char(10) not null);
insert into t1 values (1, 'a');
set @@optimizer_switch='in_to_exists=on,semijoin=off,materialization=off,subquery_cache=off';
select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1))))))))))))))))))))))))))))))));
a
1
drop table t1;
set optimizer_switch=@subselect_tmp;
set @optimizer_switch_for_subselect_test=null;

10
mysql-test/t/subselect.test
View File

@ -4768,4 +4768,14 @@ SET SESSION sql_mode=@old_sql_mode;
DEALLOCATE PREPARE stmt;
DROP TABLE t1;
--echo #
--echo # LP BUG#833777 Performance regression with deeply nested subqueries
--echo #
create table t1 (a int not null, b char(10) not null);
insert into t1 values (1, 'a');
set @@optimizer_switch='in_to_exists=on,semijoin=off,materialization=off,subquery_cache=off';
select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1 where a in (select a from t1))))))))))))))))))))))))))))))));
drop table t1;
set optimizer_switch=@subselect_tmp;

10
sql/sql_select.cc
View File

@ -14782,9 +14782,15 @@ do_select(JOIN *join,List<Item> *fields,TABLE *table,Procedure *procedure)
{
/*
HAVING will be checked after processing aggregate functions,
But WHERE should checkd here (we alredy have read tables)
But WHERE should checkd here (we alredy have read tables).
If there is join->exec_const_cond, and all tables are constant, then it
is equivalent to join->conds. exec_const_cond is already checked in the
beginning of JOIN::exec. If it is false, JOIN::exec returns zero
result already there, therefore execution reaches this point only if
exec_const_cond is TRUE. Since it is equvalent to join->conds, then
join->conds is also TRUE.
*/
if (!join->conds || join->conds->val_int())
if (!join->conds || join->exec_const_cond || join->conds->val_int())
{
error= (*end_select)(join, 0, 0);
if (error == NESTED_LOOP_OK || error == NESTED_LOOP_QUERY_LIMIT)