The benchmark should be run with the following tree:
lp:~maria-captains/maria/10.0-mdev83

This tree is based on MWL#253, which adds predicate selectivity estimates. The tree of
10.0-mdev83 implements static (done during optimization) pushdown of subquery predicates.
In order to test the new features, the following optimizer switches must be enabled:

set @@optimizer_use_condition_selectivity=3;
set @@optimizer_switch='expensive_pred_static_pushdown=on';

In addition, in order to get proper selectivity estimates that let the optimizer choose the right plan
for Q20, specifically Q20 should be run with the following indexes:
create index i_p_name on part(p_name);
create index i_n_name on nation(n_name);
Notice that these indexes do not affect execution performance, because they are not used
during execution. Once there are histograms that would allow proper selectivity estimates,
these indexes will not be needed. Make sure that these indexes exist only when Q20 is
being run.

To run the latest implementation of mdev-83 that includes histograms from MWL#253,
against already created database with previous version of 10.0:
use mysql;
drop table column_stats;
CREATE TABLE IF NOT EXISTS column_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, column_name varchar(64) NOT NULL, min_value varchar(255) DEFAULT NULL, max_value varchar(255) DEFAULT NULL, nulls_ratio decimal(12,4) DEFAULT NULL, avg_length decimal(12,4) DEFAULT NULL, avg_frequency decimal(12,4) DEFAULT NULL, hist_size tinyint unsigned, histogram varbinary(255), PRIMARY KEY (db_name,table_name,column_name) ) ENGINE=MyISAM CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Columns';

To collect table independent statistics with ANALYZE TABLE set:
set @@use_stat_tables='preferably';

In addition to other optimizer setttings, make sure that you set:
set @@optimizer_use_condition_selectivity=4;
set @@histogram_size=24;
set @@optimizer_switch='expensive_pred_static_pushdown=on';

Specifically Q20 benefits a lot from:
set @@optimizer_switch='mrr=on,mrr_cost_based=off,mrr_sort_keys=on';

To check how to re-create the statistics tables check the latest version of the file 'scripts/mysql_system_tables.sql'.
Currently the SQL DDL for the stats tables is:

use mysql;
drop table table_stats, column_stats, index_stats;

CREATE TABLE IF NOT EXISTS table_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, cardinality bigint(21) unsigned DEFAULT NULL, PRIMARY KEY (db_name,table_name) ) ENGINE=MyISAM CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Tables';

CREATE TABLE IF NOT EXISTS column_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, column_name varchar(64) NOT NULL, min_value varchar(255) DEFAULT NULL, max_value varchar(255) DEFAULT NULL, nulls_ratio decimal(12,4) DEFAULT NULL, avg_length decimal(12,4) DEFAULT NULL, avg_frequency decimal(12,4) DEFAULT NULL, hist_size tinyint unsigned, hist_type enum('SINGLE_PREC_HB','DOUBLE_PREC_HB'), histogram varbinary(255), PRIMARY KEY (db_name,table_name,column_name) ) ENGINE=MyISAM CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Columns';

CREATE TABLE IF NOT EXISTS index_stats (db_name varchar(64) NOT NULL, table_name varchar(64) NOT NULL, index_name varchar(64) NOT NULL, prefix_arity int(11) unsigned NOT NULL, avg_frequency decimal(12,4) DEFAULT NULL, PRIMARY KEY (db_name,table_name,index_name,prefix_arity) ) ENGINE=MyISAM CHARACTER SET utf8 COLLATE utf8_bin comment='Statistics on Indexes';

Then run ANALYZE ... .