*SESSION SCHEDULE INFORMATION*

Session ID: CON9283
Session Title: New MySQL Full-Text Search Features and Solutions
Venue / Room: Hilton San Francisco – Golden Gate 8
Date and Time: 9/29/12, 14:30 – 15:30

Session ID: CON8811
Session Title: In-Depth Query Optimization for MySQL
Venue / Room: Hilton San Francisco – Golden Gate 8
Date and Time: 9/30/12, 14:45 – 15:45

Full Agenda

MySQL PAM authentication uses Linux pam_ldap library to send the calls. To configure MySQL LDAP authentication we will need to configure pam_ldap on linux.

OpenLDAP Linux configuration

• Make sure that libpam-ldap/openldap is installed. If not, on RedHat/CentOS use commands:

# yum install openldap openldap-clients

• Configure /etc/ldap.conf. Sample configuration:

debug 10 # set debug level only during the initial configuration
base dc=corp,dc=company_name,dc=com
binddn cn=service_account,OU=Service Accounts,OU=US Security,DC=corp,DC=company_name,DC=com
bindpw <password>
timelimit 120
idle_timelimit 3600
uri ldaps://<LDAP URL>:<LDAP PORT>

# The following lines are needed only for Active Directory

pam_login_attribute samaccountname
pam_member_attribute member
nss_map_objectclass posixAccount user
nss_map_objectclass shadowAccount user
nss_map_attribute uid sAMAccountName
nss_map_attribute homeDirectory unixHomeDirectory
nss_map_attribute shadowLastChange pwdLastSet
nss_map_objectclass posixGroup group
nss_map_attribute uniqueMember member
pam_login_attribute sAMAccountName
pam_filter objectclass=User
pam_password ad


Make sure you can connect to ldap

# telnet <ldap server> <ldap password> (should show “connected”)

Make sure you can search ldap with ldapsearch

# ldapsearch –w <password for bind user> -x –D 'cn=USER,OU=People …' “(&(ObjectClass=user)(cn=USERNAME))”

MySQL configuration

To setup mysql follow the instructions from http://dev.mysql.com/doc/refman/5.5/en/pam-authentication-plugin.html

• Make sure you use MySQL 5.5.16+ enterprise version
• Add line:

plugin-load=authentication_pam.so

into my.cnf and restart mysql server

• Put the following in /etc/pam.d/mysql:

#%PAM-1.0
auth        required    pam_ldap.so
account     required    pam_ldap.so


Individual User Authentication

• Create user for group substitution

CREATE USER 'user_name'@'localhost'
IDENTIFIED WITH authentication_pam
AS 'mysql';

Where mysql = name of the /etc/pam.d/mysql file above, user_name should match the LDAP’s uid (samaccountname for AD).

• Login to mysql with your LDAP username and password

mysql --user=user_name --password=verysecret mydb

Group Authentication

• Create user for group substitution

CREATE USER ''@''
IDENTIFIED WITH authentication_pam
AS 'mysql, ldap_group1=mysql_user1, ldap_group2=mysql_user2';


Where mysql = name of the /etc/pam.d/mysql file above, ldap_group1 is the ldap group, mysql_user1 is the user to substitute (we need to create this user)

• Create user for each LDAP group

CREATE USER 'mysql_user1'@'localhost' IDENTIFIED BY 'very secret password';
GRANT ALL PRIVILEGES ON mydevdb.* TO 'mysql_user1'@'localhost';
GRANT PROXY ON 'mysql_user1'@'localhost' TO ''@'';

• Login to mysql with your LDAP username and passwordand make sure the user was substituted correctly:

mysql --user=user_name --password=verysecret mydb

mysql>  SELECT USER(), CURRENT_USER(), @@proxy_user;
+-------------------+-----------------------+--------------+
| USER()            | CURRENT_USER()        | @@proxy_user |
+-------------------+-----------------------+--------------+
| user_name@localhost   | mysql_user1@localhost | ''@'%'       |
+-------------------+-----------------------+--------------+


Debugging

• Add “debug 16” (or other level, 0 to 256) into /etc/ldap.conf
• Add “export AUTHENTICATION_PAM_LOG=1” to /etc/init.d/mysql and restart mysql
• Then for troubleshoot look into mysql error log (i.e. /var/log/mysqld.log).

Current issues

Due to some PAM library’s limitations the group authentication may not work with Active Directory (individual authentication works fine), probably because the “posix_group” is not fully supported by Active Directory.

Links and more info

MySQL PAM Module documentation
Download MySQL Enterprise Trial from Oracle E-delivery
We plan a session on MySQL LDAP Auth at MySQL Connect (Sept 29-30, San Francisco)

    CREATE TABLE `ontime_2010` (
      `YearD` int(11) DEFAULT NULL,
      `MonthD` tinyint(4) DEFAULT NULL,
      `DayofMonth` tinyint(4) DEFAULT NULL,
      `DayOfWeek` tinyint(4) DEFAULT NULL,
      `Carrier` char(2) DEFAULT NULL,
      `Origin` char(5) DEFAULT NULL,
      `DepDelayMinutes` int(11) DEFAULT NULL,
      `AirlineID` int(11) DEFAULT NULL,
      `Cancelled` tinyint(4) DEFAULT NULL,
    ... more fields here ...
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1

(this is not the best possible fact table as the data is not aggregated by I’ll use it for now).

And we have those dimensions tables:

 CREATE TABLE `airlines` (
  `AirlineID` int(11) NOT NULL DEFAULT '0',
  `AirlineName` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`AirlineID`),
  KEY `AirlineName` (`AirlineName`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

CREATE TABLE `date_dayofweek` (
  `code` int(11) NOT NULL DEFAULT '0',
  `description` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`code`),
  KEY `description` (`description`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> select * from date_dayofweek order by code;
+------+-------------+
| code | description |
+------+-------------+
|    1 | Monday      |
|    2 | Tuesday     |
|    3 | Wednesday   |
|    4 | Thursday    |
|    5 | Friday      |
|    6 | Saturday    |
|    7 | Sunday      |
|    9 | Unknown     |
+------+-------------+
8 rows in set (0.00 sec)

So here is the example query (find sum of cancelled flights on Sundays for the given airline group by day):

select sum(Cancelled), FlightDate, AirlineName
from ontime_2010 o, date_dayofweek dow, airlines a
where o.dayofweek=dow.code and dow.description = 'Sunday'
and a.AirlineID = o.AirlineID and a.AirlineName = 'Delta Air Lines Inc.: DL'
group by FlightDate order by FlightDate desc limit 10\G

To fix the query we can add a covered index for ontime_2010, so that all fields for ontime_2010 table will be covered:

alter table ontime_2010 add key cov2(AirlineID, dayofweek, FlightDate, Cancelled);

However we will still have “temporary table and filesort”:

mysql> explain select sum(Cancelled), FlightDate
from ontime_2010 o, date_dayofweek dow, airlines a
where o.dayofweek=dow.code and dow.description = 'Sunday'
and a.AirlineID = o.AirlineID and a.AirlineName = 'Delta Air Lines Inc.: DL'
group by FlightDate order by FlightDate desc limit 10\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: dow
         type: ref
possible_keys: PRIMARY,description
          key: description
      key_len: 258
          ref: const
         rows: 1
        Extra: Using where; Using index; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: a
         type: ref
possible_keys: PRIMARY,AirlineName
          key: AirlineName
      key_len: 258
          ref: const
         rows: 1
        Extra: Using where; Using index
*************************** 3. row ***************************
           id: 1
  select_type: SIMPLE
        table: o
         type: ref
possible_keys: DayOfWeek,covered,AirlineID,cov2
          key: cov2
      key_len: 7
          ref: ontime.a.AirlineID,ontime.dow.code
         rows: 24417
        Extra: Using where; Using index
3 rows in set (0.00 sec)

mysql> explain select sum(Cancelled), FlightDate  from ontime_2010 o
where o.dayofweek= (select code from date_dayofweek where description = 'Sunday')
and AirlineID = (select AirlineID from airlines where AirlineName = 'Delta Air Lines Inc.: DL')
group by FlightDate limit 10\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: o
         type: ref
possible_keys: DayOfWeek,covered,AirlineID,cov2
          key: cov2
      key_len: 7
          ref: const,const
         rows: 152510
        Extra: Using where; Using index
*************************** 2. row ***************************
           id: 3
  select_type: SUBQUERY
        table: airlines
         type: ref
possible_keys: AirlineName
          key: AirlineName
      key_len: 258
          ref:
         rows: 1
        Extra: Using where; Using index
*************************** 3. row ***************************
           id: 2
  select_type: SUBQUERY
        table: date_dayofweek
         type: ref
possible_keys: description
          key: description
      key_len: 258
          ref:
         rows: 1
        Extra: Using where; Using index
3 rows in set (0.00 sec)

As MySQL will use indexes when we have "field = (select .. )" and now all fields in the index belong to the single table, MySQL will use index and avoid filesort. Please note: this will not work with "field in (select ...)" and also make sure that the subselect part will return only 1 row.

Here is our table:

CREATE TABLE `internalmail` (
  `mail_id` int(10) NOT NULL AUTO_INCREMENT,
  `senderaddress_id` int(10) NOT NULL,
  `recipientaddress_id` int(10) NOT NULL,
  `mail_timestamp` timestamp NULL DEFAULT NULL,
... message body, etc ...
  PRIMARY KEY (`mail_id`),
) ENGINE=InnoDB DEFAULT CHARSET=latin1

And our query:

select * from internalmail
 where (senderaddress_id = 247 or recipientaddress_id = 247 or recipientaddress_id = 0)
and mail_timestamp > '2010-08-01 12:30:47'
order by mail_timestamp desc 

In this query we show all messages from and to user_id = 247 plus all messages to system user (user_id=0). We need to show only messages for the last 3 months and show the most recent messages first.

To speed up the query we can try creating indexes:

KEY `recipientaddress_id` (`recipientaddress_id`),
KEY `senderaddress_id` (`senderaddress_id`),
KEY `mail_timestamp` (`mail_timestamp`),

However, as the query uses “OR”, MySQL will use a filesort.

mysql> explain select * from internalmail
where (senderaddress_id = 247 or recipientaddress_id = 247 or recipientaddress_id = 0)
and mail_timestamp > '2010-08-01 12:30:47'
order by mail_timestamp desc\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: internalmail
         type: ALL
possible_keys: recipientaddress_id,senderaddress_id,mail_timestamp
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 4843257
        Extra: Using where; Using filesort
1 row in set (0.00 sec)

UPDATE: even if we will create combined indexes on (recipientaddress_id,mail_timestamp) and/or (senderaddress_id,mail_timestamp) those indexes will not be used, as the query contains “OR” in the where clause.

And original query runs for 3 seconds. To fix this query we can do 2 things:

1. Rewrite query with UNION instead of OR
2. Create combined indexes

First, we rewrite query with UNION:

(select * from internalmail where senderaddress_id = 247 and mail_timestamp > ‘2010-08-19 12:30:47′)
union
(select * from internalmail where recipientaddress_id = 247 and mail_timestamp > ‘2010-08-19 12:30:47′)
union
(select * from internalmail where recipientaddress_id = 0 and mail_timestamp > ‘2010-08-19 12:30:47′)
order by mail_timestamp desc;

Second, we create 2 indexes:

mysql> alter table internalmail add key send_dt(senderaddress_id, mail_timestamp);
mysql> alter table internalmail add key recieve_dt(recipientaddress_id, mail_timestamp);

After that, MySQL will be able to fully utilize index for each of the 3 queries in union:

mysql> explain
(select * from internalmail where senderaddress_id = 247  and mail_timestamp > '2010-08-19 12:30:47')
union
(select * from internalmail where  recipientaddress_id = 247  and mail_timestamp > '2010-08-19 12:30:47')
union
(select * from internalmail where  recipientaddress_id = 0  and mail_timestamp > '2010-08-19 12:30:47')
order by mail_timestamp desc\G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: internalmail
         type: range
possible_keys: senderaddress_id,mail_timestamp,send_dt
          key: send_dt
      key_len: 9
          ref: NULL
         rows: 5
        Extra: Using where
*************************** 2. row ***************************
           id: 2
  select_type: UNION
        table: internalmail
         type: range
possible_keys: recipientaddress_id,mail_timestamp,recieve_dt
          key: recieve_dt
      key_len: 9
          ref: NULL
         rows: 11
        Extra: Using where
*************************** 3. row ***************************
           id: 3
  select_type: UNION
        table: internalmail
         type: range
possible_keys: recipientaddress_id,mail_timestamp,recieve_dt
          key: recieve_dt
      key_len: 9
          ref: NULL
         rows: 1
        Extra: Using where
*************************** 4. row ***************************
           id: NULL
  select_type: UNION RESULT
        table: 
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
        Extra: Using filesort
4 rows in set (0.00 sec)

Although this query has to perform a final filesort it is much faster: now it runs in 0 sec compared to 3 seconds originally.

select max(DepDelayMinutes), 	carrier, dayofweek
from ontime_2010
where dayofweek = 7
group by Carrier,  dayofweek

the usual case will be adding a covered index on (dayofweek, Carrier, DepDelayMinutes). And MySQL will use this index fine (using index mean it will use the covered index):

mysql> explain select max(DepDelayMinutes), Carrier, dayofweek from ontime_2010
where dayofweek =7 group by Carrier, dayofweek\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: ontime_2010
         type: ref
possible_keys: covered
          key: covered
      key_len: 2
          ref: const
         rows: 905138
        Extra: Using where; Using index
1 row in set (0.00 sec)

However, as the dayofweek part has low number of unique values, mysql will have to scan a lots of index entries (estimated rows: 905138).

MySQL can use loose index scan. Unfortunately, a o lots of limitations apply:

• The query is over a single table.
• The GROUP BY names only columns that form a leftmost prefix of the index and no other columns.
• The only aggregate functions used in the select list (if any) are MIN() and MAX(), same column
• etc… (see docs for details)

As our example query is suitable for loose index scan, we can create another index:

mysql> alter table ontime_2010 add key lis1(Carrier, dayofweek, DepDelayMinutes);

mysql> explain select max(DepDelayMinutes), Carrier, dayofweek from ontime_2010
where dayofweek =7 group by Carrier, dayofweek \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: ontime_2010
         type: range
possible_keys: DayOfWeek,covered
          key: lis1
      key_len: 5
          ref: NULL
         rows: 19
        Extra: Using where; Using index for group-by
1 row in set (0.01 sec)

Here, Using index for group-by, means that MySQL uses loose index scan.
Also, and it is really great, it works with range on dayofweek too:

mysql> explain select max(DepDelayMinutes), Carrier, dayofweek from ontime_2010
where dayofweek > 3 group by Carrier, dayofweek \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: ontime_2010
         type: range
possible_keys: DayOfWeek,covered
          key: lis1
      key_len: 5
          ref: NULL
         rows: 19
        Extra: Using where; Using index for group-by
1 row in set (0.00 sec)

And original covered index does not work with ranges in where clause:

mysql> explain select max(DepDelayMinutes), Carrier, dayofweek from ontime_2010 use index (covered)
where dayofweek > 3 group by Carrier, dayofweek \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: ontime_2010
         type: range
possible_keys: covered
          key: covered
      key_len: 2
          ref: NULL
         rows: 2416543
        Extra: Using where; Using index; Using temporary; Using filesort

In the above example, MySQL uses index but still have to create temporary table and filesort.

Now speed comparison:
I’m using “ontime” flight performance statistics data from transtats.bts.gov
The table only consist of data for 2010.
Table size: 5M rows, ~2G in size. Table structure:

CREATE TABLE `ontime_2010` (
  `YearD` int(11) DEFAULT NULL,
  `MonthD` tinyint(4) DEFAULT NULL,
  `DayofMonth` tinyint(4) DEFAULT NULL,
  `DayOfWeek` tinyint(4) DEFAULT NULL,
  `Carrier` char(2) DEFAULT NULL,
  `Origin` char(5) DEFAULT NULL,
  `DepDelayMinutes` int(11) DEFAULT NULL,
... more fields here ...
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Results (cached index and data):
“where dayofweek = 7″ (ref)

• Loose index scan: 0 sec
• Covered index: 0.6 sec

“where dayofweek > 3″ (range)

• Loose index scan: 0 sec
• index (+ filesort): 5.53 sec

I will also present the findings from this article (among other things) during the upcoming webinar, Getting the Best MySQL Performance in Your Products: Part 3, Query Tuning, which will take place on Tuesday, Nov 23 (webinar is free, webinar registration)