The document outlines the agenda for the 8th demand seminar held by EXEM, including presentations on PostgreSQL Vacuum and MySQL locks. The PostgreSQL presentation covers the details of Vacuum including its behavior during updates, deletes, and different Vacuum commands. The MySQL presentation covers different types of locks in MySQL including global read locks, table locks, and string locks.

1. EXEM seminar report no. 008 (2016.07.12)
Research & Contents Team
제 8회 수요 세미나 자료

2. Table of Agenda
© Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
제 8회 수요 세미나
1. [PostgreSQL] Vacuum의 거의 모든 것 (4차)
2. [MySQL] Lock (1차)

3. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL] Vacuum의 거의 모든 것 (4차)
발표자: 연구컨텐츠팀 이근오

4. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
Page Layout ( 1/2 )
 Dump
0.

5. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
 SELECT * from heap_page_items(get_raw_page('t2', 0));
Page Layout ( 1/2 )
 Dump
0.
00 7c 9f c0 00 7c 9f 80

6. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
 SELECT * from heap_page_items(get_raw_page('t2', 0));
create table t2 ( c1 char(19),c2 char(8),c3 char(8) );
insert into t2 values('A1001','A','A');
insert into t2 values('A1002','A','A');
 Dump
all line pointers on a heap page
1.

7. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
 SELECT * from heap_page_items(get_raw_page('t2', 0));
update t2 set c2='B' where c1 = 'A1001' ;
 Dump
2.
새로운 위치에
변경데이타 “B” 레코드를 추가.
ItemIdData 부분도 추가됨.
“A1001” 원본은 “xmax” 값을 변경

8. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
update t2 set c2='B' where c1 = 'A1001' ;
 tab.sql
2.
select xmin,xmax,ctid,* from t1;
 select txid_current() ;
select xmin,xmax,ctid,* from t2;
“A1001” item 의 xmin 변경됨.

9. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
 SELECT * from heap_page_items(get_raw_page('t2', 0));
vacuum t2 ;
 Dump
3.
“순서가 변경됨”
• A1002가 제일 아래로 내려감
• A1001 위치도 아래로 변경
• A1001 원래 위치의 데이터는 그대로
두고 ItemIdData만 삭제.

10. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
vacuum t2 ;
 tab.sql
3.
select xmin,xmax,ctid,* from t1;
 select txid_current() ;
select xmin,xmax,ctid,* from t2;
변경안됨

11. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
vacuum freeze t2 ;
 tab.sql
4.
select xmin,xmax,ctid,* from t1;
select xmin,xmax,ctid,* from t2;
1807012522에서 변경됨

12. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
vacuum full t2 ;
 tab.sql
5.
select xmin,xmax,ctid,* from t1;
select xmin,xmax,ctid,* from t2;
136677에서 변경됨 변경안됨

13. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
vacuum full t2 ;
 Dump
5.

14. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
autovacuum_analyze_scale_factor = 0.1
autovacuum_analyze_threshold = 50
6.
1. 1000건 insert
2. autovacuum_naptime=60s 이상 wait 후 확인 : autoanalyze OK.
3. autovacuum_analyze_scale_factor=0.1 인 100건 insert  wait 후 확인 : autoanalyze NOT OK.

15. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
autovacuum_analyze_scale_factor = 0.1
autovacuum_analyze_threshold = 50
6.
4. autovacuum_analyze_threshold=50 인 50건 insert  wait 후 확인 : autoanalyze NOT OK.
5. Only 1건 insert  wait 후 확인 : autoanalyze OK.
autovacuum analyze 대상 조건
변경건수(insert포함) > ( reltuples * 0.1(autovacuum_analyze_scale_factor) )
+ 50(autovacuum_analyze_scale_factor)

16. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
autovacuum_vacuum_scale_factor = 0.2
autovacuum_vacuum_threshold = 50
7.
1. 1000건 insert  wait 후 확인 : autovacuum NOT OK.
2. autovacuum_vacuum_scale_factor=0.2 인 200건 update  wait 후 확인 : autovacuum NOT OK.

17. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[PostgreSQL]
Vacuum의 거의 모든 것(4차)
autovacuum_vacuum_scale_factor = 0.2
autovacuum_vacuum_threshold = 50
7.
3. autovacuum_vacuum_threshold=50 인 50건 update  wait 후 확인 : autovacuum NOT OK.
4. Only 1건 update  wait 후 확인 : autovacuum OK.
autovacuum vacuum 대상 조건
변경건수(insert제외) > ( reltuples * 0.2(autovacuum_vacuum_scale_factor) )
+ 50(autovacuum_vacuum_threshold)

18. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
[MySQL] Lock (1차)
발표자: 연구컨텐츠팀 이대덕

19. 02. [MySQL] MySQL Lock
2-1. Global Read Lock
2-2. Table Lock
2-3. String Lock
2-4. Name Lock
Table of Agenda

20. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
FLUSH TABLES WITH READ LOCK;
UNLOCK TABLES;
• 서버 전체(모든 스토리지 엔진)에 Global Read Lock획득
• Table Cache에 있는 테이블을 Flush (기존의 문장이 Table Lock을 가지고 수행 중이라
면 대기함)
• SELECT문을 제외한 문장은 Global Read Lock이 해제될 때까지 대기 (State: Waiting
for global read lock)
• 주로 DB전체의 일관성 있는 백업을 받기 위해 사용
2-1. Global Read Lock

21. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
(root@localhost)[(none)] 13:48:51>show status like 'open_tables';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Open_tables | 54 |
+---------------+-------+
1 row in set (0.00 sec)
(root@localhost)[(none)] 13:48:53>flush tables with read lock;
Query OK, 0 rows affected (9.06 sec)
(root@localhost)[(none)] 13:49:10>show status like 'open_tables';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Open_tables | 1 |
+---------------+-------+
1 row in set (0.00 sec)
(root@localhost)[(none)] 14:00:20>unlock tables;
Query OK, 0 rows affected (0.00 sec)
2-1. Global Read Lock

22. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
(root@localhost)[exem_i] 13:32:41>select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 14286 |
+-----------------+
1 row in set (0.00 sec)
(root@localhost)[exem_i] 13:33:16>lock table foo write;
Query OK, 0 rows affected (10.97 sec)
2-1. Global Read Lock
(root@localhost)[(none)] 13:34:37>select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 13 |
+-----------------+
1 row in set (0.00 sec)
(root@localhost)[(none)] 13:34:44>flush tables with read lock;

23. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
2-1. Global Read Lock
(root@localhost)[exem_i] 13:33:34>show processlist;
+-------+------+-----------+--------+---------+------+------------------------------+----------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-------+------+-----------+--------+---------+------+------------------------------+----------------------------+
| 13 | root | localhost | NULL | Query | 20 | Waiting for global read lock | flush table with read lock |
| 14286 | root | localhost | exem_i | Query | 0 | starting | show processlist |
+-------+------+-----------+--------+---------+------+------------------------------+----------------------------+
2 rows in set (0.00 sec)

24. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
LOCK TABLES tbl_name [[AS] alias] lock_type [, tbl_name [[AS] alias] lock_type...
UNLOCK TABLES
• Lock_type : READ [LOCAL] | [LOW_PRIORITY] WRITE
• MySQL레벨에서 테이블 단위로 설정되는 LOCK
• MyISAM, MEMORY Storage Engine에서는 데이터를 변경하는 즉시 묵시적으로 획득 후 변경 후 반환
• 해당 테이블에 LOCK_TABLE 권한 필요
• 서버 레벨이지만 MyISAM스토리지 엔진을 기준으로 한 LOCK이며 InnoDB에서는 사용을 권장하지 않음
• READ LOCK :
해당세션과 다른 세션 모두 Select문장만 수행가능하며 Local 옵션을 사용 하는 경우 Concurrent Insert가능
다른 세션에서 획득한 READ LOCK끼리 호환가능
• WRITE LOCK :
해당 세션에서만 DML,DDL이 가능하며 다른 세션의 읽기,쓰기, TABLE LOCK과 호환불가
LOW_PRIORITY 옵션을 사용하는 경우 Lock 요청 큐에서 낮은 우선순위를 갖게 됨
2-2. Table Lock

25. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
(root@localhost)[exem_i] 15:20:06>select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 14286|
+-----------------+
1 row in set (0.00 sec)
(root@localhost)[exem_i] 14:36:22>lock table foo read;
Query OK, 0 rows affected (0.00 sec)
(root@localhost)[exem_i] 14:36:51>select * from foo;
+------+------+
| c1 | c2 |
+------+------+
| 31 | a |
| 2 | b |
| 123 | a |
+------+------+
3 rows in set (0.00 sec)
(root@localhost)[exem_i] 14:36:57>update foo set c1=1 where c2='a';
ERROR 1099 (HY000): Table 'foo' was locked with a READ lock and can't be updated
2-2. Table Lock

26. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
(root@localhost)[exem_i] 15:20:06>select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 13 |
+-----------------+
1 row in set (0.00 sec)
(root@localhost)[exem_i] 15:28:07>select * from foo;
+------+------+
| c1 | c2 |
+------+------+
| 31 | a |
| 2 | b |
| 123 | a |
+------+------+
3 rows in set (0.00 sec)
(root@localhost)[exem_i] 15:28:11>update foo set c1=1 where c2='a';
(root@localhost)[exem_i] 15:27:31>show processlist;
+-------+------+-----------+--------+---------+------+---------------------------------+----------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-------+------+-----------+--------+---------+------+---------------------------------+----------------------------------+
| 13 | root | localhost | exem_i | Query | 15 | Waiting for table metadata lock | update foo set c1=1 where c2='a' |
| 14286 | root | localhost | exem_i | Query | 0 | starting | show processlist |
| 25063 | root | localhost | NULL | Sleep | 2833 | | NULL |
+-------+------+-----------+--------+---------+------+---------------------------------+----------------------------------+
3 rows in set (0.00 sec)
2-2. Table Lock

27. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
(root@localhost)[exem_i] 15:20:06>select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 13 |
+-----------------+
1 row in set (0.00 sec)
(root@localhost)[exem_i] 15:34:08>lock table foo read;
Query OK, 0 rows affected (0.00 sec)
(root@localhost)[exem_i] 15:34:15>lock table foo write;
(root@localhost)[exem_i] 15:31:58>show processlist;
+-------+------+-----------+--------+---------+------+---------------------------------+----------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-------+------+-----------+--------+---------+------+---------------------------------+----------------------+
| 13 | root | localhost | exem_i | Query | 5 | Waiting for table metadata lock | lock table foo write |
| 14286 | root | localhost | exem_i | Query | 0 | starting | show processlist |
| 31080 | root | localhost | NULL | Sleep | 147 | | NULL |
+-------+------+-----------+--------+---------+------+---------------------------------+----------------------+
3 rows in set (0.00 sec)
2-2. Table Lock

28. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
mysql> lock tables foo read;
Query OK, 0 rows affected (0.00 sec)
mysql> show open tables from exem_i;
+----------+--------+--------+-------------+
| Database | Table | In_use | Name_locked |
+----------+--------+--------+-------------+
| exem_i | t1 | 0 | 0 |
| exem_i | myisam | 0 | 0 |
| exem_i | foo | 1 | 0 |
+----------+--------+--------+-------------+
3 rows in set (0.00 sec)
mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)
mysql> show open tables from exem_i;
+----------+--------+--------+-------------+
| Database | Table | In_use | Name_locked |
+----------+--------+--------+-------------+
| exem_i | t1 | 0 | 0 |
| exem_i | myisam | 0 | 0 |
| exem_i | foo | 0 | 0 |
+----------+--------+--------+-------------+
3 rows in set (0.00 sec)
2-2. Table Lock

29. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
GET_LOCK(str,timeout)
RELEASE_LOCK(str)
IS_USED_LOCK(str)
IS_FREE_LOCK(str)
• TABLE,ROW 단위가 아닌 Client간의 String에 대한 Lock
• Client들은 동일한 String에 Lock을 획득할 수 없음.
• GET_LOCK() : 해당 Lock 획득(획득에 성공하면 1리턴 실패하면 2를 리턴)
• RELEASE_LOCK() : 해당 Lock 해제(해제에 성공하면 1리턴, LOCK이 존재하지 않으면 NULL값 리턴)
• IS_USED_LOCK() : 해당 Lock을 누가 사용 중인지 확인(사용중인 세션의 Connection_id값 반환 or NULL)
• IS_FREE_LOCK() : 특정 String이 사용가능한지 확인(가능하면 1을 리턴, 사용 중이면 0을 리턴, Error발생시 NULL)
• 주로 특정 어플리케이션들에서 서로의 데이터 중복을 확인하기 위해 사용
2-3. User Lock

30. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
mysql> select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 243397 |
+-----------------+
1 row in set (0.00 sec)
mysql> select is_used_lock('locked_string');
+-------------------------------+
| is_used_lock('locked_string') |
+-------------------------------+
| NULL |
+-------------------------------+
1 row in set (0.00 sec)
mysql> select is_free_lock('locked_string');
+-------------------------------+
| is_free_lock('locked_string') |
+-------------------------------+
| 1 |
+-------------------------------+
1 row in set (0.00 sec)
mysql> select get_lock('locked_string',600);
+-------------------------------+
| get_lock('locked_string',600) |
+-------------------------------+
| 1 |
+-------------------------------+
1 row in set (0.00 sec)
2-3. User Lock

31. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
2-3. User Lock
mysql> select connection_id();
+-----------------+
| connection_id() |
+-----------------+
| 243396 |
+-----------------+
1 row in set (0.00 sec)
mysql> select is_used_lock('locked_string');
+-------------------------------+
| is_used_lock('locked_string') |
+-------------------------------+
| 243397 |
+-------------------------------+
1 row in set (0.00 sec)
mysql> select is_free_lock('locked_string');
+-------------------------------+
| is_free_lock('locked_string') |
+-------------------------------+
| 0 |
+-------------------------------+
1 row in set (0.00 sec)
mysql> select get_lock('locked_string',600);
mysql> show processlist;
+--------+------+-----------+--------+---------+------+-----------+--------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+--------+------+-----------+--------+---------+------+-----------+--------------------------------------+
| 243396 | root | localhost | exem_i | Query | 5 | User lock | select get_lock('locked_string',600) |
| 243397 | root | localhost | exem_i | Sleep | 179 | | NULL |
| 243555 | root | localhost | exem_i | Query | 0 | starting | show processlist |
+--------+------+-----------+--------+---------+------+-----------+--------------------------------------+
3 rows in set (0.00 sec)

32. © Copyrights 2001~2016, EXEM CO.,LTD. All Rights Reserved.
RENAME TABLE tbl_name1 TO tbl_name2
DROP TABLE tbl_name
• 테이블을 RENAME하거나 DROP할 경우 묵시적으로 테이블 명에 Name Lock
• RENAME할 경우 기존이름과 변경될 이름에 모두 Lock획득
• TABLE LOCK과 호환성 없음
• SHOW OPEN TABLES 커맨드로 Name_lock이 걸린 테이블 확인 가능
mysql> show open tables from exem_i;
+----------+--------+--------+-------------+
| Database | Table | In_use | Name_locked |
+----------+--------+--------+-------------+
| exem_i | abc | 0 | 0 |
| exem_i | t1 | 0 | 0 |
| exem_i | myisam | 0 | 0 |
| exem_i | foo | 0 | 0 |
+----------+--------+--------+-------------+
4 rows in set (0.00 sec)
2-4. Name Lock

33. Research & Contents
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