从 Oracle 合并到 MySQL - NPR 实例分析 http://www.ossez.com/forum.php?mod=viewthread&tid=26869&fromuid=426 (出处: OSSEZ)

1. ®
npr.org
Migration From Oracle to MySQL
An NPR Case Study
By Joanne Garlow

2. Overview
 Background
 Database Architecture
 SQL Differences
 Concurrency Issues
 Useful MySQL Tools
 Encoding Gotchas

3. Background
 NPR (National Public Radio)
 Leading producer and distributor of radio programming
 All Things Considered, Morning Edition, Fresh Air, Wait,
Wait, Don’t Tell Me, etc.
 Broadcasted on over 800 local radio stations nationwide
 NPR Digital Media
 Website (NPR.org) with audio content from radio programs
 Web-Only content including blogs, slideshows, editorial
columns
 About 250 produced podcasts, with over 600 in directory
 Mobile apps and sites
 Syndication

4. High-Level System Architecture

5. Limitations of the Oracle
Architecture
 Reached capacity of single system to support our
load
 Replication outside our budget
 Databases crashes were becoming frequent

6. Database Architecture Goals
 Redundancy
 Scalability
 Load balancing
 Separation of concerns
 Better security

7. High-Level System Architecture

8. • Updated by our Content Management System
• Transaction Oriented
• Resource Contention
• Highly Normalized
• Isolation from main website
• Read-only by our webservers
• Horizontally scalable
Database Architecture
Main
InnoDB
AMG
MyISAM
PUBLIC
InnoDB
STATIONS
InnoDB
Main
RO slave
Main
RO slave
Content
Mgmt System
Web Servers
Scripts Backup
RO slave
• Read and updated only by our website
• Low resource contention
• Small tables or log tables
• Short Transactions
• Updated by a nightly script
• Read-only by our Content
Management System
• Need fast full text queries
(replacing Oracle Text)
• Large tables
• Updated by a quarterly script
• Read-only from our website
• Some log type information written
• Low resource contention
• No transactions

9. Issues When Converting SQL
 MySQL is case sensitive
 Oracle outer join syntax (+) -> OUTER JOIN clause
 Oracle returns a zero to indicate zero rows updated
– MySQL returns TRUE (1) to indicate it
successfully updated 0 rows
 MySQL sorts null to the top, Oracle sorts null to
the bottom
Use “order by – colName desc” for sorting asc with
nulls at bottom
 MySQL has Limit clause – YAY!

10. Replacing Oracle Sequences
 Initialize a table with a single row:
CREATE TABLE our_seq (
id INT NOT NULL
);
INSERT INTO our_seq (id) VALUES (120000000);
 Do the following to get the next number in the “sequence”:
UPDATE our_seq SET id=LAST_INSERT_ID(id+1);
SELECT LAST_INSERT_ID();

11. Replacing Oracle Sequences
 For updating many rows at once, get the total number of unique IDs you need
first:
SELECT @totalRows := COUNT(*) FROM...
 Then update npr_seq by that many rows:
UPDATE npr_seq SET id=LAST_INSERT_ID(id+@totalRows);
 and store that ID into another variable:
SELECT @lastSeqId := LAST_INSERT_ID();
 Then use the whole rownum workaround described above to get a unique value for
each row:
INSERT INTO my_table (my_primary_id . . . ) SELECT
@lastSeqId - (@rownum:=@rownum+1), . . . FROM (SELECT
@rownum:=-1) r, . . .

12. Converting Functions
 NVL() -> IFNULL() or COALESCE()
 DECODE() -> CASE() or IF()
 Concatenating strings || -> CONCAT()
 ‘test’ || null returns ‘test’ in Oracle
 CONCAT(‘test’,null) returns null in MySQL
 LTRIM and RTRIM -> TRIM()
 INSTR() works differently.
 Use LOCATE() for Oracle’s INSTR() with occurrences =
1.
 SUBSTRING_INDEX() and REVERSE() might also work.

13. Converting Dates
 sysdate -> now()
 Adding or subtracting
 In Oracle “– 1” subtracts a day
 In MySQL “- 1” subtracts a milisecond – must
use “interval”
 TRUNC() -> DATE()
 TO_DATE and TO_CHAR -> STR_TO_DATE and
DATE_FORMAT

14. Update Differences
 You can't update a table that is used in the WHERE
clause for the update (usually in an "EXISTS" or a
subselect) in mysql.
UPDATE tableA SET tableA.col1 = NULL
WHERE tableA.col2 IN
(SELECT tableA.col2
FROM tableA A2, tableB
WHERE tableB.col3 = A2.col3 AND
tableB.col4 = 123456);
 You can join tables in an update like this (Much
easier!):
UPDATE tableA
INNER JOIN tableB ON tableB.col3 = tableA.col3
SET tableA.col1 = NULL
WHERE tableB.col4 = 123456;

15. RANK() and DENSE_RANK()
 We really found no good MySQL equivalent for
these functions
 We used GROUP_CONCAT() with an ORDER BY and
GROUP BY to get a list in a single column over a
window of data

16. Collation
 You can set collation at the server, database,
table or column level.
 Changing the collation at a higher level (say on
the database) won’t change the collation for
preexisting tables or column.
 Backups will use the original collation unless
you specify all the way down to column level.

17. Concurrency Issues
 In our first round of concurrency testing, our
system ground to a halt!
 Deadlocks
 Slow Queries
 MySQL configuration
 sync_binlog = 1 // sync to disk, slow but safe
 innodb_flush_log_at_trx_commit = 1 // write each
commit
 transaction_isolation = READ-COMMITTED

18. Useful MySQL Tools
 MySQL Enterprise Monitor
http://www.mysql.com/products/enterprise/
 MySQL GUI Tools Bundle:
http://dev.mysql.com/downloads/gui-tools/5.0.html
 MySQL Query Browser similar to Oracle’s SQL
Developer
 MySQL Administrator

19. Innotop and innoDB Status
 innotop
http://code.google.com/p/innotop
 Helped us identify deadlocks and slow queries
(don’t forget the slow query log!)
 In mysql, use
show engine innodb statusG;
 Useful for contention and locking issues

20. Query Profiling
 Try the Query Profiler with Explain Plan when
debugging slow queries
http://dev.mysql.com/tech-resources/articles/using-
new-query-profiler.html

21. Concurrency Solution
 Tuning our SQL and our server configuration
helped
 Turns out that the RAID card we were using had
no write cache at all. Fixing that allowed us
to go live.

22. Encoding Gotcha’s
 Switched from ISO-8859-1 to UTF-8
 Migration Tool
 Issues with characters that actually were not
ISO-8859-1 in our Oracle database
 Lack of documentation for the LUA script produced
by the migration GUI
 Update encoding end to end
 JSPs, scripts (Perl), PHP, tomcat (Java)

23. Continuing Issues
 Bugs with innodb locking specific records (as
opposed to gaps before records)
 Uncommitted but timed out transactions
 Use innotop or “show engine innodb statusG; “
and look for threads waiting for a lock but no
locks blocking them
 Requires MySQL reboot

24. Questions?
 Joanne Garlow
 jgarlow@npr.org
 http://www.npr.org/blogs/inside