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Migration From Oracle to MySQL

An NPR Case Study

By Joanne Garlow

Overview

Background

Database Architecture

SQL Differences

Concurrency Issues

Useful MySQL Tools

Encoding Gotchas

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

High-Level System Architecture

Limitations of the Oracle Architecture

Reached capacity of single system to support our

load

Replication outside our budget

Databases crashes were becoming frequent

Database Architecture Goals

Redundancy

Scalability

Load balancing

Separation of concerns

Better security

High-Level System Architecture

• 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

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!

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();

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, . . .

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.

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

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;

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

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.

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

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

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 status\G;

Useful for contention and locking issues

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

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.

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)

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 status\G; “

and look for threads waiting for a lock but no

locks blocking them

Requires MySQL reboot

Questions?

Joanne Garlow

jgarlow@npr.org

http://www.npr.org/blogs/inside