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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
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! No sequences - DOH! Continued….
Replacing Oracle Sequences Initialize a table with a single row: CREATE TABLE our_seq ( id INT NOT NULL ); INSERT INTO our_seq (id) VALUES ( ); 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: := COUNT(*) FROM... Then update npr_seq by that many rows: UPDATE npr_seq SET and store that ID into another variable: := 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... ) - FROM 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 Oracles 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 useinterval 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 = ); 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 = ;
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) wont 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 MySQL GUI Tools Bundle: MySQL Query Browser similar to Oracles SQL Developer MySQL Administrator
Innotop and innoDB Status innotop Helped us identify deadlocks and slow queries (dont 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 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 Gotchas Switched from ISO to UTF-8 Migration Tool Issues with characters that actually were not ISO 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