This is the 182nd edition of Log Buffer, the weekly review of database blogs. Make sure to read the whole edition so you do not miss where to submit your SQL limerick!

This week started out with me posting about International Women’s Day, and has me personally attending Confoo (Montreal) which is an excellent conference I hope to return to next year. I learned a lot from confoo, especially the blending nosql and sql session I attended.

This week was also the Hotsos Symposium. Doug’s Oracle Blog has a series of posts about Hotsos. If all this talk about conferences has gotten you excited, Joshua Drake notes that 14 days and the hotel is almost full for postgresql conference east which is March 25th-28th in Philadelphia. And the Oracle database insider notes that the Oracle OpenWorld call for papers is now open.

According to Susan Visser this week (ending tomorrow) is also read an e-book week. So if you have not already done so, read an e-book! She links a coupon for an e-book in the post.

Craig Mullins notes that the mainframe is a good career choice in Mainframes: The Safe IT Career Choice. He notes that the mainframe is still not dead:

People having been predicting the death of the mainframe since the advent of client/server in the late 1980s. That is more than 20 years! Think of all the things that have died in that timespan while the mainframe keeps on chugging away: IBM’s PC business, Circuit City, Koogle peanut butter, public pay phones, Johnny Cash… the list is endless.

In other career-related news, Antonio Cangiano is looking for [2] top-notch student hackers for a 16-month internship at IBM in Toronto starting in May. All the details, including how to apply, are in Cangiano’s blog post.

Willie Favero wants to know how you “solve the batch dilemma” for issues like “shrinking your batch window, designing your batch to play nicely with … OLTP” in how’s your batch workload doing? Perhaps Favero should read the updated batch best practices posted by Anthony Shorten.

Bryan Smith surveys a more personal question by asking if you go both ways and “manage both DB2 for Linux, UNIX, and Windows and DB2 for z/OS” in don’t ask, don’t tell, bi-platform DBAs. This week’s Log Buffer editor admits to being a tri-platform DBA — she has tried many platforms, and in fact, many databases (MySQL, Oracle, DB2, SQL Server, Sybase, Postgres and Ingres)!

Hari Prasanna Srinivasan promotes a patching survey in Oracle really wants to hear from you! Patching Survey.

Henrik Loeser explains what a deadlock and a hot spot are by using a real life analogy taken from a police report in deadlock and hot spot in real life.

Jamie Thomson asks why do you abbreviate schema names?. Shlomi Noach tries to solve the issue that “there is no consistent convention as for how to write [about table aliases in] an SQL query” in proper sql table alias use conventions. Noach also gives us a tip: faster than truncate.

Leons Petrazickis reminds us that “rulesets are chains” and it is important to have your rulesets in the proper order in iptables firewall pitfall.

Anyone interested in the history of MySQL AB will be informed after reading Dries Buytaert’s article.
Gavin Towey shares his software that helps centrally manage 120 MySQL servers in qsh.pl: distributed query tool For those who want to learn more about column-oriented databases, particularly in MySQL, Robin Schumacher of the InfiniDB blog announces that there is a MySQL University session recording on MySQL column databases now available. MySQL join-fu expert Jay Pipes has moved his blog to www.joinfu.com and starts with An SQL Puzzle and of course a follow up on the sql puzzle.

Ivan Zoratti is happy that finally, slides posted for the MySQL DW breakfast. Venu Anuganti gives you tips on one of the most common MySQL frustrations: optimizing subqueries in how to improve subqueries derived tables performance. Justin Swanhart posts the way in which he Gets Linux performance information from your MySQL database without shell access and emulates a ‘top’ CPU summary using /proc/stat and MySQL using the same method.

The Oracle Apps blog has an introduction to Oracle user productivity kit (UPK). Even though in this editor’s opinion the article is very sales-pitchy, it has valuable information, and does indeed live up to its promise:

UPK is a software tool that can capture all the steps in a system process. It records every keystroke, every click of the mouse, each menu option chosen and each button pressed. All this is done in the UPK Recorder by going through the transaction and pressing “printscreen” after every user action. From this, without any further effort from the developer, UPK builds a number of valuable outputs.

Allen White gives a great tip on how to optimize queries in keep your data clean.

Mike Dietrich reminds you to remove “old” parameters and events from your init.ora when upgrading, “as keeping them will definitely slow down the database performance in the new release.” He shows evidence of slowness when this is not done. Dietrich also shows how you can be gathering workload statistics “to give the optimizer some good knowledge about how powerful your IO-system might be”, especially “a few days after upgrading to the new release…while a real workload is running.”

Brian Aker shows the exciting features coming soon in Drizzle in Drizzle, Cherry, Roadmap for our Next Release.

Maybe you are thinking of migrating, not upgrading…..The O’Reilly Radar shows how to asses an Oracle to MySQL migration in MySQL migration and risk management. Actually, that article interviews Ronald Bradford on the subject — Bradford has been prolific lately, updating free my.cnf advice series and “Don’t Assume”: MySQL for the Oracle DBA series. Nick Quarmby also talks about migrating Oracle, but not to a new database, just to a new platform, in his primer on migrating Oracle Applications to new platforms. And the big news comes from Carlos of dataprix that Twitter will migrate from MySQL to Cassandra DB.

Paul S. Randal explains his way of benchmarking: 1 Tb table population on SQL Server.

Pete Finnigan shares his slides from a webinar on how to secure oracle, and Denis Pilipchuk shares his approaches for discovering security vulnerabilities in software applications.

Jeff Davis shares his thoughts about scalability and the relational model. Robert Treat responds actually, the relational model doesn’t scale and Baron Schwartz counters with NoSQL doesn’t mean non-relational.

Buck Woody explains “whenever you want to know something about SQL Server’s configuration, whether that’s the Instance itself or a database, you have a few options” — and of course what those options are — in system variables, stored procedures or functions for meta data.

This week’s T-SQL Tuesday topic was I/O. There are many links to great blog posts in the comments; three random posts I chose to highlight: Michael Zilberstein talks about IO capacity planning, while Kalen Delaney talks about using STATISTICS IO in I/O, you know, and Merrill Aldrich chimes in with information on real world SSD’s. Aldrich also begs folks not to waste resources and make more work for developers and DBAs in dear ISV, you’re keeping me awake nights with your VARCHAR() dates.

And we end with a bit of fin: Paul Nielsen wants us all to have a bit of fun; he has posted an SQL limerick and asks readers to create there own in there once was in Dublin a query.

在Oracle10g之后，提供了DBMS_ADVANCED_REWRITE包，具有强大的查询重写功能，可以让我们在数据库层面实现很多微妙的调整。假设我们有一个应用，但是现在无法直接修改应用程序的编码，但是又想能够让应用程序的某些SQL产生我们想要的变化，那么就可以使用DBMS_ADVANCED_REWRITE包。

drop table t;
create table t as select object_id,object_name from dba_objects;
drop table t1;
create table t1 as select object_id,object_name from dba_objects where 1=0;

SQL> select count(*) from t;

  COUNT(*)
----------
     16636

SQL> select count(*) from t1;

  COUNT(*)
----------
         0

现在我们有表T和T1，表结构相同，但是表T中有1.6万记录，而表T1中没有记录，如果说我们的应用中有一个SQL多次地查询表T的总记录数，占用了大量的CPU和逻辑读，而这样的count记录数又是完全没有用处的，但是我们无法修改应用程序去掉这个SQL，那么我们就可以通过DBMS_ADVANCED_REWRITE包来讲查询表T的SQL转变为查询表T1，这样就大大减少了这条SQL的逻辑读。

首先DBMS_ADVANCED_REWRITE包的执行权限必须显式赋给需要的用户。

CONN sys/password AS SYSDBA

GRANT EXECUTE ON DBMS_ADVANCED_REWRITE TO kamus;

CONN kamus/password

BEGIN
  SYS.DBMS_ADVANCED_REWRITE.declare_rewrite_equivalence (
     name             => 't_rewrite',
     source_stmt      => 'SELECT count(*) FROM t',
     destination_stmt => 'SELECT count(*) FROM t1',
     validate         => FALSE,
     rewrite_mode     => 'TEXT_MATCH');
END;
/

然后需要设置会话层面的QUERY_REWRITE_INTEGRITY参数，该参数默认值为ENFORCED，表示只有重写后的SQL输出结果跟原结果完全一样时，查询才会被真正重写，在这里需要修改为TRUSTED。

SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY = TRUSTED;
SQL> set autot on
SQL> select count(*) from t;

  COUNT(*)
----------
         0

Execution Plan
----------------------------------------------------------
Plan hash value: 238181912

----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |     1 |    13 |     2   (0)| 00:00:01 |
|   1 |  VIEW               |      |     1 |    13 |     2   (0)| 00:00:01 |
|   2 |   SORT AGGREGATE    |      |     1 |       |            |          |
|   3 |    TABLE ACCESS FULL| T1   |     1 |       |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        417  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY = ENFORCED;
SQL> select count(*) from t;

  COUNT(*)
----------
     16636

Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    22   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    | 16639 |    22   (0)| 00:00:01 |
-------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         69  consistent gets
          0  physical reads
          0  redo size
        420  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL>

可以看到在重写之后，执行计划中显示直接去查询表T1，而consistent gets也从查询表T需要的69减少为3。

可以从[USER|ALL|DBA]_REWRITE_EQUIVALENCES视图中获得查询重写的信息。

SQL> select * from user_rewrite_equivalences;

OWNER  NAME       SOURCE_STMT               DESTINATION_STMT          REWRITE_MO
------ ---------- ------------------------- ------------------------- ----------
KAMUS  T_REWRITE  SELECT count(*) FROM t    SELECT count(*) FROM t1   TEXT_MATCH

DBMS_ADVANCED_REWRITE包也有限制，不可以重写牵涉到SYS用户对象的SQL。

drop table t1;
create table t1 as select * from all_tables where 1=0;

SQL> BEGIN
  2    SYS.DBMS_ADVANCED_REWRITE.declare_rewrite_equivalence (
  3       name             => 't_rewrite',
  4       source_stmt      => 'SELECT count(*) FROM all_tables',
  5       destination_stmt => 'SELECT count(*) FROM t1',
  6       validate         => FALSE,
  7       rewrite_mode     => 'TEXT_MATCH');
  8  END;
  9  /
BEGIN
*
ERROR at line 1:
ORA-30354: Query rewrite not allowed on SYS relations
ORA-06512: at "SYS.DBMS_ADVANCED_REWRITE", line 29
ORA-06512: at "SYS.DBMS_ADVANCED_REWRITE", line 185
ORA-06512: at line 2

在测试中，如果destination_stmt中包含SYS用户对象，是可以成功创建查询重写的，但是在执行SQL的时候却会报ORA-03113错误，后台出现ORA-07445错误，无法正常执行。

SQL> BEGIN
  2    SYS.DBMS_ADVANCED_REWRITE.declare_rewrite_equivalence (
  3       name             => 't_rewrite',
  4       source_stmt      => 'SELECT count(*) FROM t1',
  5       destination_stmt => 'SELECT count(*) FROM all_tables',
  6       validate         => FALSE,
  7       rewrite_mode     => 'TEXT_MATCH');
  8  END;
  9  /

PL/SQL procedure successfully completed.

SQL> select * from user_rewrite_equivalences;

OWNER  NAME       SOURCE_STMT               DESTINATION_STMT          REWRITE_MO
------ ---------- ------------------------- ------------------------- ----------
KAMUS  T_REWRITE  SELECT count(*) FROM t1   SELECT count(*) FROM all_ TEXT_MATCH
                                            tables

SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY = TRUSTED;

Session altered.

SQL> select count(*) from t1;
select count(*) from t1
*
ERROR at line 1:
ORA-03113: end-of-file on communication channel
Process ID: 8360
Session ID: 135 Serial number: 22

This is not really a call to action, but rather the name of a ☞ cool website that allows you to try out Redis commands through a web browser and follow a quick tutorial. It is very similar to ☞ Try MongoDB.

1. An emergency release from Riak fixing a couple of bugs in the 0.9.0 release ☞
2. Redis has released 1.2.5 fixing replication with multiple connected slaves ☞
3. You can watch live the NoSQL Live from Boston event ☞. And yes, myNoSQL is an official memdia partner of the event.
4. There seems to be a new ☞ Hypertable.com site.