2009-02-24

Idiocy

People are idiots, some more than others. At least people are idiots in different aspects of life and issues. A person is an idiot differently each day. At least I feel like that. Other days it is quite obvious to see things exists and other days you cannot figure out a simplest case. Well maybe it is not idiotism to be clueless about a thing that one has never figured out before. Most often it is really nice to be working with persons that are less idiots than me. At least in a issue that one tried to figure out and the other has a ready answer to.

I have set up a 10g data quard manually and tried to register the databases to Grid Control. 
Stand by instance seemed to be quite hard to get registered to the agent. I did not figure out that the standby node is in mount state and dbsnmp monitoring user is trying to reach the database as a normal user. Just change the monitoring user to connect to the database as sysdba and voila grid control is showing roles of the instances correctly. Creating a stand by instance with Grid Control handles that ok. 

This week I have had a pleasure to work with a person who could figure out thigs while problems arise and a oraganization that has scheduled HA evironment testing well before going to production. After setting up Data Guard environment we had plenty of time to try out different kinds of switchower and failover situations and there is still time in the project plan to try out even more complex situations. This possibility and capability should exist in every project having a HA environment. 

2009-02-21

A week on the other side (part3 DB2)

Missing Oracle function based indexes. Got involved in a project having a DB2 database environment. There is a need for case insensitive predicate in a where clause. In Oracle it is possible to create function based index:

create table t(n number(8) primary key, last_name varchar(100));

insert into t (n,last_name)
select rownum, object_name
from all_objects;

select * from t where upper(last_name) = :ln;

Performs a full table scan to table t.

create index t_last_name_upper on t(upper(last_name));

select * from t where upper(last_name) = :ln;

And the index is used if the table is big enough.

In DB2 there is no such thing as a function based index. But in a article on IBM site there is described alternatives to do this. Generated columns and index extensions. Generated columns seems like a valid approach. Even though data is duplicated on a row. No need for triggers seems like a good thing and the original query do not need any modifications.

This was about the first time that I am using db2cc Control Center to do things in DB2 environment. Execution plans were found easily from Access Plan page. At least with a small table implementation and tests went ok. Allthough adding a column in a table was not so straight forward because the column was a GENERATED ALLWAYS AS column.

SET INTEGRITY FOR t OFF;

alter table t

add column last_name_u varchar(100)

GENERATED ALWAYS AS ( UPPER(last_name));


SET INTEGRITY FOR t IMMEDIATE CHECKED FORCE GENERATED;


create index t_lastname_upper_idx on t(last_name_u);



How much easier would that have been in Oracle 11g. Just add a virtual column...

alter table t add last_name_u as (upper(last_name));

create index t_last_name_upper on t(last_name_u);

select * from t where upper(last_name) = :ln;


Similarily no need to change the query. Index is used. And the column is virtual, no need for storage. The index extensions approach might be the way to avoid unneeded storing of the data in DB2.

Needed also reduce a size of column and got surpriced how often REORG TABLE is needed in DB2. Or did I miss something in the documentation.

A week on the other side (part2 SQLServer)


I had a pleasure to spend more than a hour installing SQLServer for evaluation purposes. Installed SQLServer 2008 for the first time. I have now 180 days to do test with AdventureWorks databases.

A couple weeks ago I went to see a presentation about SQLServer 2008 to Oracle specialists mainly presented by Marko Hotti. Nice overview about the product. There was a question from the audience about isolation levels in SQLServer. There is still a common understanding among Oracle specialist that readers block writers. There exists snapshot isolation level in SQLServer. It was presented allready in version 2005.

2009-02-20

A week on the other side (part1 PostgreSQL)

For a Oracle oriented person like me, it was nice to see that indexing in PostgreSql environment has the same kind of possibility to boost performance of a query. In a OLTP environment the first star was the most meaningfull this time. Query execution time deminisched from minutes to part of a second. Talking about stars I mean the stars described in a book Relational Database Index Design and the Optimizers writen by Tapio Lahdenmäki. Recommended reading, at least the chapter DERIVING THE IDEAL INDEX FOR A SELECT. There was three separately indexed foreign key columns in a half a million rows table. All those columns were predicates in a query. All that was needed was to create a single index having all those three columns in it.

Dropping an index in postgre was not an online operation. Need to figure out why.

2009-02-16

unindex



###########
# updated link to Tom Kyte blog.
# Comment about Toms script in this post is not valid anymore.
# The script itself is valid.
###########

Until now Tom Kytes unindex has been satisfying my needs on searching unindexed foreign key constraints. There came a day when there became need to improve the query. I came across "enq: TM - contention" wait events - cause there was no fk indexing. After a sneak overview on other indexing and queries on the table I figured that it would be nice to have indexing in other order than the constraint is defined.

--example

create table a(a1 number(8)
, a2 number(8)
, x varchar2(2)
, primary key (a1,a2));

create table b(b number(8) primary key
, a1 number(8)
, a2 number(8)
, foreign key (a1,a2) references a);

create index fk_idx on b(a2,a1);

Toms unindex is giving me false negative four stars.
In my opinion fk_idx is satisfying the unindex need. At least TM Enq Wait events dismished from the environment after creating the index with different order of columns than fk.

Here is an alternative approach to search for unindexed foreign keys implemented in a single sql clause. It is operational at least with Oracle 10.2.0.1 and 11.1.0.7. There exists a bug involved with usage of connect_by_root in versions 10.2.0.3, 10.2.0.4 and 11.1.0.6. For those versions you need to set the _optimizer_connect_by_cost_based parameter to false.

alter session set "_optimizer_connect_by_cost_based" = false;


select case when i.index_name is not null
then 'OK'
else '****'
end ok
, c.table_name
, c.constraint_name
, c.cols,i.index_name
from (
select table_name, constraint_name
, max(sys_connect_by_path(column_name, ' ' )) cols
from (
select a.table_name
, b.constraint_name
, column_name
, position
, row_number() over
(partition by b.constraint_name
order by column_name) rn
from user_constraints a, user_cons_columns b
where a.constraint_name = b.constraint_name
and a.constraint_type = 'R'
)
start with rn = 1
connect by prior rn = rn-1
and prior constraint_name = constraint_name
group by table_name, constraint_name
) c
left outer join
(
select table_name
, index_name
, cr
, max(sys_connect_by_path(column_name, ' ' )) cols
from (
select table_name
, index_name
, column_position
, column_name
, cr
, row_number() over (partition by index_name, cr
order by column_name) rn
from (
select table_name
, index_name
, column_position
, column_name
, connect_by_root(column_name) cr
from user_ind_columns
connect by prior column_position-1 = column_position
and prior index_name = index_name
)
)
start with rn = 1
connect by prior rn = rn-1
and prior index_name = index_name
and prior cr = cr
group by table_name, index_name, cr
) i on c.cols = i.cols and c.table_name = i.table_name
;

About Me

My photo
I am Timo Raitalaakso. I have been working since 2001 at Solita Oy as a Senior Database Specialist. I have received Oracle ACE nomination. My main focus is on projects involving Oracle database. In this Rafu on db blog I write some interesting issues that evolves from my interaction with databases. Mainly Oracle.