When i create a primary key in oracle table, why does it create a 'clustered' index by default. What is the reason for this automatic creation of a clustered index on creation of a primary key? is it just the Oracle designer's preference that he designed oracle in this way?
Oracle will create an index to police an unique constraint where no pre-existing index is suitable. Without the index, Oracle would need to serialize operations (such as a table lock) whenever someone tries to insert or delete a row (or update the PK).
Contrarily to MS-SQL Server, this index is not clustered on heap tables (default table organization), i.e. this index won't change the underlying table structure and natural order. The rows won't be reordered when Oracle creates the index. The index will be a B-tree index and will exist as a separate entity where each entry points to a row in the main table.
Oracle doesn't have clustered index as MS SQL, however indexed-organized tables share some properties with cluster-indexed tables. The PK is an integral part of such tables and has to be specified during creation.
(Oracle also has table clusters, but they are a completely different concept).
Creating Index is basic functionality of Primary key, it is also in SQL Server and MySQL, Clustered Index makes your searches faster.
The Database Engine automatically creates a unique index to enforce the uniqueness of the PRIMARY KEY constraint. If a clustered index does not already exist on the table or a nonclustered index is not explicitly specified, a unique, clustered index is created to enforce the PRIMARY KEY constraint.
Read this:
http://www.sqlskills.com/blogs/kimberly/the-clustered-index-debate-continues/
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I am coming from MS sql server where clustered and non clustered index used to play important role.
But looks like there are nothing of this sort under oracle. I am sure there must be some equivalent of
clustered and non clustered index in oracle . Can somebody throw light on this ?
When we say create index in oracle is it equivalent to non clustered index ?
Index Organised table in Oracle stores data of the whole table sorted on the basis of say - primary key. So, this is the closest thing to have as clustered Index in Oracle, other than that all other indexes are non-clustered but the Index key for all other indexes are sorted too with Rowid in-front of them which points to the actual data.
I've recently stopped to think that Primary Keys are not indexes, they're a combination of Unique and Null constraints. And till now, I've never created index for PK columns. My question is if I should create index for PK columns if this column is going to be used in the WHERE part of many queries.
Oracle will create an index for you, or can use an existing one. Whether a unique or non-unique index is used is up to you.
http://docs.oracle.com/cd/B28359_01/server.111/b28310/indexes003.htm#i1006566
A primary key itself is not an index, and nor is a unique constraint -- they are both constraints. However an index is used to support them.
A unique index is rather different as it can exist in the absence of a unique or primary key constraint, and neither constraint type require that the index supporting it be unique.
I have a table Customer_Chronics in Oracle 11g.
The table has three key columns as shown below :
branch_code
customer_id
period
I have partitioned by table by list of branch_code, and now I'm having dilemma. Which is better:
Create unique index indexNumberOne on Customer_Chronics (PERIOD, CUSTOMER_ID);
Create unique index indexNumberTwo on Customer_Chronics (branch_code, PERIOD, CUSTOMER_ID);
The actual data must be unique by period, customer_id. If I put a unique index only on these two columns Oracle will check all partitions on the table when inserting new records?
The only way to enforce uniqueness is with a unique constraint on the columns of interest. So that's your first option. The database will check all values across all partitions it this case. But as it's a unique index that shouldn't take too long no matter how big the table gets (if that's your concern).
Yes, If you put unique index on that two columns only, Oracle will create a global index and will check all partitions. This is one of challenges I face sometime because we prefer local indexs for big tables (small tables should be OK).
I'm currenly developing on Oracle. I have several tables for which I defined FOREIGN KEY constraints. I have already read this SQL Server-oriented and this MySQL-oriented questions but I could find none about Oracle.
So the question is always the same: in order to optimize query performance, for those columns for which I create a FOREIGN KEY constraint, do I also have to create an explicit secondary index? Doesn't Oracle automatically create an index on FOREIGN KEYed columns to boost performances during JOINs?
I usually perform queries in which the WHERE clause compare against those columns.
No, Oracle doesn't automatically create indexes on foreign key columns, even though in 99% of cases you probably should. Apart from helping with queries, the index also improves the performance of delete statements on the parent table.
I have a table in SQL Server that needs its normal pk index to be replaced by a clustered index on two fields. These other fields are not part of the primary key.
I need to do something similar in Oracle. As far as I know, this can be done using index-ordered tables, but my guess is these indexes are only constructed on the primary key.
Is there any way I can get a similar behavior to SQLServer's clustered index in Oracle?
Index organized tables are the concept of oracle which is near to clustered indexes in sql server. I found a discussion about the topic in oracle forum and one on asktom.
My question is: why do you want to adapt the behavior? What is the benefit you whant to obtain?
A clustered index in sql server is mostly the primary key index. Rowdata is stored into the index node. The conecpt on oracle to store row data into the index is an index organized table.
On oracle the iot is used to avoid a second lookup for row data into the table after index lookup.
The purpose of clustered index on sql server is to store the rowdata. A table can have only one clustered index. This index will hold the rowdata. Any other index is a non clustered index.
IMHO the concept of clustered index is bound to sql server data storage and there is no need to rebuild this behavior in oracle. Oracle has other concepts to store data.
Answer:
A regular index on oracle is all to solve your problem.