I have read some articles and oracle documentation on unique and nonunique indexes. What I understood is Index is Index and constraint is constraint.
Constraint uses the index to ensure data integrity.
The primary key uses the unique or nonunique index to ensure that column is unique and not null.
The same goes for unique constrain.
But constraints can use a non-unique index if we want them deferrable.
The question comes for a unique index, it is just an index and I can not see any constraint associated with it so what is going on internally so the unique index won't allow for duplicate values, it is some internal semantics working as a constraint to ensure index uniqueness?
And let's say a unique index has the ability to be unique without any constraint then I want to understand the relation of constraints with indexes in every sense.
"There really is no such thing as a nonunique entry in a B*Tree index. In a nonunique index, Oracle simply stores the rowid by appending it to the key as an extra column with a length byte to make the key unique. For example, an index such as CREATE INDEX I ON T(X,Y) is conceptually CREATE UNIQUE INDEX I ON T(X,Y,ROWID). In a unique index, as defined by you, Oracle does not add the rowid to the index key. In a nonunique index, you will find that the data is sorted first by index key values (in the order of the index key) and then by rowid ascending. In a unique index, the data is sorted only by the index key values".
Though unique, primary key constraints and unique indexes both help with uniqueness, their objectives differ.
A unique or primary key constraints are meant to enforce data integrity. They might create a unique index implicitly and then it uses unique index to maintain integrity or it can use nonunique index as the key enforcement. Anyway index (unique or nonunique) always used to support primary key.
A unique index serves to make data access efficient. Its primary purpose is to help performance, but it does maintain uniqueness as a corollary and primary key may use this.
Related
I am using Oracle 12c and I have an IDENTITY column set as GENERATED ALWAYS.
CREATE TABLE Customers
(
id NUMBER GENERATED ALWAYS AS IDENTITY,
customerName VARCHAR2(30) NULL,
CONSTRAINT "CUSTOMER_ID_PK" PRIMARY KEY ("ID")
);
Since the ID is automatically from a sequence it will be always unique.
Do I need a PK on the ID column, and if yes, will it impact the performance?
Would an index produce the same result with a better performance on INSERT?
No, you don't need a primary key necessarily, but you should always provide the optimiser as much information about your data as possible - including a unique constraint whenever possible.
In the case of a surrogate key (like your ID), it's almost always appropriate to declare it as a Primary Key, since it's the most likely candidate for referential constraints.
You could use an ordinary index on ID, and performance of lookups will be comparable depending on data volume - but there is virtually no good reason in this case to use a non-unique index instead of a unique index - and there is no good reason in this case to avoid the constraint which will require an index anyway.
Yes, you always should have a Uniqueness constraint (with rare exceptions) on an ID column if indeed it is (and should be) unique, regardless of the method by which it is populated - whether the value is provided by your application code or via an IDENTITY column.
I have a table with a long value as primary key.
Now i think that oracle by default will create a index on it.And i dont need to
create a index explicityly.
The question is :Is primary key by default indexed by oracle in this case?
Yes, a primary key (or any unique column constraint) will create an index, if there is not already one present.
This is the case for almost all databases. Otherwise the uniqueness constraint cannot be efficiently enforced.
As per my knowledge, when a primary key is defined in a table an index is created for that column. So I don't have to create an index explicitly.
Are there any other way an implicit indexes added by oracle?
Only for primary key constraints and unique key constraints.
Note that this is an implementation detail only so that those constraints can be enforced efficiently.
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/
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.