So a table I am looking at has a unique constraint AND a unique index over multiple columns, and the exact same columns for both.
Is there a use for this or is the unique constraint redundant?
I agree that the existence of unique constraints and unique indexes does look redundant at first. It seems like a violation of Don't Repeat Yourself, allowing for confusing differences. But there are at least two reasons both exist - management features and allowing existing duplicates.
Management Features
In theory, a logical constraint can be created without worrying about the implementation. The constraint specifies what must be true, along with some options such as deferring the constraint until a commit.
In practice, constraints have such a large performance and storage penalty that the implementation must be considered. An index is required or else a single insert would require O(n) time instead of O(log(n)). Those indexes can take up a huge amount of space; someone might want to specify where it's stored, how it's compressed, etc.
Most of the time those features aren't important and using all the index defaults is fine. But sometimes storage and performance are critical and people will want to tweak the index without caring about the constraint.
Allow Existing Duplicates
There is at least one case where a unique constraint does not have a unique index. It's possible to allow existing duplicate valuess but prevent any future duplicates by setting the constraint to NOVALIDATE and using a non-unique index.
--Create table and insert duplicate values.
create table test1(a number);
insert into test1 values(1);
insert into test1 values(1);
commit;
--Add a non-validated unique constraint, with a non-unique index.
alter table test1
add constraint test1_uq unique(a)
using index (create /* Not unique!*/ index test1_uq on test1(a)) novalidate;
--Now multiple inserts raise: ORA-00001: unique constraint (JHELLER.TEST1_UQ) violated
insert into test1 values(2);
insert into test1 values(2);
The physical index must allow duplicates, but the logical constraint knows to not allow any more duplicates. Although this is a rare feature and I'm not sure if I've ever seen it in production code.
Related
let's say I have a large table.
This table not need to be queried, I just want to save the data inside for a while.
I want to prevent duplicates rows in the table, so I want to add an unique
constraint (or PK) on the table.
But the auto-created unique index is realy unnecessary.
I don't need it, and it's just wasting space in disk and require a maintenance
(regardless of the long time to create it).
Is there a way to create an unique constraint without index (any index - unique or nonunique)?
Thank you.
No, you can't have a UNIQUE constraint in Oracle without a corresponding index. The index is created automatically when the constraint is added, and any attempt to drop the index results in the error
ORA-02429: cannot drop index used for enforcement of unique/primary key
Best of luck.
EDIT
But you say "Let's say I have a large table". So how many rows are we talking about here? Look, 1TB SSD's are under $100. Quad-core laptops are under $400. If you're trying to minimize storage use or CPU burn by writing a bunch of code with minimal applicability to "save money" or "save time" my suggestion is that you're wasting both time and money. I repeat - ONE TERABYTE of storage costs the same as ONE HOUR of programmer time. A BRAND SPANKING NEW COMPUTER costs the same as FOUR LOUSY HOURS of programmer time. You are far, far better off doing whatever you can to minimize CODING TIME, rather than the traditional optimization targets of CPU time or disk space. Thus, I submit that the UNIQUE index is the low cost solution.
But the auto-created unique index is really unnecessary.
In fact, UNIQUEness in an Oracle Database is enforced/guaranteed via an INDEX. That's why your primary key constraints come with a UNIQUE INDEX.
Per the Docs
UNIQUE Key Constraints and Indexes
Oracle enforces unique integrity constraints with indexes.
Maybe Index-Organized Tables is what you need ?.
But strictly the index organized table is the table stored in the structure of the index - one can say that there is the index alone without the table, while yor requirement is to have the table without the index, so this is the opposite :)
CREATE TABLE some_name
(
col1 NUMBER(10) NOT NULL,
col2 NUMBER(10) NOT NULL,
col3 VARCHAR2(50) NOT NULL,
col4 VARCHAR2(50) NOT NULL,
CONSTRAINT pk_locations PRIMARY KEY (col1, col2)
)
ORGANIZATION INDEX
I want to create a table with a column that references the name of a sequence I've also created. Ideally, I'd like to have a foreign key constraint that enforces this. I've tried
create table testtable (
sequence_name varchar2(128),
constraint testtableconstr
foreign key (sequence_name)
references user_sequences (sequence_name)
on delete set null
);
but I'm getting a SQL Error: ORA-01031: insufficient privileges. I suspect either this just isn't possible, or I need to add something like on update cascade. What, if anything, can I do to enforce this constraint when I insert rows into this table?
I assume you're trying to build some sort of deployment management system to keep track of your schema objects including sequences.
To do what you ask, you might explore one of the following options:
Run a report after each deployment that compares the values in your table vs. the data dictionary view, and lists any discrepancies.
Create a DDL trigger which does the insert automatically whenever a sequence is created.
Add a trigger to the table which does a query on the sequences view and raises an exception if not found.
I'm somewhat confused at what you are trying to achieve here - a sequence (effectively) only has a single value, the next number to be allocated, not all the values that have been previously allocated.
If you simply want to ensure that an attribute in the relation is populated from the sequence, then a trigger would be the right approach.
I have a few non-unique constraints that I want to alter into unique constraints ( business rules have changed since the data model was made ). Is there any way to do it with out dropping and recreating as a unique constraint? I was thinking there would be an option in the alter constraint command, but I have not found anything.
Thanks!!
You cannot convert a non-unique index into a unique index.
(It's difficult to say what cannot be done. I'm basing this answer on looking at the ALTER INDEX page of the SQL Language Reference, searching for the word UNIQUE, and not finding any relevant hints. I looked at 11g instead of 10g, but that's probably better in this case because there are a few features that exist in 10g but are only documented in 11g.)
However, you can use a non-unique index for a unique constraint. But there are some performance considerations: a unique index would be smaller and faster.
create table my_table(a number);
create index my_table_index on my_table(a);
alter table my_table add constraint my_table_unique unique (a)
using index my_table_index;
You can't modify a constraint in the way you wish you can only drop and recreate it. If you want to do this with no downtime then look into the DBMS_REDEFINITION package.
In my case, just do drop and re-create the index:
DROP INDEX index_name;
CREATE UNIQUE INDEX index_name ON table_name (col01,col02) TABLESPACE indx;
I'm trying to create a constraint on the OE.PRODUCT_INFORMATION table which is delivered with Oracle 11g R2.
The constraint should make the PRODUCT_NAME unique.
I've tried it with the following statement:
ALTER TABLE PRODUCT_INFORMATION
ADD CONSTRAINT PRINF_NAME_UNIQUE UNIQUE (PRODUCT_NAME);
The problem is, that in the OE.PRODUCT_INFORMATION there are already product names which currently exist more than twice.
Executing the code above throws the following error:
an alter table validating constraint failed because the table has
duplicate key values.
Is there a possibility that a new created constraint won't be used on existing table data?
I've already tried the DISABLED keyword. But when I enable the constraint then I receive the same error message.
You can certainly create a constraint which will validate any newly inserted or updated records, but which will not be validated against old existing data, using the NOVALIDATE keyword, e.g.:
ALTER TABLE PRODUCT_INFORMATION
ADD CONSTRAINT PRINF_NAME_UNIQUE UNIQUE (PRODUCT_NAME)
NOVALIDATE;
If there is no index on the column, this command will create a non-unique index on the column.
If you are looking to enforce some sort of uniqueness for all future entries whilst keeping your current duplicates you cannot use a UNIQUE constraint.
You could use a trigger on the table to check the value to be inserted against the current table values and if it already exists, prevent the insert.
http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14251/adfns_triggers.htm
or you could just remove the duplicate values and then enfoce your UNIQUE constraint.
EDIT: After Jonearles and Jeffrey Kemp's comments, I'll add that you can actually enable a unique constraint on a table with duplicate values present using the NOVALIDATE clause but you'd not be able to have a unique index on that constrained column.
See Tom Kyte's explanation here.
However, I would still worry about how obvious the intent was to future people who have to support the database. From a support perspective, it'd be more obvious to either remove the duplicates or use the trigger to make your intent clear.
YMMV
You can use deferrable .
ALTER TABLE PRODUCT_INFORMATION
ADD CONSTRAINT PRINF_NAME_UNIQUE UNIQUE (PRODUCT_NAME)
deferrable initially deferred NOVALIDATE;
Could someone clarify what is the purpose of having unique index without unique constraint (Oracle)?
For example,
create table test22(id int, id1 int, tmp varchar(20));
create unique index idx_test22 on test22(id);
insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // ok
insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // fails, ORA-00001: unique
// constraint (TEST.IDX_TEST22) violated
So far it looks like there is a constraint. But
create table test33(id int not null primary key,
test22_id int not null,
foreign key(test22_id) references test22(id) );
also fails with "ORA-02270: no matching unique or primary key for this column-list".
I'm totally confused by this behaviour. Is there a constraint or not?
There are many articles that explain why it's possible to have a unique constraint without unique index; that is clear and makes perfect sense. However, I don't understand the reason for unique index without constraint.
A constraint and an index are separate logical entities. A unique constraint, for example, is visible in USER_CONSTRAINTS (or ALL_CONSTRAINTS or DBA_CONSTRAINTS). An index is visible in USER_INDEXES (or ALL_INDEXES or DBA_INDEXES).
A unique constraint is enforced by an index though it is possible (and sometimes necessary) to enforce a unique constraint using a non-unique index. A deferrable unique constraint, for example, is enforced using a non-unique index. If you create a non-unique index on a column and subsequently create a unique constraint, you can also use that non-unique index to enforce the unique constraint.
In practice, a unique index acts very much like a unique, non-deferrable constraint in that it raises the same error that a unique constraint raises since the implementation of unique constraints uses the index. But it is not quite the same because there is no constraint. So, as you've seen, there is no unique constraint so you cannot create a foreign key constraint that references the column.
There are cases where you can create a unique index that you cannot create a unique constraint. A function-based index, for example, that enforces conditional uniqueness. If I wanted to create a table that supported logical deletes but ensure that COL1 is unique for all non-deleted rows
SQL> ed
Wrote file afiedt.buf
1 CREATE TABLE t (
2 col1 number,
3 deleted_flag varchar2(1) check( deleted_flag in ('Y','N') )
4* )
SQL> /
Table created.
SQL> create unique index idx_non_deleted
2 on t( case when deleted_flag = 'N' then col1 else null end);
Index created.
SQL> insert into t values( 1, 'N' );
1 row created.
SQL> insert into t values( 1, 'N' );
insert into t values( 1, 'N' )
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.IDX_NON_DELETED) violated
SQL> insert into t values( 1, 'Y' );
1 row created.
SQL> insert into t values( 1, 'Y' );
1 row created.
But if we're talking about a straight unique non-function based index, there are probably relatively few cases where it really makes more sense to create the index rather than creating the constraint. On the other hand, there are relatively few cases where it makes much difference in practice. You'd almost never want to declare a foreign key constraint that referenced a unique constraint rather than a primary key constraint so you rarely lose something by only creating the index and not creating the constraint.
As was already explained in other answers: constraints and the indexes are different entities. But they lack precise definitions and official comments on the topic. Before we discuss the relationship between these two entities lets take a look at their purpose independent of each other.
Purpose of a constraint1:
Use a constraint to define an integrity constraint-- a rule that restricts the values in a database.
The purposes of an index2:
You can create indexes on columns to speed up queries. Indexes provide faster access to data for operations that return a small portion of a table's rows.
In general, you should create an index on a column in any of the following situations:
The column is queried frequently.
A referential integrity constraint exists on the column.
A UNIQUE key integrity constraint exists on the column.
Now we know what constraints and indexes are, but what is the relationship between them?
The relationship between indexes and constraints is3:
a constraint MIGHT create an index or use an existing index to efficient enforce itself. For example, a PRIMARY KEY constraint will either create an index (unique or non-unique depending) or it will find an existing suitable index and use it.
an index has nothing to do with a constraint. An index is an index.
So, a constraint MIGHT create/use and index. An INDEX is an INDEX, nothing more, nothing less.
So sum this up and directly address the following sentence from your question:
However, I don't understand the reason for unique index without constraint.
Indexes speed up queries and integrity checks (constraints). Also for conditional uniqueness a unique (functional) index is used as this cannot be achieved with a constraint.
Hopefully this brings a little bit more clarification to the whole topic, but there is one aspect of the original question that remains unanswered:
Why did the following error occur when no constraint existed:
ORA-00001: unique constraint (TEST.IDX_TEST22) violated
The answer is simple: there is no constraint and the error message misnames it!
See the official "Oracle Ask TOM" comment 4 on the same problem:
It isn't a constraint. the error message "misnames" it.
If it were a constraint, you could create a foreign key to it -- but you cannot.
Hope it helps.
Links:
1 Oracle 10g Documentation on Constraints
2 Oracle 10g Documentation on Selecting an Index Strategy
3 4 "Oracle Ask TOM" answer to a similar problem
Another point which may be useful in this context is :
Disabling/Dropping an existing unique constraint do not drop the underlying unique index. You got to drop the unique index explicitly.
You can not make conditional uniqueness by declaring a unique constraint, But you can do it by declaring a unique index.
Supporse if you try to execute below:
alter table test22
add constraint test22_u
unique (id, case when tmp = 'aaa' then null else tmp end);
ORA-00904: : invalid identifier
But if you can do it by using the unique index
create unique index test22_u
on test22 ( customer_id,
case when is_default = 'Y' then null else address_id end)