I have a programming situation in which I have to check the uniqueness of a column of the table.
Say My table is employee table and I have added a column that is code. Now I have to prevent the insertion of MGR and mgr.
So that means I can not have 2 rows having case insensitive values.
How to add the check constraints ?
If you are using Oracle, you may add a unique index on the lowercase of your column.
create unique index <index_name> on <tablename>(lower(<column_name>))
Related
I have a datamart with 5 dimension table and a fact table.
I'm trying to clean a dimension table that has few rows (4000 rows). But, the fact table have millions rows (25GB)(Indexes and partitions).
When I try to delete a row in the table dimension, the process becomes very slow. It's just as slow despite the absence of relationship with a row in the fact table (cascade delete).
Is there any way to optimize this?. Thanks in advance.
Presumably, there is a cascading delete of some sort between the dimension table and the fact table.
Adding an index on the key column in the fact table may be sufficient. Then Oracle can immediately tell if/where any given value is.
If that doesn't work, drop the foreign key constraint altogether. Delete the unused values and add the constraint back in.
You could try these strategies as well :
create another copy of the fact table but, without the dim foreign key column of the table to be cleaned.
create fact_table_new as
select dim1_k, dim2_k, dim3_k, dim4_k, dim5_k (not this column), fact_1, fact_2, ...
from fact_table ;
or
update fact_table
set dim5_fk_col = null
where dim5_fk_col in (select k_col from dim5_table) ;
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).
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)
I have a db table with a column that is a String. I do not consider the case to be significant (e.g. "TEST == "test"). Unfortunately, it appears that JPA2 does, because both values are inserted into my table; I would like the second one to be rejected.
Is there a generic way to annotate an "ignore-case" unique constraint on a string column?
As an alternative, I could also consider putting a unique "ignore-case" constraint on the actual db column. Is that possible in Oracle 10?
What I don't want to do is write code, because this occurs often in this particular db.
All help is greatly appreciated.
you can achieve this with a function-based unique index
create unique index <index_name> on <table_name> (UPPER(<column_name>));
for Example
create table t111( col varchar2(10));
create unique index test_idx on t111 (UPPER(col));
insert into t111 values('test');
insert into t111 values ('TEST');
I have this table in an Oracle DB which has a primary key defined on 3 of the data columns. I want to drop the primary key constraint to allow rows with duplicate data for those columns, and create a new column, 'id', to contain an auto-incrementing integer ID for these rows. I know how to create a sequence and trigger to add an auto-incrementing ID for new rows added to the table, but is it possible to write a PL/SQL statement to add unique IDs to all the rows that are already in the table?
Once you have created the sequence:
update mytable
set id = mysequence.nextval;
If you're just using an integer for a sequence you could update the id with the rownum. e.g.
update
table
set id = rownum
You then need to reset the sequence to the next valid id.
Is this what you need?
UPDATE your_table
SET id = your_seq.nextval;
This assumes you don't care what order your primary keys are in.
First you should check your PCTFREE... is there enough room for every row to get longer?
If you chose a very small PCTFREE or your data has lots of lenght-increasing updates, you might begin chaining every row to do this as an update.
You almost certainly better to do this as a CTAS.
Create table t2 as select seq.nextval, t1.* from t1.
drop t1
rename t2 to t1.