I have written query to generate a random 8 digit nummber and then i am inserting values into a table. The numbers generated should be unique.
Example: Say below is query, this generates random 8 digit number, and then i am inserting it into table temp. Now everytime before inserting into the table how do i need to check if the number generated from below query is alredy in temp table?
Using for loop doesnt seem to be useful as it will cause lag when there are going to be lot of transactions at same time. Any other suggestions like goto, if number is already in table can we use goto and then redirect it to same query until we get a unique value?
Query:
select trunc(dbms_random.value(10000000,99999999))
into v_customer_nbr
from dual;
If you are looking for unique values then you should rather consider using SYS_GUID instead. SYS_GUID generates and returns a globally unique identifier made up of 16 bytes.
Related
I know this question has been asked more than once here. But I am not able to resolve my issue so posting it again for help.
I have a table called Transaction in Oracle database (11g) with 2.7 million records. There is a not-null varchar2(20) (txn_id) column which contains numeric values. This is not the primary key of the table, and most of the values are unique. By most of the values I mean there are cases where one value can be there 3-4 times in the table.
If I perform a simple query of select based on TXN_ID it take about 5 seconds or more to return the result.
Select * from Transaction t where t.txn_id = 245643
I have an index created on this column, but when I check the explain plan for above query, it is using full table scan. This query is being used many times in the application which is making the application slow.
Can you please provide some help what might be causing this issue?
You are comparing a varchar column with a numeric literal (245643). This forces Oracle to convert one side of the equality, and off hand, it seems as though it's choosing the "wrong" side. Instead of having to guess how Oracle will handle this conversion, use a character literal:
SELECT * FROM Transaction t WHERE t.txn_id = '245643'
I have a massive table in which I can't do any more partitioning or sub-partitioning, nor am allowed to do any alter. I want to query its records by batches, and thought a good way would be using the last two digits from the account numbers (wouldn't have any other field splitting records as evenly).
I guess I'd need to at least index that somehow (remember I can't alter table to add a virtual column either).
Is there any kind of index to be used in such situation?
I am using Oracle 11gR2
You can use function based index:
create index two_digits_idx on table_name (substr(account_number, -2));
This index will work only in queries like that:
select ...
from table_name t ...
where substr(account_number, -2) = '25' -- or any other two digits
For using index, you need to use in a query the same expression like in an index.
I have a table that I expect to get 7 million records a month on a pretty wide table. A small portion of these records are expected to be flagged as "problem" records.
What is the best way to implement the table to locate these records in an efficient way?
I'm new to Oracle, but is a materialized view an valid option? Are there such things in Oracle such as indexed views or is this potentially really the same thing?
Most of the reporting is by month, so partitioning by month seems like an option, but a "problem" record may be lingering for several months theorectically. Otherwise, the reporting shuold be mostly for the current month. Would you expect that querying across all month partitions to locate any problem record would cause significant performance issues compared to usinga single table?
Your general thoughts of where to start would be appreciated. I realize I need to read up and I'll do that but I wanted to get the community thought first to make sure I read the right stuff.
One more thought: The primary key is a GUID varchar2(36). In order of magnitude, how much of a performance hit would you expect this to be relative to using a NUMBER data type PK? This worries me but it is out of my control.
It depends what you mean by "flagged", but it sounds to me like you would benefit from a simple index, function based index, or an indexed virtual column.
In all cases you should be careful to ensure that all the index columns are NULL for rows that do not need to be flagged. This way your index will contain only the rows that are flagged (Oracle does not - by default - index rows in B-Tree indexes where all index column values are NULL).
Your primary key being a VARCHAR2 GUID should make no difference, at least with regards to the specific flagging of rows in this question, indexes will point to rows via Oracle internal ROWIDs.
Indexes support partitioning, so if your data is already partitioned, your index could be set to match.
Simple column index method
If you can dictate how the flagging works, or the column already exists, then I would simply add an index to it like so:
CREATE INDEX my_table_problems_idx ON my_table (problem_flag)
/
Function-based index method
If the data model is fixed / there is no flag column, then you can create a function-based index assuming that you have all the information you need in the target table. For example:
CREATE INDEX my_table_problems_fnidx ON my_table (
CASE
WHEN amount > 100 THEN 'Y'
ELSE NULL
END
)
/
Now if you use the same logic in your SELECT statement, you should find that it uses the index to efficiently match rows.
SELECT *
FROM my_table
WHERE CASE
WHEN amount > 100 THEN 'Y'
ELSE NULL
END IS NOT NULL
/
This is a bit clunky though, and it requires you to use the same logic in queries as the index definition. Not great. You could use a view to mask this, but you're still duplicating logic in at least two places.
Indexed virtual column
In my opinion, this is the best way to do it if you are computing the value dynamically (available from 11g onwards):
ALTER TABLE my_table
ADD virtual_problem_flag VARCHAR2(1) AS (
CASE
WHEN amount > 100 THEN 'Y'
ELSE NULL
END
)
/
CREATE INDEX my_table_problems_idx ON my_table (virtual_problem_flag)
/
Now you can just query the virtual column as if it were a real column, i.e.
SELECT *
FROM my_table
WHERE virtual_problem_flag = 'Y'
/
This will use the index and puts the function-based logic into a single place.
Create a new table with just the pks of the problem rows.
I have a huge table with one int PRIMARY KEY IDENTITY column.
I guess making the SELECT query using that primary key is the fastest way for the database to find the row in the table isn't it?
If that is true i still have a question.
Is that query as fast as a call to a dictionary by key or the database still has to read all the rows from the beginning (the Primary Key column) till it finds the row itself?
Thanks in advance ^^
Using primary key is obviously the fastest way to access a particular row.
If you want to understand how it works, you have to understand how index works.
In general it works like that :
Let's say you have a table t1(col1,col2...col10) and you have an index on col1.
Index on col1 means that you have some data structure which contains pairs (col1, rec_id)
and rec_id allows direct access to row with appropriate col1.
The data structure is ordered by col1 and therefore allows efficient searching by col1.
I think searching in dictionary works per dictionary search algorithm which should be more like binary search kind.
When you declare a column as Primary key in table, then that column is indexed, hence it should be working based on hashing principle, so searching is definitely NOT row by row as you mentioned.
Finally, yes it is the common and fast way, but you should be selective about the number of columns and rows you need in your sql query. Avoid fetching large number of rows per select call.
I've got to store multiple text fields of variable length in an oracle database. I'd need to define them as columns of the same table to be able to order the results when I query it.
I can't know the max size of the field contents, most of them will be less than 100 characters but there could be some of thousands of chars. Furthermore the number of fields changes dinamically.
I was thinking of defining a table with multiple NCLOB columns that would allow me to store anything in them (very short and very long texts) But I wonder if this would be the right design.
Summary:
Variable number of fields (metadata of the same object)
Variable length of the content
I need to order the results
Thanks
KL
When you need variable number of fields, it's better to split the table into parent and child. Then you can effectively have any number of fields. And you can add order column to store ordering information to order the result. You can query by joining the two table and use order by clause to order the result. Also you can add foreign key constraint to make sure the relationship and data integrity.
In the case of variable length of contents, you can use varchar2(or nvarchar2) to store text date. Varchar2 can hold characters up to 4000 bytes. If you know that the maximum length of the content can be longer than 4000 bytes, you should use CLOB(or NCLOB).