I want to separete my Oracle table into 5 parts, these parts records will be selected randomly from the original table. Parts can contain the same results, it is not a problem.
How can I do that?
You could use ORDER BY dbms_random.value and then work out the number of total records and divide by 5 and use this to limit the number of row returned:
SELECT * FROM
( SELECT * FROM mytable
ORDER BY dbms_random.value
)
WHERE rownum <= (SELECT count(*)/5 from mytable)
Related
I know that OFFSET 10000 LIMIT 20 in Mysql reads all data from first to 10020, and discards first 10000 data.
Does OFFSET 10000 ROWS FETCH NEXT 20 ROWS ONLY in Oracle work the same?
No, but of course it must read the proportionate of rows to satisfy the ordering sequence,
eg
select * from emp
order by empno
reads every row so they can be sorted, and
select * from emp
where deptno = 10
order by empno
reads every row for department of 10 and then can sort them.
(There's a few special cases where we can skip that with indexes etc, but that's a separate discussion).
Coming back to FETCH FIRST / OFFSET, in effect we are amending the query to satisfy that need:
select ...
from mytable
order by col
offset 100 fetch 20
becomes
select ..., row_number() over ( order by col) as r
from mytable
because then we can do
select
from ( select ..., row_number() over ( order by col) as r
from mytable
)
where r between 100 and 120
to match your FETCH/OFFSET needs.
I have a very large table MY_TABLE (100 million rows). I wish to select a sample of 5, say, records from this table.
What I can think of is getting 5 arbitrary primary keys as follows, this uses fast full scan as the explain plan shows:
select MY_PRIMARY_KEY_COLUMN from (select MY_PRIMARY_KEY_COLUMN, rownum as rn from MY_TABLE) where rn <=5
and then getting the records corresponding to these primary keys.
However this is still very very slow..
Can it be done more efficiently?
As it looks, I got confused. As the commenters noticed, there should have been no problem with the query
select * from MY_TABLE where rownum <=5
but I somehow started to look at
select MY_PRIMARY_KEY_COLUMN from (select MY_PRIMARY_KEY_COLUMN, rownum as rn from MY_TABLE) where rn <=5
which indeed runs very slowly..
Sorry for wasting everyone's time, the select * from MY_TABLE where rownum <=5 works perfectly.
I have written this query on oracle
SELECT itemcode,itemname from hmis.shopitemcode order by dbms_random.value
Its working fine and I got random record but my question is I just want only 5 random record not all the record from database table
when I try to use this query
SELECT itemcode,itemname from hmis.shopitemcode where ROWNUM <= 5 order by dbms_random.value
this query randomize top 5 record not get 5 random record from whole database table
hope this helps!
select * from (
SELECT itemcode,itemname from hmis.shopitemcode order by dbms_random.value
)
where rownum <= 5;
Problem: I need write stored procedure(s) that will return result set of a single page of rows and the number of total rows.
Solution A: I create two stored procedures, one that returns a results set of a single page and another that returns a scalar -- total rows. The Explain Plan says the first sproc has a cost of 9 and the second has a cost of 3.
SELECT *
FROM ( SELECT ROW_NUMBER() OVER ( ORDER BY D.ID DESC ) AS RowNum, ...
) AS PageResult
WHERE RowNum >= #from
AND RowNum < #to
ORDER BY RowNum
SELECT COUNT(*)
FROM ...
Solution B: I put everything in a single sproc, by adding the same TotalRows number to every row in the result set. This solution feel hackish, but has a cost of 9 and only one sproc, so I'm inclined to use this solution.
SELECT *
FROM ( SELECT ROW_NUMBER() OVER ( ORDER BY D.ID DESC ) RowNum, COUNT(*) OVER () TotalRows,
WHERE RowNum >= from
AND RowNum < to
ORDER BY RowNum;
Is there a best-practice for pagination in Oracle? Which of the aforementioned solutions is most used in practice? Is any of them considered just plain wrong? Note that my DB is and will stay relatively small (less than 10GB).
I'm using Oracle 11g and the latest ODP.NET with VS2010 SP1 and Entity Framework 4.4. I need the final solution to work within the EF 4.4. I'm sure there are probably better methods out there for pagination in general, but I need them working with EF.
If you're already using analytics (ROW_NUMBER() OVER ...) then adding another analytic function on the same partitioning will add a negligible cost to the query.
On the other hand, there are many other ways to do pagination, one of them using rownum:
SELECT *
FROM (SELECT A.*, rownum rn
FROM (SELECT *
FROM your_table
ORDER BY col) A
WHERE rownum <= :Y)
WHERE rn >= :X
This method will be superior if you have an appropriate index on the ordering column. In this case, it might be more efficient to use two queries (one for the total number of rows, one for the result).
Both methods are appropriate but in general if you want both the number of rows and a pagination set then using analytics is more efficient because you only query the rows once.
In Oracle 12C you can use limit LIMIT and OFFSET for the pagination.
Example -
Suppose you have Table tab from which data needs to be fetched on the basis of DATE datatype column dt in descending order using pagination.
page_size:=5
select * from tab
order by dt desc
OFFSET nvl(page_no-1,1)*page_size ROWS FETCH NEXT page_size ROWS ONLY;
Explanation:
page_no=1
page_size=5
OFFSET 0 ROWS FETCH NEXT 5 ROWS ONLY - Fetch 1st 5 rows only
page_no=2
page_size=5
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY - Fetch next 5 rows
and so on.
Refrence Pages -
https://dba-presents.com/index.php/databases/oracle/31-new-pagination-method-in-oracle-12c-offset-fetch
https://oracle-base.com/articles/12c/row-limiting-clause-for-top-n-queries-12cr1#paging
This may help:
SELECT * FROM
( SELECT deptno, ename, sal, ROW_NUMBER() OVER (ORDER BY ename) Row_Num FROM emp)
WHERE Row_Num BETWEEN 5 and 10;
A clean way to organize your SQL code could be trough WITH statement.
The reduced version implements also total number of results and total pages count.
For example
WITH SELECTION AS (
SELECT FIELDA, FIELDB, FIELDC FROM TABLE),
NUMBERED AS (
SELECT
ROW_NUMBER() OVER (ORDER BY FIELDA) RN,
SELECTION.*
FROM SELECTION)
SELECT
(SELECT COUNT(*) FROM NUMBERED) TOTAL_ROWS,
NUMBERED.*
FROM NUMBERED
WHERE
RN BETWEEN ((:page_size*:page_number)-:page_size+1) AND (:page_size*:page_number)
This code gives you a paged resultset with two more fields:
TOTAL_ROWS with the total rows of your full SELECTION
RN the row number of the record
It requires 2 parameter: :page_size and :page_number to slice your SELECTION
Reduced Version
Selection implements already ROW_NUMBER() field
WITH SELECTION AS (
SELECT
ROW_NUMBER() OVER (ORDER BY FIELDA) RN,
FIELDA,
FIELDB,
FIELDC
FROM TABLE)
SELECT
:page_number PAGE_NUMBER,
CEIL((SELECT COUNT(*) FROM SELECTION ) / :page_size) TOTAL_PAGES,
:page_size PAGE_SIZE,
(SELECT COUNT(*) FROM SELECTION ) TOTAL_ROWS,
SELECTION.*
FROM SELECTION
WHERE
RN BETWEEN ((:page_size*:page_number)-:page_size+1) AND (:page_size*:page_number)
Try this:
select * from ( select * from "table" order by "column" desc ) where ROWNUM > 0 and ROWNUM <= 5;
I also faced a similar issue. I tried all the above solutions and none gave me a better performance. I have a table with millions of records and I need to display them on screen in pages of 20. I have done the below to solve the issue.
Add a new column ROW_NUMBER in the table.
Make the column as primary key or add a unique index on it.
Use the population program (in my case, Informatica), to populate the column with rownum.
Fetch Records from the table using between statement. (SELECT * FROM TABLE WHERE ROW_NUMBER BETWEEN LOWER_RANGE AND UPPER_RANGE).
This method is effective if we need to do an unconditional pagination fetch on a huge table.
Sorry, this one works with sorting:
SELECT * FROM (SELECT ROWNUM rnum,a.* FROM (SELECT * FROM "tabla" order by "column" asc) a) WHERE rnum BETWEEN "firstrange" AND "lastrange";
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Paging with Oracle
I try to select data starting from 11 row. and i used
select e_name from copy where rownum>10;
this will not display's anything..
please help me to select 11th row to 15th row in my table
You cannot use rownum like that, you need to wrap everything into a derived table:
select *
from (
select *,
rownum as rn
form your_table
order by some_column
)
where rn between 11 and 15
You should use an order by in the inner query because otherwise you will not get consistent results over time. Rows in a relational table do not have any ordering so the database is free to return the rows in any order it feels approriate.
Please read the manual for more details. The reason your query isn't working is documented there with examples.
http://docs.oracle.com/cd/E11882_01/server.112/e26088/pseudocolumns009.htm#i1006297
You have to use like
select e_name
from (select e_name,rownum rno from copy)
where rno > 10 and rno < 16
Sample Example
you could use analytic function row_number() as well. Please consider http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions137.htm
First write the query which will select all the rows like this:-
select ename from employee
Now add a rownum column to your query, rownum will help you to identify the number of row in your query result
select rownum r,ename from employee
Now make your query as a sub query and apply the range on 'r' (rownum)
select * from (selecr rownum r, ename from employee) subq where subq.r between 11 and 15