Using ColdFusion 10 on Windows, I have a datasource connected to Oracle 11g. I can submit a query as follows:
<cfquery name="qry_Test" datasource="dsn_orcl" maxrows="100">
SELECT TRANSID FROM TBL_TRANS
</cfquery>
This will return my 100 rows of transaction IDs. But as I increase the number of columns in the query, the amount of rows I can successfully return goes down.
<cfquery name="qry_Test" datasource="dsn_orcl" maxrows="50">
SELECT TRANSID, TRANSDATE FROM TBL_TRANS
</cfquery>
The maxrows=50 setting is arbitrary, but if I exceed a certain number, say 50, the page just hangs and hangs. So, as the query width increases, its depth decreases. Never seen this before.
Anybody ever experienced this?
Instead of using tag for limiting the number of rows returned you can try limiting it from the database side.
Try this code:-
<cfquery name="qry_Test" datasource="dsn_orcl">
SELECT * FROM (SELECT TRANSID, TRANSDATE
FROM TBL_TRANS ORDER BY TRANSID) TB_TRANSACTION
WHERE rownum <= 50
ORDER BY rownum;
</cfquery>
Please let me know if this helps.
Related
We have a code to sort data and paginate the same and render the data to a Jqgrid. The code works fine when it is connected to an SQL server. That is on paginating each page returns distinct data as expected. But on connecting to an oracle server after some point of time the duplicate data are rendered. Both Oracle and SQL server has same data. Parameters in the Jqgrid page and the number of pages are working as expected on the server-side. That is on paging the start point and chunk size is correctly transferred to the server-side. The duplicate values are observed after sorting columns that are of type varchar in the database but hold numeric also. The database status column holds values of 3 and A, after sorting with the status column the duplicate data when the paginating issue is observed. Duplicate data in the sense, that data on page 2 will be the same as data on page 3. Any help will be appreciated. Thanks in advance...
Query One:-
select * from ( select row_.*, rownum rownum_ from ( Select x,y,z,status FROM tablename c WHERE status IN('in condition seperated with status') ORDER BY status asc ) row_ where rownum <= 30 ) where rownum_ > 20;
Query Two:-
select * from ( select row_.*, rownum rownum_ from ( Select x,y,z,status FROM tablename c WHERE status IN('in condition seperated with status') ORDER BY status asc ) row_ where rownum <= 20 ) where rownum_ > 10;
Here the query 1 and 2 always return the same results.
Where two or more values in the column of your ORDER BY clause are the same, you must always provide another secondary column to rank. Otherwise, data return has only a probability of fetching correct result as we expect. The possibility of getting a correct answer will be same as rolling a dice. The secondary column must be unique for accurate results. While you might be able to assume that they will sort themselves based on order entered
select * from( select row_.*, rownum rownum_ from( Select x,y,z,status FROM tablename c WHERE status IN('in condition seperated with status') ORDER BY status,x asc ) row_ where rownum <= 30 ) where rownum_ > 20;
Hoping x is a unique value. DBMS_RANDOM.VALUE can also be used in case if is an oracle specific query other than adding extra order by clause
I need help from DBAs here.
I have a query that fetches around 1800 records from DB.
However, it is observed that oracle's temp table space is getting filled up and makes oracle to respond too slow.
I have identified the query that is causing the issue and the query is something like this.
SELECT * FROM A a, B b WHERE a.id = b.fieldId AND b.col1 = :1 AND b.col2 = :2 ORDER BY TO_NUMBER(b.col3) ASC
This is query is returning around 1800 records and DBA segments show that 44 GB out of 50 GB of data is occupied.
I am not sure what could be solution for this.
I am using Oracle 12.1
Please look into this and suggest if i have to rewrite the query.
Thanks in Advance.
It is hard to how resource intensive query is without checking query plan at least.
This might be not your query at all who "ate" all the TEMP space. Here is how to get top 20 session with highest TEMP usage.
select round(u.blocks*8192/1024/1024,2) "TEMP usage, Mb",
s.sid, s.osuser, s.machine, s.module, s.action, s.status, s.event, s.LAST_CALL_ET, s.WAIT_TIME, s.sql_id, s.sql_child_number
from v$session s,
v$sort_usage u
where s.saddr = u.session_addr
order by u.blocks desc
fetch first 20 rows with ties
So I have a simple table with 5 or so columns, one of which is a clob containing some JSON data.
I am running
SELECT * FROM BIG_TABLE
SELECT * FROM BIG_TABLE WHERE ROWNUM < 2
SELECT * FROM BIG_TABLE WHERE ROWNUM = 1
SELECT * FROM BIG_TABLE WHERE ID=x
I expect that any fractionally intelligent relational database would return the data immediately. We are not imposing order by/group by clauses, so why not return the data as and when you find it?
Of all the forms of SELECT statements above, only 4. returned in a sub-second manner. This is unexpected for 1-3 which are returning between 1 and 10 minutes before the query shows any responses in SQL Developer. SQL Developer has the standard SQL Array Fetch Size of 50 (JDBC Fetch size of 50 rows) so at a minimum, it is taking 1-10 minutes to return 50 rows from a simple table with no joins on a super high-performance RAC cluster backed by fancy 4-tiered EMC disk subsystem.
Explain plans show a table scan. Fine, but why should I wait 1-10 minutes for the results with rownum in the WHERE clause?
What is going on here?
OK - I found the issue. ROWNUM does not operate like I thought it did and in the code above it never stops the full table scan.
This is because:
RowNum is assigned during the predicate operation (where clause evaluation) and incremented afterwards, i.e.: your row makes it into the result set and then gets rownum assigned.
In order to filter by rownum you need to already have it exist, something like ...
SELECT * FROM (SELECT * FROM BIG_TABLE) WHERE ROWNUM < 1
In effect what this means is that there is no way to filter out the top 5 rows from a table without having first selected the entire table if no other filter criteria are involved.
I solved my problem like this...
SELECT * FROM (SELECT * FROM BIG_TABLE WHERE
DATE_COL BETWEEN :Date1 AND :Date2) WHERE ROWNUM < :x;
I need to select rows randomly from an Oracle DB.
Ex: Assume a table with 100 rows, how I can randomly return 20 of those records from the entire 100 rows.
SELECT *
FROM (
SELECT *
FROM table
ORDER BY DBMS_RANDOM.RANDOM)
WHERE rownum < 21;
SAMPLE() is not guaranteed to give you exactly 20 rows, but might be suitable (and may perform significantly better than a full query + sort-by-random for large tables):
SELECT *
FROM table SAMPLE(20);
Note: the 20 here is an approximate percentage, not the number of rows desired. In this case, since you have 100 rows, to get approximately 20 rows you ask for a 20% sample.
SELECT * FROM table SAMPLE(10) WHERE ROWNUM <= 20;
This is more efficient as it doesn't need to sort the Table.
SELECT column FROM
( SELECT column, dbms_random.value FROM table ORDER BY 2 )
where rownum <= 20;
In summary, two ways were introduced
1) using order by DBMS_RANDOM.VALUE clause
2) using sample([%]) function
The first way has advantage in 'CORRECTNESS' which means you will never fail get result if it actually exists, while in the second way you may get no result even though it has cases satisfying the query condition since information is reduced during sampling.
The second way has advantage in 'EFFICIENT' which mean you will get result faster and give light load to your database.
I was given an warning from DBA that my query using the first way gives loads to the database
You can choose one of two ways according to your interest!
In case of huge tables standard way with sorting by dbms_random.value is not effective because you need to scan whole table and dbms_random.value is pretty slow function and requires context switches. For such cases, there are 3 additional methods:
1: Use sample clause:
https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6
https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/SELECT.html#GUID-CFA006CA-6FF1-4972-821E-6996142A51C6
for example:
select *
from s1 sample block(1)
order by dbms_random.value
fetch first 1 rows only
ie get 1% of all blocks, then sort them randomly and return just 1 row.
2: if you have an index/primary key on the column with normal distribution, you can get min and max values, get random value in this range and get first row with a value greater or equal than that randomly generated value.
Example:
--big table with 1 mln rows with primary key on ID with normal distribution:
Create table s1(id primary key,padding) as
select level, rpad('x',100,'x')
from dual
connect by level<=1e6;
select *
from s1
where id>=(select
dbms_random.value(
(select min(id) from s1),
(select max(id) from s1)
)
from dual)
order by id
fetch first 1 rows only;
3: get random table block, generate rowid and get row from the table by this rowid:
select *
from s1
where rowid = (
select
DBMS_ROWID.ROWID_CREATE (
1,
objd,
file#,
block#,
1)
from
(
select/*+ rule */ file#,block#,objd
from v$bh b
where b.objd in (select o.data_object_id from user_objects o where object_name='S1' /* table_name */)
order by dbms_random.value
fetch first 1 rows only
)
);
To randomly select 20 rows I think you'd be better off selecting the lot of them randomly ordered and selecting the first 20 of that set.
Something like:
Select *
from (select *
from table
order by dbms_random.value) -- you can also use DBMS_RANDOM.RANDOM
where rownum < 21;
Best used for small tables to avoid selecting large chunks of data only to discard most of it.
Here's how to pick a random sample out of each group:
SELECT GROUPING_COLUMN,
MIN (COLUMN_NAME) KEEP (DENSE_RANK FIRST ORDER BY DBMS_RANDOM.VALUE)
AS RANDOM_SAMPLE
FROM TABLE_NAME
GROUP BY GROUPING_COLUMN
ORDER BY GROUPING_COLUMN;
I'm not sure how efficient it is, but if you have a lot of categories and sub-categories, this seems to do the job nicely.
-- Q. How to find Random 50% records from table ?
when we want percent wise randomly data
SELECT *
FROM (
SELECT *
FROM table_name
ORDER BY DBMS_RANDOM.RANDOM)
WHERE rownum <= (select count(*) from table_name) * 50/100;
I use SET ROWCOUNT 27900 And then select two columns:
Select
emp.employeeid,
empd.employeedetailid
From
employee emp (NOLOCK)
join
employeedetail empd (NOLOCK) on emp.employeeid = empd.employeeid
This query executes in 3 sec
If I use SET ROWCOUNT 27950 then the same query takes 20 sec to execute.
I am not a sql DBA, why there is a difference of 17 sec for just 50 rows. Is this anything related to page size or index?
Can anyone help me to fine tune the query?
Have you tried doing this using TOP instead of SET ROWCOUNT, and adding an ORDER BY?
SELECT TOP 27900 emp.employeeid, ...
ORDER BY ...;
This will give the optimizer a much better chance at optimizing. In simplest terms, SET ROWCOUNT is applied after the entire query has been processed, and only affects the rows that are sent back to the caller...