I have a quite complicated view (using several layers of views across several database links) which takes a second to return all of it's rows. But, when I ask for distinct rows, it takes considerably more time. I stopped waiting after 4 minutes.
To make my self as clear as possible:
select a, b from compicated_view; -- takes 1 sec (returns 6 rows)
select distinct a, b from compicated_view; -- takes at least 4 minutes
I find that pretty weird, but hey, that's how it is. I guess Oracle messed something up when planing that query. Now, is there a way to force Oracle to first finish the select without distinct, and then do a "select distinct *" on the results? I looked into optimizer hints, but I can't find anything about hinting the order in which distinct is applied (this is first time I'm optimizing a query, obviously :-/).
I'm using Oracle SQl Developer on Oracle 10g EE.
Try:
SELECT DISTINCT A,B FROM (
SELECT A,B FROM COMPLICATED_VIEW
WHERE rownum > 0 );
this forces to materialize the subquery and prevents from view merging/predicate pushing, and likely from changing the original plan of the view.
You may also try NO_MERGE hint:
SELECT /*+ NO_MERGE(alias) */
DISTINCT a,b
FROM (
SELECT a,b FROM COMPLICATED_VIEW
) alias
Since you haven't posted details... try the following:
SELECT DISTINCT A,B
FROM
(SELECT A,B FROM COMPLICATED_VIEW);
Related
I am facing a big performance problem when trying to get a list of objects with pagination from an oracle11g database.
As far as I know and as much as I have checked online, the only way to achieve pagination in oracle11g is the following :
Example : [page=1, size=100]
SELECT * FROM
(
SELECT pagination.*, rownum r__ FROM
(
select * from "TABLE_NAME" t
inner join X on X.id = t.id
inner join .....
where ......
order
) pagination
WHERE rownum <= 200
)
WHERE r__ > 100
The problem in this query, is that the most inner query fetching data from the table "TABLE_NAME" is returning a huge amount of data and causing the overall query to take 8 seconds (there are around 2 Million records returned after applying the where clause, and it contains 9 or 10 join clause).
The reason of this is that the most inner query is fetching all the data that respects the where clause and then the second query is getting the 200 rows, and the third to exclude the first 100 to get the second pages' data we need.
Isn't there a way to do that in one query, in a way to fetch the second pages' data that we need without having to do all these steps and cause performance issues?
Thank you!!
It depends on your sorting options (order by ...): database needs to sort whole dataset before applying outer where rownum<200 because of your order by clause.
It will fetch only 200 rows if you remove your order by clause. In some cases oracle can avoid sort operations (for example, if oracle can use some indexes to get requested data in the required order). Btw, Oracle uses optimized sorting operations in case of rownum<N predicates: it doesn't sort full dataset, it just gets top N records instead.
You can investigate sort operations deeper using sort trace event: alter session set events '10032 trace name context forever, level 10';
Furthermore, sometimes it's better to use analytic functions like
select *
from (
select
t1.*
,t2.*
,row_number()over([partition by ...] order by ...) rn
from t1
,t2
where ...
)
where rn <=200
and rn>=100
because in some specific cases Oracle can transform your query to push sorting and sort filter predicates to the earliest possible steps.
I have to join 3 tables to retrieve data and looks like full outer join is a potential solution but during my try it took more than a hour to execute the query .
Any alternatives would be helpful .
thank you.
Not sure what your query looks like however, add indexes on the table if these tables are newly being created.
However, answering your question using UNION ALL will be faster, as it simply passes the first SELECT statement, and then parses the second SELECT statement and adds the results to the end of the output table. Even a Normal UNION is faster than a join.
The UNION's will make better use of indexes which could result in a faster query.
In my company's application there is a query in oracle using parallel execution (configured to 4 servers), it wasn't me who built it, but the developer put it that way for performance.
The query makes joins between views and tables and the weirdest thing is: sometimes it returns 11k results (incorrect), sometimes 27k results (correct).
After much research I found out that if I removed this parallel thing, it always returns the correct number: 27k. And if I increase the number of server to 6 or 7, it always returns the incorrect number: 11k.
The layout of the query is like this:
SELECT /*+ PARALLEL(NAME, 4) */ * FROM(
SELECT DISTINCT COLUMNS
FROM VIEW
JOIN TABLE1 ON (....)
JOIN TABLE2 ON (....)
JOIN TABLE3 ON (....)
ORDER BY 3
) NAME
Anyone has any idea why? I don't know much about this subject.
I am having a problem with a simple query of a view. The view itself is nasty having union_all and most of its fields evaluated by functions. The problem is that even a very simple query can get the whole view evaluated which then takes forever.
To simplify my problem I have made this two queries
select * from horrible_view where id in (1, 2, 3); -- this works super quick
select * from horrible_view where id in (select id from my_temp_table); -- this takes forever
select * from my_temp_table t left join horrible_view h on t.id=h.id -- this also takes forever
So even though my_temp_table contains the same 1, 2, 3 (3 rows with just one column) the second and third query makes the optimizer go bananas, so that the horrible view is evaluated completly and then returns just the 3 rows are returned.
Is there anything I could do?
IE in the second example make optimizer run the "select id from my_temp_table" first and then the query.
Oracle's version is 11.2
Further info:
First off I know only a little about Oracle. (So is the Explain plan from sqlDeveloper the thing you want? If so how can you print it, its huge?) Well here is the screenshot: here
I have created the temp table just in order to test. Its actually named TMP_R (and it has 10 rows)
I want a query that selects the number of rows in each table
but they are NOT updated statistically .So such query will not be accurate:
select table_name, num_rows from user_tables
i want to select several schema and each schema has minimum 500 table some of them contain a lot of columns . it will took for me days if i want to update them .
from the site ask tom he suggest a function includes this query
'select count(*)
from ' || p_tname INTO l_columnValue;
such query with count(*) is really slow and it will not give me fast results.
Is there a query that can give me how many rows are in table in a fast way ?
You said in a comment that you want to delete (drop?) empty tables. If you don't want an exact count but only want to know if a table is empty you can do a shortcut count:
select count(*) from table_name where rownum < 2;
The optimiser will stop when it reaches the first row - the execution plan shows a 'count stopkey' operation - so it will be fast. It will return zero for an empty table, and one for a table with any data - you have no idea how much data, but you don't seem to care.
You still have a slight race condition between the count and the drop, of course.
This seems like a very odd thing to want to do - either your application uses the table, in which case dropping it will break something even if it's empty; or it doesn't, in which case it shouldn't matter whether it has (presumably redundant) and it can be dropped regardless. If you think there might be confusion, that sounds like your source (including DDL) control needs some work, maybe?
To check if either table in two schemas have a row, just count from both of them; either with a union:
select max(c) from (
select count(*) as c from schema1.table_name where rownum < 2
union all
select count(*) as c from schema2.table_name where rownum < 2
);
... or with greatest and two sub-selects, e.g.:
select greatest(
(select count(*) from schema1.table_name where rownum < 2),
(select count(*) from schema2.table_name where rownum < 2)
) from dual;
Either would return one if either table has any rows, and would only return zero f they were both empty.
Full Disclosure: I had originally suggested a query that specifically counts a column that's (a) indexed and (b) not null. #AlexPoole and #JustinCave pointed out (please see their comments below) that Oracle will optimize a COUNT(*) to do this anyway. As such, this answer has been altered significantly.
There's a good explanation here for why User_Tables shouldn't be used for accurate row counts, even when statistics are up to date.
If your tables have indexes which can be used to speed up the count by doing an index scan rather than a table scan, Oracle will use them. This will make the counts faster, though not by any means instantaneous. That said, this is the only way I know to get an accurate count.
To check for empty (zero row) tables, please use the answer posted by Alex Poole.
You could make a table to hold the counts of each table. Then, set a trigger to run on INSERT for each of the tables you're counting that updates the main table.
You'd also need to include a trigger for DELETE.