I have a view that is very slow if you fetch all rows. But if I select a subset (providing an ID in the where clause) the performance is very good. I cannot hardcode the ID so I create a sub select to get the ID from another table. The sub select only returns one ID. Now the performance is very slow and it seems like Oracle is evaluating the whole view before using the where clause. Can I somehow help Oracle so SQL 2 and 3 have the same performance? I’m using Oracle 10g
1 slow
select * from ci.my_slow_view
2 fast
select * from ci.my_slow_view where id = 1;
3 slow
select * from ci.my_slow_view where id in (select id from active_ids)
How about
select * from ci.my_slow_view where id = (select id from active_ids)
Replacing the "in" with an "=" will tell Oracle that you expect the "select id from active_ids" to return only a single row.
This is the expected behavior...
3 is slow because Oracle will perform a "full table scan", which means that your indexes are not helping there (your where clause does not contain any constant or range and is unbounded, which implies that whatever index you use, all the rows are potentially candidates for the join condition.
Possible improvment:
First, check that the indexes are ok on your join/pk columns (id in my_slow_view and active_ids). This is necessary for the second step:
Second, generate table statistics for your table and views, that will make the Oracle cache memory optimizer kicks in.
(It should work because it is assumed that your active_ids table is small enough to be fully in memory.)
Second approach:
Write a stored procedure in PL/SQL where your id is an in parameter and rewrite your SQL so that it is used a bound parameter.
That should give you the flexibility you need (no hard coded ids), and the speed of the fastest query.
I cannot hardcode the ID so I create a
sub select to get the ID from another
table. The sub select only returns one
ID.
Most likely, gathering statistics on the small table (while it contains a single row) will help, since that should help Oracle realize that it is small and encourage it to use the index on ID.
However, it sounds like this is really not the right solution to your original problem. Generally, when one wants to perform a query repeatedly with a different lookup value, the best way is to use a bind variable. The basic method of doing this in SQLPlus would be:
SQL> variable id number
SQL> exec :id := 1;
SQL> select * from ci.my_slow_view where id = :id ;
SQL> exec :id := 2;
SQL> select * from ci.my_slow_view where id = :id ;
The details of implementing this depend on the environment you are developing in.
Or:
select * from ci.my_slow_view, active_ids
where my_slow_view.id = active_ids.id;
Related
I created a table in oracle like
CREATE TABLE suppliers AS (SELECT * FROM companies WHERE id > 1000);
I would like to know the complete select statement which was used to create this table.
I have already tried get_ddl but it is not giving the select statement. Can you please let me know how to get the select statement?
If you're lucky one of these statements will show the DDL used to generate the table:
select *
from gv$sql
where lower(sql_fulltext) like '%create table suppliers%';
select *
from dba_hist_sqltext
where lower(sql_text) like '%create table%';
I used the word lucky because GV$SQL will usually only have results for a few hours or days, until the data is purged from the shared pool. DBA_HIST_SQLTEXT will only help if you have AWR enabled, the statement was run in the last X days that AWR is configured to hold data (the default is 8), the statement was run after the last snapshot collection (by default it happens every hour), and the statement ran long enough for AWR to think it's worth saving.
And for each table Oracle does not always store the full SQL. For security reasons, DDL statements are often truncated in the data dictionary. Don't be surprised if the text suddenly cuts off after the first N characters.
And depending on how the SQL is called the case and space may be different. Use lower and lots of wildcards to increase the chance of finding the statement.
TRY THIS:
select distinct table_name
from
all_tab_columns where column_name in
(
select column_name from
all_tab_columns
where table_name ='SUPPLIERS'
)
you can find table which created from table
I am using Oracle SQL Developer and have a rather large query built. The query is going to be run on a monthly or quarterly basis. I was wondering if there was a way that I can do a declare statment up top and then in the code just reference these variables created. That way when someone wants to run the query they can just change the dates at the top of the code rather then have to dig through all of it. I am kind of new to Oracle SQL Developer but I know in other sql codes I built I could simply declare the variable and then set it and then in the code call the variable name. Below is an example of what I know how to do but i am having trouble in Oracle SQL Developer.
Example: I have a data base that contains the columns Business, business type(small,medium,large) number of deposits, deposit amount and deposit date. I want to build a query that outputs a quarterly summary of the number of deposits and the deposit amount and be able to change the quarter and size of the business.
Example Code from my previous SQL expereince this is an example of what I am trying to do since i can not disclose my code with the table names etc in them.
Declare #busstype,#qbegindate,#qenddate
Set #busstype = 'small'
Set #qbegindate = '01-JAN-2013'
Set #qenddate = '01-MAR-2013'
Select business,numberofdeposits,depositamount
From business_transactions
Where ('#qbegindate'<=depositdate<='#qenddate'
And businesstype = '#busstype')
Group By Business
The results would list out the businesses name and then the total deposits and total amount.
I know this code is not right but its just an example of what I am looking to do in Oracle SQL Developer. The query I have built is working fine I just find it a pain to dig through the code to change dates and criteria and was wondering how I would do something like this since i have figured out that I am not able to do this in ORACLE Sql Developer.
Here is an example with predefined variable:
set feedback off
var abc varchar2
begin
:abc := 'abc';
end;
/
select :abc as a from dual;
Output:
A
--------------------------------
abc
Common table expressions allow variables to be defined at the top of the query. For performance and style reasons this is generally not a good way to
use common table expressions. The advantage is this query can be run in any IDE and it is completely self-contained.
--Variables - change these before running.
with busstype as (select 'small' value from dual),
qbegindate as (select date '2013-01-01' value from dual),
qenddate as (select date '2013-03-01' value from dual)
--Query - do not modify code below.
select business,numberofdeposits,depositamount
from business_transactions
where depostiddate between
(select value from qbegindate)
and
(select value from qenddate)
and businesstype = (select value from busstype)
group by business, numberofdeposits,depositamount;
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.
I was trying to get the count from a table with millions of entries. My query looks somewhat like this:
Select count(*)
from Users
where status = 'A' and office_id = '000111' and user_type = 'C'
Status can be A or C, User Type can be C or R.
Status, Office_id and User_type are Strings
The result has around 10 million rows, and its taking a lot of time. I just want the total count.
Would appreciate if anyone could tell me why its taking this much time, and workaround if any.
Do let me know in case of any more details required.
The database engine is Oracle 11g
Edit: I Added index for all three columnns. Still theres no improvement. Also tried the below query, but it always returns the total count in the table without checking the conditions.
SELECT COUNT(office_id_key)
FROM Users
WHERE EXISTS (SELECT * FROM Users WHERE status = 'A' AND office_id = '000111' AND user_type = 'C')
Why not just simply create indexes on the table on age and place this way your search will be faster then simply scanning the entire table for these values.
CREATE INDEX age_index ON Employee(age);
CREATE INDEX place_index ON Employee(place);
This should speed up the process.
AMENDED BASED ON QUERY CHANGE
CREATE INDEX status_index ON Users(status);
CREATE INDEX office_id_index ON Users(office_id);
CREATE INDEX user_type_index ON Users(user_type);
You'll want to create the following multi-column index on the Users table to improve the query:
(office_id, status, user_type)
The database can use a "covering" index with COUNT(*). Create the index with the columns in that order, due to cardinality.
After adding the indexes, I think changing where to where exists and a subquery may help as well.
Edit2: removed exists as it was returning all valid, usually the subquery has multiple joins, but I guess the case with one table returns all true. I read that count is optimized to act similar to exists when it has only one table and no where clause, so I treat the results as a table. Hopefully, this will have the same quick results.
select count(1) from
(select 1 from Employee where age = '25' and place = 'bricksgate')
Edit: When you use 'where exists' the db server doesn't load your data into memory and also takes advantage of the indexes because you will be reading values from the indexes not doing costly table lookups. You may also want to change count(*) to count(place) - that way it will limit the fields to an indexed field as well.
In your original query, your data was doing table lookups and then loading them into memory just to be counted.
count(1) works faster than count(*)
I am trying to execute a query like
select * from tableName where rownum=1
This query is basically to fetch the column names of the table.There are more than million records in the table.When I put the above condition its taking so much time to fetch the first row.Is there any alternate to get the first row.
This question has already been answered, I will just provide an explanation as to why sometimes a filter ROWNUM=1 or ROWNUM <= 1 may result in a long response time.
When encountering a ROWNUM filter (on a single table), the optimizer will produce a FULL SCAN with COUNT STOPKEY. This means that Oracle will start to read rows until it encounters the first N rows (here N=1). A full scan reads blocks from the first extent to the high water mark. Oracle has no way to determine which blocks contain rows and which don't beforehand, all blocks will therefore be read until N rows are found. If the first blocks are empty, it could result in many reads.
Consider the following:
SQL> /* rows will take a lot of space because of the CHAR column */
SQL> create table example (id number, fill char(2000));
Table created
SQL> insert into example
2 select rownum, 'x' from all_objects where rownum <= 100000;
100000 rows inserted
SQL> commit;
Commit complete
SQL> delete from example where id <= 99000;
99000 rows deleted
SQL> set timing on
SQL> set autotrace traceonly
SQL> select * from example where rownum = 1;
Elapsed: 00:00:05.01
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=7 Card=1 Bytes=2015)
1 0 COUNT (STOPKEY)
2 1 TABLE ACCESS (FULL) OF 'EXAMPLE' (TABLE) (Cost=7 Card=1588 [..])
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
33211 consistent gets
25901 physical reads
0 redo size
2237 bytes sent via SQL*Net to client
278 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
As you can see the number of consistent gets is extremely high (for a single row). This situation could be encountered in some cases where for example, you insert rows with the /*+APPEND*/ hint (thus above high water mark), and you also delete the oldest rows periodically, resulting in a lot of empty space at the beginning of the segment.
Try this:
select * from tableName where rownum<=1
There are some weird ROWNUM bugs, sometimes changing the query very slightly will fix it. I've seen this happen before, but I can't reproduce it.
Here are some discussions of similar issues: http://jonathanlewis.wordpress.com/2008/03/09/cursor_sharing/ and http://forums.oracle.com/forums/thread.jspa?threadID=946740&tstart=1
Surely Oracle has meta-data tables that you can use to get column names, like the sysibm.syscolumns table in DB2?
And, after a quick web search, that appears to be the case: see ALL_TAB_COLUMNS.
I'd use those rather than go to the actual table, something like (untested):
SELECT COLUMN_NAME
FROM ALL_TAB_COLUMNS
WHERE TABLE_NAME = "MYTABLE"
ORDER BY COLUMN_NAME;
If you are hell-bent on finding out why your query is slow, you should revert to the standard method: asking your DBMS to explain the execution plan of the query for you. For Oracle, see section 9 of this document.
There's a conversation over at Ask Tom - Oracle that seems to suggest the row numbers are created after the select phase, which may mean the query is retrieving all rows anyway. The explain will probably help establish that. If it contains FULL without COUNT STOPKEY, then that may explain the performance.
Beyond that, my knowledge of Oracle specifics diminishes and you will have to analyse the explain further.
Your query is doing a full table scan and then returning the first row.
Try
SELECT * FROM table WHERE primary_key = primary_key_value;
The first row, particularly as it pertains to ROWNUM, is arbitrarily decided by Oracle. It may not be the same from query to query, unless you provide an ORDER BY clause.
So, picking a primary key value to filter by is as good a method as any to get a single row.
I think you're slightly missing the concept of ROWNUM - according to Oracle docs: "ROWNUM is a pseudo-column that returns a row's position in a result set. ROWNUM is evaluated AFTER records are selected from the database and BEFORE the execution of ORDER BY clause."
So it returns ANY row that it consideres #1 in the result set which in your case will contain 1M rows.
You may want to check out a ROWID pseudo-column: http://psoug.org/reference/pseudocols.html
I've recently had the same problem you're describing: I want one row from the very large table as a quick, dirty, simple introspection, and "where rownum=1" alone behaves very poorly. Below is a remedy which worked for me.
Select the max() of the first term of some index, and then use it to choose some small fraction of all rows with "rownum=1". Suppose my table has some index on numerical "group-id", and compare this:
select * from my_table where rownum = 1;
-- Elapsed: 00:00:23.69
with this:
select * from my_table where rownum = 1
and group_id = (select max(group_id) from my_table);
-- Elapsed: 00:00:00.01