I have an Oracle table with 20 million users,
I would like to query the table for users with first name Like "Patel" or "Pat" performance when querying using "like clause" is very bad.
select * from users where first name like '%Patel%'
Or
select * from users where first name like '%Pat%'
And as far as I know if I will restrict the results by rownum - it will happen only after the LIKE - so I have a full table scan...
I don't want to scan the entire 20 Million records
select * from users where first name like '%Pat%' where rownum<100
Is it possible to tell oracle to stop after finding 100 rows?
Oracle 12c (finally) introduced the fetch first syntax, which should perform a bit better:
SELECT *
FROM users
WHERE first_name LIKE '%Pat%'
FETCH FIRST 100 ROWS ONLY
select * from users where first name like '%Pat%' where rownum<100
Oracle is smart enough to do everything for You. Execution plan for this query is:
SELECT STATEMENT, GOAL = ALL_ROWS
COUNT STOPKEY
TABLE ACCESS FULL
COUNT STOPKEY means that full scan will be stopped when Oracle will find enough records to satisfy the condition.
Since the question is tagged Oracle 11g, I'll give an answer that works in 11g.
Use the optimizer hint for first_rows_100 and wrap it into an inline view.
Example:
select *
from (select /*+ opt_param('optimizer_mode','first_rows_100') */
u.*, rownum as rn
from users u
where instr (name, 'Pat') > 0 or instr (name, 'Patel') > 0) inlineview
where rn <= 100
Regards
Olafur
Related
I am using DBeaver to query a PostgreSQL database.
I have this query, it simply selects the highest id per Enterprise_Nbr. The query works but is really slow. Is there any way I can rewrite the query to improve performance.
I am using the querytool DBeaver because I don't have direct access to PostgreSQL. The ultimate goal is to link the PostgreSQL with PowerBi.
select *
from public.address
where "ID" in (select max("ID")
from public.address a
group by "Enterprise_Nbr")
Queries for greatest-n-per-group problems are typically faster if done using Postgres' proprietary distinct on () operator
select distinct on ("Enterprise_Nbr") *
from public.address
order by "Enterprise_Nbr", "ID" desc;
Your query could rewrite as: per each value of Enterprise_Nbr, retrieve row which there is not exists other rows that have same Enterprise_Nbr and greater ID.
SELECT *
FROM public.address a
WHERE NOT EXISTS (
SELECT 1
FROM public.address b
WHERE b.Enterprise_Nbr = a.Enterprise_Nbr AND b.ID > a.ID
)
I need to fetch 4 random values from each category. What should be the correct sql syntax for maria db. I have attached one image of table structure.
Please click here to check the structure
Should i write some procedure or i can do it with basic sql syntax?
You can do that with a SQL statement if you only have a few rows:
SELECT id, question, ... FROM x1 ORDER BY rand() LIMIT 1
This works fine if you have only a few rows - as soon as you have thousands of rows the overhead for sorting the rows becomes important, you have to sort all rows for getting only one row.
A trickier but better solution would be:
SELECT id, question from x1 JOIN (SELECT CEIL(RAND() * (SELECT(MAX(id)) FROM x1)) AS id) as id using(id);
Running EXPLAIN on both SELECTS will show you the difference...
If you need random value for different categories combine the selects via union and add a where clause
http://mysql.rjweb.org/doc.php/groupwise_max#top_n_in_each_group
But then ORDER BY category, RAND(). (Your category is the blog's province.)
Notice how it uses #variables to do the counting.
If you have MariaDB 10.2, then use one of its Windowing functions.
SELECT column FROM table WHERE category_id = XXX
ORDER BY RAND()
LIMIT 4
do it for all categories
I have this SQL query:
SELECT * FROM [table] WHERE 2=3;
Obviously, the query will return 0 rows, but my question is as follows. Does the SQL Server engine evaluate the where condition before executing the selection?
In other words, does SQL Server detect the where condition is impossible and return 0 rows automatically?
If you examine the execution plan for a simple query using such a predicate you will see that SQL Server uses a Constant Scan to determine that zero results will return. No indexes or tables are even touched.
SELECT * FROM [Production].[Product];
SELECT * FROM [Production].[Product] WHERE 1=0;
Basically order is given here
This is order of query execution at SQL:
FROM
ON
OUTER
WHERE
GROUP BY
CUBE | ROLLUP
HAVING
SELECT
DISTINCT 10 ORDER BY
TOP
I'm using sql-server 2005 and ASP.NET with C#.
I have Users table with
userId(int),
userGender(tinyint),
userAge(tinyint),
userCity(tinyint)
(simplified version of course)
I need to select always two fit to userID I pass to query users of opposite gender, in age range of -5 to +10 years and from the same city.
Important fact is it always must be two, so I created condition if ##rowcount<2 re-select without age and city filters.
Now the problem is that I sometimes have two returned result sets because I use first ##rowcount on a table. If I run the query.
Will it be a problem to use the DataReader object to read from always second result set? Is there any other way to check how many results were selected without performing select with results?
Can you simplify it by using SELECT TOP 2 ?
Update: I would perform both selects all the time, union the results, and then select from them based on an order (using SELECT TOP 2) as the union may have added more than two. Its important that this next select selects the rows in order of importance, ie it prefers rows from your first select.
Alternatively, have the reader logic read the next result-set if there is one and leave the SQL alone.
To avoid getting two separate result sets you can do your first SELECT into a table variable and then do your ##ROWCOUNT check. If >= 2 then just select from the table variable on its own otherwise select the results of the table variable UNION ALLed with the results of the second query.
Edit: There is a slight overhead to using table variables so you'd need to balance whether this was cheaper than Adam's suggestion just to perform the 'UNION' as a matter of routine by looking at the execution stats for both approaches
SET STATISTICS IO ON
Would something along the following lines be of use...
SELECT *
FROM (SELECT 1 AS prio, *
FROM my_table M1 JOIN my_table M2
WHERE M1.userID = supplied_user_id AND
M1.userGender <> M2.userGender AND
M1.userAge - 5 >= M2.userAge AND
M1.userAge + 15 <= M2.userAge AND
M1.userCity = M2.userCity
LIMIT TO 2 ROWS
UNION
SELECT 2 AS prio, *
FROM my_table M1 JOIN my_table M2
WHERE M1.userID = supplied_user_id AND
M1.userGender <> M2.userGender
LIMIT TO 2 ROWS)
ORDER BY prio
LIMIT TO 2 ROWS;
I haven't tried it as I have no SQL Server and there may be dialect issues.
Is there a way to only show the first N lines of output from an SQL query? Bonus points, if the query stops running once the N lines are outputted.
I am most interested in finding something which works in Oracle.
It would be helpful if you specify what database you are targetting. Different databases have different syntax and techniques to achieve this:
For example in Oracle you can ahieve this by putting condition on RowNum (select ... from ... where ... rownum < 11 -> would result in outputting first 10 records)
In MySQL you can use you can use limit clause.
Microsoft SQL Server => SELECT TOP 10 column FROM table
PostgreSQL and MySQL => SELECT column FROM table LIMIT 10
Oracle => select * from (SELECT column FROM table ) WHERE ROWNUM <= 10 (thanks to stili)
Sybase => SET rowcount 10 SELECT column FROM table
Firebird => SELECT FIRST 10 column FROM table
NOTE: Modern ORM tools such as Hibernate give high level API (Query, Restriction, Condition interfaces) that abstract the logic of top n rows based on the dialect you choose.
For Oracle the suggested and accepted solution is wrong. Try using an order clause, and the results will be unpredictable. The SQL will need to be nested to accomplish this in Oracle.
select name, price
from (
select name, price, row_number() over (order by price) r
from items
)
where r between 1 and 5;
The example above was borrowed from http://www.adp-gmbh.ch/ora/sql/examples/first_rows.html which has a good discussion on this topic.
I know it with MySQL but I don't know if it's standard SQL :
end you Query with 'limit X', X = n. of lines you want to get.
Example :
SELECT NAME FROM EMPLOYEES ORDER BY SALARY DESC LIMIT 10;
For Oracle, you can try this
select /*+ FIRST_ROWS(10) */ * from table;