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
Related
I have two queries:
select * from PRE_DETAIL_REPORT a where item = (select item from apple_skus);
select * from PRE_DETAIL_REPORT a where item IN ('100299122');
the table: APPLE_SKUS
only has one item: 100299122
When I run the first query, it takes 2 minutes to execute
When I run the second query, it takes 3 seconds to execute
What can be the reason?
you can rewrite it in this way:
select a.* from PRE_DETAIL_REPORT a
join apple_skus t on t.item = a.item;
Its the way a sql query syntax works
You have manual values for selection in your 2nd query but in the first case you have subquery specified so again a
FROM CLAUSE, N THEN SELECT so, Querying a table will take
more time than a hardcode value even when theres a single record
You could try EXISTS as it uses correlated subquery which would be much faster
Select * from table t1 where exists (select 1 from table
where
Item =t1.item)
It’s very likely that the difference is due to a different access to PRE_DETAIL_REPORT; and as mentioned earlier by someone, an explain plan (or SQL Monitor report) will tell you the answer.
But until you provide the diagnostic, this is just a guess…
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
I am trying to execute the following query in pgSQL.
select count(id) from master
where act1=true or act2=true
and regn_no>2000;
But the query return all rows who satisfies the 'act1=true or act2=true' condition. It doesn't check act1=true or act2=true.Why?
SQLFIDDLE:
http://sqlfiddle.com/#!9/23b1f/1
because AND priority is higher than OR then sql parse your where clause as
(act1=true) or (act2=true
and regn_no>2000);
you must rewrite your query as follow
select count(id) from master
where (act1=true or act2=true)
and regn_no>2000;
I want to check whether a table contains a row or not. Which is faster?
IF EXISTS(SELECT * FROM TABLE)
or
IF EXISTS(SELECT TOP 1 * FROM TABLE)
There is no difference between the queries!
The columns in the select don't get evaluated.
If you recall Logical Query processing, the from clause is executed first. The select clause is executed in the last step (actually Order By is, but that is a cosmetic thing).
So when the from clause gets executed, there are rows returned, regardless of the column names.
You have to add columnnames because otherwise you get syntax errors
IF EXISTS(SELECT 1 FROM TABLE)
is faster
there are some more suggestions
IF EXISTS(SELECT null FROM TABLE)
Obviously SELECT TOP 1 * FROM TABLE is faster.
since the index scan is reduced to one,number of rows return is one and
the estimated operation cost is also much less.
but if there is only on row in the table both the queries will show same operation cost.
SELECT *
FROM (SELECT top 1 *
FROM ms_data) temp
SELECT TOP 1 *
FROM ms_data
both the above queries have same operation cost.
What's the execute order of the different parts of a SQL select statement? Such as
distinct
from
order by
group by
having
multiline function(count, avg, max, min...)
top(sql server) or limit(mysql)
other parts
Does the different databases have the same execution order?
Great thanks.
Have a look at
SQL SERVER – Logical Query Processing Phases – Order of Statement Execution
FROM
ON
OUTER
WHERE
GROUP BY
CUBE | ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
Also, for some good info see Logical Query Processing
The above answer addresses the question but there is one exception to the above mentioned order
when you have
select top n ............
order by
Then, order by will be executed before select. ( the entries are ordered first and then the top n entries are selected)