After googling i came to know that Index seek is better than scan.
How can I write the query that will yield to seek instead of scan. I am trying to find this in google but as of now no luck.
Any simple example with explanation will be appreciated.
Thanks
Search by the primary key column(s)
Search by column(s) with index(es) on them
An index is a data structure that improves the speed of data retrieval operations on a database table. Most dbs automatically create an index when a primary key is defined for a table. SQL Server creates an index for primary key (composite or otherwise) as a "clustered index", but it doesn't have to be the primary key - it can be other columns.
NOTE:
LIKE '%'+ criteria +'%' will not use an index; LIKE criteria +'%' will
Related reading:
SQL SERVER – Index Seek vs. Index Scan
Index
Which is better: Bookmark/Key Lookup or Index Scan
Extending rexem's feedback:
The clustered index idea for pkeys isn't arbitrary. It's simply a default to make the pkey clustered. And clustered means that values will be physically placed near each other on a Sql Server 8k page thus assuming that if you fetch one value by pkey, you will probably be interested in its neighbors. i don't think it's a good idea to do that for pkeys since they're usually unique but arbitrary identifiers. Better to cluster on more useful data. One clustered index per table btw.
In a nutshell: If you can filter your query on a clustered index column (that makes sense) then all the better.
An index seek is when SQL Server can use a binary search to quickly find the row. The rows in an index are sorted in a particular order, and your query has to specify enough information in the WHERE clause to allow SQL Server to make use of the sorted index.
An index scan is when SQL Server cannot use the sort order of the index, but can still use the index itself. This makes sense if the table rows are very large, but the index is relatively small. SQL Server will only have to read the smaller index from disk.
As a simple example, take a phonebook table:
id int identity primary key
lastname varchar(50)
phonenumber varchar(15)
Say that there is an index on (lastname). Then this query will result in an index seek:
select * from phonebook where lastname = 'leno'
This query will result in an index scan:
select * from phonebook where lastname like '%no'
The analogy with a real life phonebook is that you can't look up people whose name ends in 'no'. You have to browse the entire phonebook.
Related
There is scenario, I have table with 40 columns and I have to select all data of a table (including all columns). I have created a clustered index on the table and its including Clustered Index Scan while fetching full data set from the table.
I know that without any filter or join key, SQL Server will choose Clustered Index Scan instead of Clustered Index Seek. But, I want to have optimize execution plan by optimizing Clustered Index Scan into Clustered Index Seek. Is there any solution to achieve this? Please share.
Below is the screenshot of the execution plan:
Something is not quite right in the question / request, because what you are asking for will perform badly. I suspect it comes from mis-understanding what a clustered index is.
The clustered index - which is perhaps better stated as a clustered table - is the table of data, its not separate to the table, it is the table. If the order of the data on the table is already based on ITEM ID then the scan is the most efficient access method for your query (especially given the select *) - you do not want to seek in this scenario at all - and I don't believe that it is your scenario due to the sort operator.
If the clustered table is ordered based on another field, then you would need an additional non-clustered index to provide the correct order. You would then try to force a plan which was a non-clustered index scan, nested loop to a clustered index seek. That can be achieved using query hints, most likely an INNER LOOP JOIN would cause the seek - but a FORCESEEK also exists which can be used.
Performance wise this second option is never going to win - you are in effect looking at a tipping point notion (https://www.sqlskills.com/blogs/kimberly/the-tipping-point-query-answers/)
Well, I was trying to achieve the same, I wanted an index seek instead of an index scan on my top query.
SELECT TOP 5 id FROM mytable
Here is the execution plan being shown for the query:
I even tried the Offset Fetch Next approach, the plan was same.
To avoid a index scan, I included a fake primary key filter like below:
SELECT TOP 5 id FROM mytable where id != 0
I know, I won't have a 0 value in my primary key, so I added it in top query, which was resolved to an index seek instead of index scan:
Even though, the query plan comparison gives operation cost as similar to other, for index seek and scan in this regard. But I think to achieve index seek this way, it is an extra operation for the db to perform because it has to compare whether the id is 0 or not. Which we entirely do not need it to do if we want the top few records.
I have two different queries in SQL Server and I want to clarify
how the execution plan would be different, and
which of them is more efficient
Queries:
SELECT *
FROM table_name
WHERE column < 2
and
SELECT column
FROM table_name
WHERE column < 2
I have a non-clustered index on column.
I used to use Postgresql and I am not familiar with SQL Server and these kind of indexes.
As I read many questions here I kept two notes:
When I have a non-clustered index, I need one more step in order to have access to data
With a non-clustered index I could have a copy of part of the table and I get a quicker response time.
So, I got confused.
One more question is that when I have "SELECT *" which is the influence of a non-clustered index?
1st query :
Depending on the size of the data you might face lookup issues such as Key lookup and RID lookups .
2nd query :
It will be faster because it will not fetch columns that are not part of the index , though i recommend using covering index ..
I recommend you check this blog post
The first select will use the non-clustered index to find the clustering key [clustered index exists] or page and slot [no clustered index]. Then that will be used to get the row. The query plan will be different depending on your STATS (the data).
The second query is "covered" by the non-clustered index. What that means is that the non-clustered index contains all of the data that you are selecting. The clustering key is not needed, and the clustered index and/or heap is not needed to provide data to the select list.
I have a query that fails to execute with "Could not allocate a new page for database 'TEMPDB' because of insufficient disk space in filegroup 'DEFAULT'".
On the way of trouble shooting I am examining the execution plan. There are two costly steps labeled "Clustered Index Scan (Clustered)". I have a hard time find out what this means?
I would appreciate any explanations to "Clustered Index Scan (Clustered)" or suggestions on where to find the related document?
I would appreciate any explanations to "Clustered Index Scan
(Clustered)"
I will try to put in the easiest manner, for better understanding you need to understand both index seek and scan.
SO lets build the table
use tempdb GO
create table scanseek (id int , name varchar(50) default ('some random names') )
create clustered index IX_ID_scanseek on scanseek(ID)
declare #i int
SET #i = 0
while (#i <5000)
begin
insert into scanseek
select #i, 'Name' + convert( varchar(5) ,#i)
set #i =#i+1
END
An index seek is where SQL server uses the b-tree structure of the index to seek directly to matching records
you can check your table root and leaf nodes using the DMV below
-- check index level
SELECT
index_level
,record_count
,page_count
,avg_record_size_in_bytes
FROM sys.dm_db_index_physical_stats(DB_ID('tempdb'),OBJECT_ID('scanseek'),NULL,NULL,'DETAILED')
GO
Now here we have clustered index on column "ID"
lets look for some direct matching records
select * from scanseek where id =340
and look at the Execution plan
you've requested rows directly in the query that's why you got a clustered index SEEK .
Clustered index scan: When Sql server reads through for the Row(s) from top to bottom in the clustered index.
for example searching data in non key column. In our table NAME is non key column so if we will search some data in the name column we will see clustered index scan because all the rows are in clustered index leaf level.
Example
select * from scanseek where name = 'Name340'
please note: I made this answer short for better understanding only, if you have any question or suggestion please comment below.
Expanding on Gordon's answer in the comments, a clustered index scan is scanning one of the tables indexes to find the values you are doing a where clause filter, or for a join to the next table in your query plan.
Tables can have multiple indexes (one clustered and many non-clustered) and SQL Server will search the appropriate one based upon the filter or join being executed.
Clustered Indexes are explained pretty well on MSDN. The key difference between clustered and non-clustered is that the clustered index defines how rows are stored on disk.
If your clustered index is very expensive to search due to the number of records, you may want to add a non-clustered index on the table for fields that you search for often, such as date fields used for filtering ranges of records.
A clustered index is one in which the terminal (leaf) node of the index is the actual data page itself. There can be only one clustered index per table, because it specifies how records are arranged within the data page. It is generally (and with some exceptions) considered the most performant index type (primarily because there is one less level of indirection before you get to your actual data record).
A "clustered index scan" means that the SQL engine is traversing your clustered index in search for a particular value (or set of values). It is one of the most efficient methods for locating a record (beat by a "clustered index seek" in which the SQL Engine is looking to match a single selected value).
The error message has absolutely nothing to do with the query plan. It just means that you are out of space on TempDB.
I have been having issues with performance and timeouts due to a clustered index scan. However another seemingly identical database did not have the same issue.
Turns out the COMPATIBILITY_LEVEL flag on the db was different... the version with COMPATIBILITY_LEVEL 100 was using the scan, the db with level 130 wasn't. Performance difference is huge (from more than 1 minute to less that 1 second for same query)
ALTER DATABASE [mydb] SET COMPATIBILITY_LEVEL = 130
If you hover over the step in the query plan, SSMS displays a description of what the step does. That will give you a baseline understanding of "Clustered Index Scan (Clustered)" and all other steps involved.
So, we have a table, InventoryListItems, that has several columns. Because we're going to be looking for rows at times based on a particlar column (g_list_id, a foreign key), we have that foreign key column placed into a non-clustered index we'll call MYINDEX.
So when I search for data like this:
-- fake data for example
DECLARE #ListId uniqueidentifier
SELECT #ListId = '7BCD0E9F-28D9-4F40-BD67-803005179B04'
SELECT *
FROM [dbo].[InventoryListItems]
WHERE [g_list_id] = #ListId
I expected that it would use the MYINDEX index to find just the needed rows, and then look up the information in those rows. So not as good as just finding everything we need in the index itself, but still a big win over doing a full scan of the table.
But instead it seems that I'm still getting a clustered index scan. I can't figure out why that would happen.
If I do something like SELECTing only the values in the included columns of the index, it does what I would expect, an index seek, and just pulls everything from the index.
But if I SELECT *, why does it just bail on the index and do a scan when it seems like it would still benefit greatly from using it because it's referenced in the WHERE clause?
Since you're doing a SELECT * and thus you retrieve all columns, SQL Server's query optimizer may have decided it's easier and more efficient to just do a clustered index scan - since it needs to go to the clustered index leaf level to get all the columns anyway (and doing a seek first, and then a key lookup to actually get the whole data page, is quite an expensive operation - scan might just be more efficient in this setup).
I'm almost sure if you try
SELECT g_list_id
FROM [dbo].[InventoryListItems]
WHERE [g_list_id] = #ListId
then there will be an index seek instead (since you're only retrieving a single column - not everything).
That's one of the reasons why I would recommend to be extra careful when using SELECT * .... - try to avoid it if ever possible.
I have a table similar you can see below
Table Keywords
Column ID
Column Keyword
Column Keyword2
the first query is
select keyword from keywords with (nolock) where keyword = keyword
another query for the same tabel is
select keyword2 from with (nolock) keywords where keyword2 Like 'keyword%'
My question is what index type to set on which columns int this table
to make a select process more sufficient? Is it should be clustered index or non-clustered? and on which columns I need to set it?
This table contains about 600k rows and it constantly growing.
Another question I'm getting a dead-lock error when I trying to insert a new record to Keywords table. What can be the problem? I'm selecting records with nolock.
Thank you
Since your two queries are on totally separate columns, you will need two separate non-clustered indices:
one index on keyword to speed up the first query
a second index on keyword2 to speed up the second query
And assuming you're using SQL Server: neither of them really makes a good clustered index, I would say - but a good clustered index would be really beneficial!
A good clustered index should be:
unique
small
stable (never changing)
ever-increasing
Your best bet would be on an INT IDENTITY field. See Kimberly Tripp's outstanding blog post Ever-increasing clustering key - the Clustered Index Debate..........again! for more detailed background on the requirements for a good clustering key.
If we are really seeing the only use cases, you want a clustered key on keyword2 and then hope your DBMS is smart enough to optimize index use with LIKE operator. Clustering helps when the returned rows from a typical query are adjacent in the DB, so keeping the table in alphabetical order on keyword2 will mean fewer pages have to be scanned on the SELECT. Clustering a table where access is pretty much random (e.g., user names) won't give you any more than a standard index.