I'm working on a database system and it's indexes, but I'm having a really hard time seing the clear difference between a covering index and a clustered index.
I've googled my way around but hasn't got a clear cut answer on:
What is the differences between the two types of indexes
When do I use Covering index and when do I use Clustered index.
I hope someone can explain it to me in a almost children-like answer :-)
Sincerely Mestika
By the way, I'm using IBM DB2 version 9.7
I cannot speak to DB2, but the following applies to SQL Server.
When all of the required columns are part of the index the index is called a a "covering index". SQL Server 2005 introduced this type of index by allowing you to have "included columns" in the index. This allows you to include additional columns in the index over the 16 column limit or columns that would be too large to include.
While you can only have one clustered index per table, you can have up to 249 non-clustered indexes per table.
By having a covering index available to satisfy a query, SQL Server won't need to go back to the clustered index to retrieve the rest of the data required by the query.
Randy
Related
I try both indexes in SQL Server 2012 but confused with syntax, is there any difference between both type of index or both are same?
If same than why they create new keyword columnstore in sql.
Thank you in advance.
As shown in the simplified picture below, both indexes are organized as columns but NCCI is created on an existing rowstore table as shown on the right side in the picture below while a table with CCI does not have a rowstore table. Both tables can have one or more btree nonclustered indexes.
Other than this, the physical structures on how data is stored in delta and compressed rowgroups are identical and both kinds of indexes have same performance optimizations including batchmode operators. However, there some key differences between these and the table below lists the main differences.
Source :- https://blogs.msdn.microsoft.com/sqlserverstorageengine/2016/07/18/columnstore-index-differences-between-clusterednonclustered-columnstore-index/
I am very beginner in SQL Server 2005 and I am learning it from online tutorial, here is some of my question:
1: What is the difference between Select * from XYZ and Select ALL * from XYZ.
2: The purpose of Clustered index is like to make the search easier by physically sorting the table [as far as I kknow :-)]. Let say if have primary column on a table than is it good to create a clustered index on the table? because we have already a column which is sorted.
3: Why we can create 1 Clustered Index + 249 Nonclustered Index = 250 Index on a table? I understand the requirement of 1 clustered index. But why 249?? Why not more than 249?
No difference SELECT ALL is the default as opposed to SELECT DISTINCT
Opinion varies. For performance reasons Clustered indexes should ideally be small, stable, unique, and monotonically increasing. Primary keys should also be stable and unique so there is an obvious fit there. However clustered indexes are well suited for range queries. Looking up individual records by PK can perform well if the PK is nonclustered so some authors suggest not "wasting" the clustered index on the PK.
In SQL Server 2008 you can create up to 999 NCIs on a table. I can't imagine ever doing so but I think the limit was raised as potentially with "filtered indexes" there might be a viable case for this many. Indexes add a cost to data modification operations though as the changes need to be propagated in multiple places so I would imagine it would only be largely read only (e.g. reporting) databases that ever achieve even double figures of non clustered non filtered indexes.
For 3:
Everytime when you insert/delete record in the table ALL indexes must be updated. If you will have too many indexes it takes too long time.
If your table have more then 5-6 indexes I think you need take the time and check yourself.
If I'm am trying to squeeze every last drop of performance out of a query what affect does having these types of index's being used by my joins.
clustered index.
non-clustered index.
clustered or non-clustered index with extra columns that may not be involved in the join.
Will I gain any performance if I go through and create clustered index's that only contain the columns involved in my joins and nothing else?
(I realize I may have to move the clustered index from another index(making that index non-clustered) since it can only have one.)
In addition to Gareth Saul's answer a tiny clarification:
Non-clustered indexes repeat the
included fields, with pointer to the
rows that have that value.
This pointer to the actual data value is the column (or the set of columns) that are in your clustering key.
That's one of the main reasons why you should try and keep the clustering key small and static - small because otherwise you'll waste a lot of space, on disk and in your server's RAM, and static because otherwise, you'll have to update not just your clustering index, but also all your non-clustered indices as well, if your value changes.
This "lookup pointer is the clustering key" feature has been in SQL Server since version 7, as Kim Tripp will explain in great detail here:
What is a clustered index?
In SQL Server 7.0 and higher the
internal dependencies on the
clustering key CHANGED. (Yes, it's
important to know that things CHANGED
in 7.0... why? Because there are still
some folks out there that don't
realize how RADICAL of a change
occurred in the internals (wrt to the
clustering key) in SQL Server 7.0).
What changed is that the clustering
key gets used as the "lookup" value
from the nonclustered indexes.
Will I gain any performance if I go through and create clustered index's that only contain the columns involved in my joins and nothing else?
Not as I understand. The point of a clustered index is that it then sorts the data on disk around that index (hence why you can only have the one), so if your join data isn't being sorted by those exact columns as well, I don't think it'd make any difference. Plus by putting data that might change (as opposed to the key) into the clustered index, you make it more likely that things will need rebuilding peridically, slowing the overall database down.
Sorry if this sounds a daft question, but have you tried running your query through the index tuning wizard? Not foolproof by any stretch but I've had some decent improvements from it in the past.
You only get one clustered index - this is what controls the physical storage of the table on disk / in memory.
Non-clustered indexes repeat the included fields, with pointer to the rows that have that value. Having an index on the columns being used in your joins should improve performance. You can further optimise by using "included columns" in your index - this duplicates the row information directly into the index, which can remove the performance penalty of having to look up the row itself to perform the select.
It is useful to pay attention to the order in which your joins occur - the sequence of columns in your index should match up to this. Remember that the SQL engine may optimise and re-order your query internally - profiling may be helpful.
In most situations, you can just use the Database Engine Tuning Advisor - the recommendations it provides are pretty much spot on.
If you can your best bet is for a non-clustered index that has all the element of your join in it and if possible the field you are selecting.
This will create a spanning index meaning that all the fields SQL requires to perform are on one index.
If possible have an index which has no unnessasery field in it. Every field added makes the an individual index record larger, the smaller each index record the more you get in each Page. The more index items you get in each page the less you have to go to the Disk.
Clustered Index - Will mean the table is layed out in the order specified in the Index, this means that you will get better performance for select * from TABLE where INDEXFIELD = 3. Unless you are selecting lots of large data items this should not be required.
I am working on optimizing a SQL query that goes against a very wide table in a legacy system. I am not able to narrow the table at this point for various reasons.
My query is running slowly because it does an Index Seek on an Index I've created, and then uses a Bookmark Lookup to find the additional columns it needs that do not exist in the Index. The bookmark lookup takes 42% of the query time (according to the query optimizer).
The table has 38 columns, some of which are nvarchars, so I cannot make a covering index that includes all the columns. I have tried to take advantage of index intersection by creating indexes that cover all the columns, however those "covering" indexes are not picked up by the execution plan and are not used.
Also, since 28 of the 38 columns are pulled out via this query, I'd have 28/38 of the columns in the table stored in these covering indexes, so I'm not sure how much this would help.
Do you think a Bookmark Lookup is as good as it is going to get, or what would another option be?
(I should specify that this is SQL Server 2000)
OH,
the covering index with include should work. Another option might be to create a clustered indexed view containing only the columns you need.
Regards,
Lieven
You could create an index with included columns as another option
example from BOL, this is for 2005 and up
CREATE NONCLUSTERED INDEX IX_Address_PostalCode
ON Person.Address (PostalCode)
INCLUDE (AddressLine1, AddressLine2, City, StateProvinceID);
To answer this part "I have tried to take advantage of index intersection by creating indexes that cover all the columns, however those "covering" indexes are not picked up by the execution plan and are not used."
An index can only be used when the query is created in a way that it is sargable, in other words if you use function on the left side of the operator or leave out the first column of the index in your WHERE clause then the index won't be used. If the selectivity of the index is low then also the index won't be used
Check out SQL Server covering indexes for some more info
If your ID column on a table is a unique identifier (Guid), is there any point creating a clustered primary key on the ID column?
Given that they are globally unique, how would the sorting work?
GUIDs as they are are terrible for performance since they are effectively random values (this "breaks" clustered index), and they are awful for indexes, since less entries fit on a single page/extent (SQL Server terms). SQL Server 2005 introduces newsequentialid() which helps solving first problem.
I strongly advise against using clustered Guid key... We had big performance issues on SQL server because of such poor design a few years ago.
Also check out: Improving performance of cluster index GUID primary key
Putting a clustered index on a guid column is not such a good idea (unless you're making use of sequential guids).
The clustered index determines the physical order of how the records are stored.
This means that, if you put a clustered index on a column that does not sequentially grow, SQL Server will have some work making sure that the records are correctly ordered physically when you insert new records.
The idea of having a sorted index is very good in itself as searching then becomes very efficient.
The problem however is that in case of a GUID one never searches with "WHERE GUID = xyz". So the whole concept is wasted. So I would suggest to rather have a clustered index on a column which is used most often as a SARG to increase the query efficiency.