While studying for the 70-433 exam I noticed you can create a covering index in one of the following two ways.
CREATE INDEX idx1 ON MyTable (Col1, Col2, Col3)
-- OR --
CREATE INDEX idx1 ON MyTable (Col1) INCLUDE (Col2, Col3)
The INCLUDE clause is new to me. Why would you use it and what guidelines would you suggest in determining whether to create a covering index with or without the INCLUDE clause?
If the column is not in the WHERE/JOIN/GROUP BY/ORDER BY, but only in the column list in the SELECT clause is where you use INCLUDE.
The INCLUDE clause adds the data at the lowest/leaf level, rather than in the index tree.
This makes the index smaller because it's not part of the tree
INCLUDE columns are not key columns in the index, so they are not ordered.
This means it isn't really useful for predicates, sorting etc as I mentioned above. However, it may be useful if you have a residual lookup in a few rows from the key column(s)
Another MSDN article with a worked example
You would use the INCLUDE to add one or more columns to the leaf level of a non-clustered index, if by doing so, you can "cover" your queries.
Imagine you need to query for an employee's ID, department ID, and lastname.
SELECT EmployeeID, DepartmentID, LastName
FROM Employee
WHERE DepartmentID = 5
If you happen to have a non-clustered index on (EmployeeID, DepartmentID), once you find the employees for a given department, you now have to do "bookmark lookup" to get the actual full employee record, just to get the lastname column. That can get pretty expensive in terms of performance, if you find a lot of employees.
If you had included that lastname in your index:
CREATE NONCLUSTERED INDEX NC_EmpDep
ON Employee(DepartmentID)
INCLUDE (Lastname, EmployeeID)
then all the information you need is available in the leaf level of the non-clustered index. Just by seeking in the non-clustered index and finding your employees for a given department, you have all the necessary information, and the bookmark lookup for each employee found in the index is no longer necessary --> you save a lot of time.
Obviously, you cannot include every column in every non-clustered index - but if you do have queries which are missing just one or two columns to be "covered" (and that get used a lot), it can be very helpful to INCLUDE those into a suitable non-clustered index.
This discussion is missing out on the important point: The question is not if the "non-key-columns" are better to include as index-columns or as included-columns.
The question is how expensive it is to use the include-mechanism to include columns that are not really needed in index? (typically not part of where-clauses, but often included in selects). So your dilemma is always:
Use index on id1, id2 ... idN alone or
Use index on id1, id2 ... idN plus include col1, col2 ... colN
Where:
id1, id2 ... idN are columns often used in restrictions and col1, col2 ... colN are columns often selected, but typically not used in restrictions
(The option to include all of these columns as part of the index-key is just always silly (unless they are also used in restrictions) - cause it would always be more expensive to maintain since the index must be updated and sorted even when the "keys" have not changed).
So use option 1 or 2?
Answer: If your table is rarely updated - mostly inserted into/deleted from - then it is relatively inexpensive to use the include-mechanism to include some "hot columns" (that are often used in selects - but not often used on restrictions) since inserts/deletes require the index to be updated/sorted anyway and thus little extra overhead is associated with storing off a few extra columns while already updating the index. The overhead is the extra memory and CPU used to store redundant info on the index.
If the columns you consider to add as included-columns are often updated (without the index-key-columns being updated) - or - if it is so many of them that the index becomes close to a copy of your table - use option 1 I'd suggest! Also if adding certain include-column(s) turns out to make no performance-difference - you might want to skip the idea of adding them:) Verify that they are useful!
The average number of rows per same values in keys (id1, id2 ... idN) can be of some importance as well.
Notice that if a column - that is added as an included-column of index - is used in the restriction: As long as the index as such can be used (based on restriction against index-key-columns) - then SQL Server is matching the column-restriction against the index (leaf-node-values) instead of going the expensive way around the table itself.
Basic index columns are sorted, but included columns are not sorted. This saves resources in maintaining the index, while still making it possible to provide the data in the included columns to cover a query. So, if you want to cover queries, you can put the search criteria to locate rows into the sorted columns of the index, but then "include" additional, unsorted columns with non-search data. It definitely helps with reducing the amount of sorting and fragmentation in index maintenance.
One reason to prefer INCLUDE over key-columns if you don't need that column in the key is documentation. That makes evolving indexes much more easy in the future.
Considering your example:
CREATE INDEX idx1 ON MyTable (Col1) INCLUDE (Col2, Col3)
That index is best if your query looks like this:
SELECT col2, col3
FROM MyTable
WHERE col1 = ...
Of course you should not put columns in INCLUDE if you can get an additional benefit from having them in the key part. Both of the following queries would actually prefer the col2 column in the key of the index.
SELECT col2, col3
FROM MyTable
WHERE col1 = ...
AND col2 = ...
SELECT TOP 1 col2, col3
FROM MyTable
WHERE col1 = ...
ORDER BY col2
Let's assume this is not the case and we have col2 in the INCLUDE clause because there is just no benefit of having it in the tree part of the index.
Fast forward some years.
You need to tune this query:
SELECT TOP 1 col2
FROM MyTable
WHERE col1 = ...
ORDER BY another_col
To optimize that query, the following index would be great:
CREATE INDEX idx1 ON MyTable (Col1, another_col) INCLUDE (Col2)
If you check what indexes you have on that table already, your previous index might still be there:
CREATE INDEX idx1 ON MyTable (Col1) INCLUDE (Col2, Col3)
Now you know that Col2 and Col3 are not part of the index tree and are thus not used to narrow the read index range nor for ordering the rows. Is is rather safe to add another_column to the end of the key-part of the index (after col1). There is little risk to break anything:
DROP INDEX idx1 ON MyTable;
CREATE INDEX idx1 ON MyTable (Col1, another_col) INCLUDE (Col2, Col3);
That index will become bigger, which still has some risks, but it is generally better to extend existing indexes compared to introducing new ones.
If you would have an index without INCLUDE, you could not know what queries you would break by adding another_col right after Col1.
CREATE INDEX idx1 ON MyTable (Col1, Col2, Col3)
What happens if you add another_col between Col1 and Col2? Will other queries suffer?
There are other "benefits" of INCLUDE vs. key columns if you add those columns just to avoid fetching them from the table. However, I consider the documentation aspect the most important one.
To answer your question:
what guidelines would you suggest in determining whether to create a covering index with or without the INCLUDE clause?
If you add a column to the index for the sole purpose to have that column available in the index without visiting the table, put it into the INCLUDE clause.
If adding the column to the index key brings additional benefits (e.g. for order by or because it can narrow the read index range) add it to the key.
You can read a longer discussion about this here:
https://use-the-index-luke.com/blog/2019-04/include-columns-in-btree-indexes
The reasons why (including the data in the leaf level of the index) have been nicely explained. The reason that you give two shakes about this, is that when you run your query, if you don't have the additional columns included (new feature in SQL 2005) the SQL Server has to go to the clustered index to get the additional columns which takes more time, and adds more load to the SQL Server service, the disks, and the memory (buffer cache to be specific) as new data pages are loaded into memory, potentially pushing other more often needed data out of the buffer cache.
An additional consideraion that I have not seen in the answers already given, is that included columns can be of data types that are not allowed as index key columns, such as varchar(max).
This allows you to include such columns in a covering index. I recently had to do this to provide a nHibernate generated query, which had a lot of columns in the SELECT, with a useful index.
There is a limit to the total size of all columns inlined into the index definition. That said though, I have never had to create index that wide.
To me, the bigger advantage is the fact that you can cover more queries with one index that has included columns as they don't have to be defined in any particular order. Think about is as an index within the index.
One example would be the StoreID (where StoreID is low selectivity meaning that each store is associated with a lot of customers) and then customer demographics data (LastName, FirstName, DOB):
If you just inline those columns in this order (StoreID, LastName, FirstName, DOB), you can only efficiently search for customers for which you know StoreID and LastName.
On the other hand, defining the index on StoreID and including LastName, FirstName, DOB columns would let you in essence do two seeks- index predicate on StoreID and then seek predicate on any of the included columns. This would let you cover all possible search permutationsas as long as it starts with StoreID.
When inserting large sets of data into a table (from another table, in no particular order), how do you optimize a multi-column index so that the index is updated in the fastest possible way?
Assume the index is never used in any SELECT, DELETE or UPDATE query.* Assume also that the distinct counts for the columns as follows (for example):
COLUMN | DISTINCT COUNT
col1 | 634
col2 | 9,923
col3 | 2,357
col4 | 3
* Reason for not using the index in selecting data is this is a primary key index or a unique constraint index. The index is in place so that inserts violating the constraint should fail.
I have read that the most selective column should come first. Is that correct, and is the index then to be created as follows?
(col2, col3, col1, col4)
If that is wrong, how do you determine the best order for column in an index which will only see bulk INSERTs into the corresponding table? The goal is to speed up the updating of the index during the bulk INSERT.
The quickest way is to DROP INDEX, then do the bulk inserts and CREATE INDEX when you are done inserting.
The proper structure of the index does not have so much to do with the distribution of values in the columns but with the retrieval strategies, presumably for UPDATE and DELETE only, and then specifically when you do partial filtering on some but not all always all columns of the index. Those more frequent filters should come first in your index columns. But you probably want to reconsider your indexing strategy more radically if this is the case: it may be better to have two or more indexes to match your typical retrieval strategies.
Ignoring your call for ignorance: why would you not apply the index to SELECT statements? Indexes are useful only for selecting subsets of data from your tables, whether that is for SELECT or a qualified UPDATE or DELETE. There is no functional difference for using indexes in any of these three operations.
Addendum after comments from OP: Indexes are useful for many purposes but their maintenance is relatively expensive, where "relatively" becomes "impossibly" very quickly with increasing table size. In your case you have to compare every record from your source table with every record in your destination table, or O(m*n) order. That is unworkable with tables of a large size, even with an index. Your best bet is to drop the index, do the inserts, create an index which is not unique, find and delete all duplicates, drop the index, finally create a new unique index.
The order of the columns is not terribly important for a uniqueness enforcement purposes. But it would be unusual for a unique index to not also happen to be useful for some queries, and so I would order the columns to take advantage of that.
For bulk inserting into this index rapidly, I'd try inserting in index order. So add an order by (col2, col3, col1, col4) to the select part of your insert. This leads to more efficient IO.
Suppose that I created a compound primary key on col1, col2, col3, will indices be created on each of the column?
I know that the primary key constraint will create index for the combination of (col1, col2, col3) so that the search on these 3 columns will be faster. But I'm not sure if the database will create index on each of the column so that the search on individual column, like search on column2, will be speed up.
Can anybody tell me what happens on these columns in term of index?
It depends. Generally one index will be created so only queries using the "left most" columns can make use of it, i.e. in your example "SELECT * from T where COL2='x'" will not use the index. That said, Oracle can use an index "skip" where the first column is kind of compressed and the plan can jump past it looking at the next column in.
Your best approach would be to look at the manual and try it in the DBMS of your choice then look at the query plan.
Summary: it depends.
How to check if all fields are unique in Oracle?
SELECT myColumn, COUNT(*)
FROM myTable
GROUP BY myColumn
HAVING COUNT(*) > 1
This will return to you all myColumn values along with the number of their occurence if their number of occurences is higher than one (i.e. they are not unique).
If the result of this query is empty, then you have unique values in this column.
An easy way to do this is to analyze the table using DBMS_STATS. After you do, you can look at dba_tables... Look at the num_rows column. The look at dab_tab_columns. Compare the num_distinct for each column to the number of rows. This is a round about way of doing what you want without performing a full table scan if you are worried about affecting a production system on a huge table. If you want direct results, do what the the others suggest by running the query against the table with a group by.
one way is to create a unique index.
if the index creation fails, you have existing duplicate info, if an insert fails, it would have produced a duplicate...
I have a table that maps a user's permissions to a given object. So, it is essentially a join table to 3 different tables. (Object, User, and Permission)
The values of each row will always be unique for all 3 columns, but not any 2.
I need to create a non-clustered index. I want to put the index on the foreign keys to the object and user, but I am wondering if I should put it on all 3 columns.
"The values of each row will always be unique for all 3 columns"
You might be interested to know that SQL Server unique constraints are implemented as indexes. So if you have (or want) a constraint backing up that unique-claim of yours, you already have an index on all 3.
CREATE UNIQUE NONCLUSTERED INDEX idx_unique_perms ON UserPermissions
(
ObjectId ASC,
UserId ASC,
PermissionID ASC
)
If you make one, just remember to order your columns for high selectivity.
If you have some doubts, formulate the query(ies) you intend to execute against these tables, and run the SSMS Query Tuning Wizard. That should help you get started in the right direction.
One thing to consider is the number of rows in these three tables. If the row counts will be small, it might not even be worthwhile adding indexes. A table scan would probably be done anyway.