I want to execute the next delete query:
delete from MyTable where userId = 5
What of the below indexed will have better performance on this query, or will they both perform the same?
All the mentioned fields in here are BigInt
CREATE INDEX MyTable_UserId_UserBalance_index ON Main.dbo.MyTable (UserId, UserBalance);
CREATE INDEX MyTable_UserId_index ON Main.dbo.MyTable (UserId);
For that particular query they should perform roughly the same. Since you're looking up the first item in the compound index, finding the records should be the same as if there were another index on the single column.
I'm surprised at many of the answers here - for the purpose of deletion, there won't be any benefit using either one of these indexes over the other. For if the record exists in either index, you'll need to find it and remove it.
The purpose of indexes is for reads, not deleting data. If you're trying to read data, you would ask a question like this one, as one index has the potential to return the data quicker than the other. For deletes, you need to delete from all indexes, including the NC indexes.
It seems as if some enlightenment into the world of indexes is being called for.
Some great (free) documentation from well-known DBA - Brent Ozar: https://www.brentozar.com/archive/2016/10/think-like-engine-class-now-free-open-source/
CREATE INDEX MyTable_UserId_index ON Main.dbo.MyTable (UserId);
Both the indexes would work the same way unless the order is reversed for below index.
From:
CREATE INDEX MyTable_UserId_UserBalance_index ON Main.dbo.MyTable (UserId, UserBalance);
To:
CREATE INDEX MyTable_UserId_UserBalance_index ON Main.dbo.MyTable (UserBalance, UserId);
In second case, server may not see this index as useful as UserId is at second level.
Also, why would you create two indexes with same column in it? If you know that your table will be queried frequently with both UserId and UserBalance then, probably it is best to create an index with both columns in it.
Again, just make sure which column gets utilized the most.
I have a table with several million rows. It has over a dozen columns, many of which are long. It is a large table.
I have a lot of common operations where I need to select data from 2 columns based on one of the columns as a lookup.
I have several indexes tuned to a handful of operations ; including ones that only contain an ID and some boolean fields. This has largely worked out well.
I just ran into a problem, where an "IN()" select on a field that contains the md5 sum of another field became a bottleneck; deferring to a sequential scan and ignoring all indexes that had the md5 sum in it.
A normal scan took 45seconds. Turning enable_seqscan off took a few milliseconds.
After playing around for a bit, I realized that this index would work:
CREATE INDEX speed_idx_YAY ON table( field_md5 );
But having any other fields in the index would fail:
CREATE INDEX speed_idx_BOO ON table( field_md5 , field_other );
The shift from using a multi-column index to a sequential scan happened "overnight", as the database grew. At one time it worked, and then it didn't.
Does anyone have tips on how to best prepare for potential situations like this ? Part of me is tempted to create single-column indexes for every indexed field on some tables as a backup.
Referenced:
Why doesn't Postgresql use index for IN query?
PostgreSQL: Why is this query not using my index?
Why isn't Postgres using the index?
Have you thought about creatign indexes based on md5_field and include the other fields
CREATE NONCLUSTERED INDEX IX_speed_idx
ON speed_idx (md5_field)
INCLUDE (field_a,field_b,field_c);
Can any one give a quick overview about Column Store Index and Row Store Index?
I have searched over the internet and got confused, they say an Index can be applied to a Field(Column), while they also say there exist a row store index?
Need clarification.
Many thanks!
In SQL Server, prior to 2012 all indexes were row-store, you specify what fields you want to index, and the value of the rows are evaluated to improve performance (over-simplified). These indexes speed up finding/filtering rows.
Columnstore indexes introduced to SQL Server in 2012, also require you to specify what fields you want to index (Microsoft recommends including all fields in a Columnstore index). These indexes aren't like traditional indexes, they're more like pre-aggregated statistics.
More in-depth descriptions of the index types are available, but your basic confusion seemed to be row vs column. All indexes are applied to columns, but how the index works and what it stores is different between the two types.
Are there any techniques which can be used to simulate multiple clustered indices on a table in Sybase 12.5 ? thanks
I dont think, you can simulate multiple clustered indices. Because when you have one clustered index created on a table, the data gets rearranged according to the data in the clustered index column. Logically you cannot arrange the data in another order in a table according to another column. All that you can do is create non clustered index for other columns
Other thing you can do is combine two or more columns and create a clustered index.
The only close approximation I can think of for this would be to create non-clustered indexes that include all columns from the table. In that way, the non-clustered index would contain all of the data.
However, to achieve that, the entire table would have to fit within any constraints imposed on non-clustered indexes. (E.g. for SQL Server, there's a limit on some column datatypes, and the whole size in bytes - probably similar restrictions apply in any product).
Best way is to create as many tables as you want and try different clustered indexes,which you want to simulate, on these tables. Then maybe run queries against these tables to check which one is performing better ,if your main motive is just to check which column would make a better clustered index. But i would advice that you should run full workload or all the queries which you will be executing against this table so that you will be in better position to see which combinations will be best for you.
Also, when is it appropriate to use one?
An index is used to speed up searching in the database. MySQL has some good documentation on the subject (which is relevant for other SQL servers as well):
http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html
An index can be used to efficiently find all rows matching some column in your query and then walk through only that subset of the table to find exact matches. If you don't have indexes on any column in the WHERE clause, the SQL server has to walk through the whole table and check every row to see if it matches, which may be a slow operation on big tables.
The index can also be a UNIQUE index, which means that you cannot have duplicate values in that column, or a PRIMARY KEY which in some storage engines defines where in the database file the value is stored.
In MySQL you can use EXPLAIN in front of your SELECT statement to see if your query will make use of any index. This is a good start for troubleshooting performance problems. Read more here:
http://dev.mysql.com/doc/refman/5.0/en/explain.html
A clustered index is like the contents of a phone book. You can open the book at 'Hilditch, David' and find all the information for all of the 'Hilditch's right next to each other. Here the keys for the clustered index are (lastname, firstname).
This makes clustered indexes great for retrieving lots of data based on range based queries since all the data is located next to each other.
Since the clustered index is actually related to how the data is stored, there is only one of them possible per table (although you can cheat to simulate multiple clustered indexes).
A non-clustered index is different in that you can have many of them and they then point at the data in the clustered index. You could have e.g. a non-clustered index at the back of a phone book which is keyed on (town, address)
Imagine if you had to search through the phone book for all the people who live in 'London' - with only the clustered index you would have to search every single item in the phone book since the key on the clustered index is on (lastname, firstname) and as a result the people living in London are scattered randomly throughout the index.
If you have a non-clustered index on (town) then these queries can be performed much more quickly.
An index is used to speed up the performance of queries. It does this by reducing the number of database data pages that have to be visited/scanned.
In SQL Server, a clustered index determines the physical order of data in a table. There can be only one clustered index per table (the clustered index IS the table). All other indexes on a table are termed non-clustered.
SQL Server Index Basics
SQL Server Indexes: The Basics
SQL Server Indexes
Index Basics
Index (wiki)
Indexes are all about finding data quickly.
Indexes in a database are analogous to indexes that you find in a book. If a book has an index, and I ask you to find a chapter in that book, you can quickly find that with the help of the index. On the other hand, if the book does not have an index, you will have to spend more time looking for the chapter by looking at every page from the start to the end of the book.
In a similar fashion, indexes in a database can help queries find data quickly. If you are new to indexes, the following videos, can be very useful. In fact, I have learned a lot from them.
Index Basics
Clustered and Non-Clustered Indexes
Unique and Non-Unique Indexes
Advantages and disadvantages of indexes
Well in general index is a B-tree. There are two types of indexes: clustered and nonclustered.
Clustered index creates a physical order of rows (it can be only one and in most cases it is also a primary key - if you create primary key on table you create clustered index on this table also).
Nonclustered index is also a binary tree but it doesn't create a physical order of rows. So the leaf nodes of nonclustered index contain PK (if it exists) or row index.
Indexes are used to increase the speed of search. Because the complexity is of O(log N). Indexes is very large and interesting topic. I can say that creating indexes on large database is some kind of art sometimes.
INDEXES - to find data easily
UNIQUE INDEX - duplicate values are not allowed
Syntax for INDEX
CREATE INDEX INDEX_NAME ON TABLE_NAME(COLUMN);
Syntax for UNIQUE INDEX
CREATE UNIQUE INDEX INDEX_NAME ON TABLE_NAME(COLUMN);
First we need to understand how normal (without indexing) query runs. It basically traverse each rows one by one and when it finds the data it returns. Refer the following image. (This image has been taken from this video.)
So suppose query is to find 50 , it will have to read 49 records as a linear search.
Refer the following image. (This image has been taken from this video)
When we apply indexing, the query will quickly find out the data without reading each one of them just by eliminating half of the data in each traversal like a binary search. The mysql indexes are stored as B-tree where all the data are in leaf node.
INDEX is a performance optimization technique that speeds up the data retrieval process. It is a persistent data structure that is associated with a Table (or View) in order to increase performance during retrieving the data from that table (or View).
Index based search is applied more particularly when your queries include WHERE filter. Otherwise, i.e, a query without WHERE-filter selects whole data and process. Searching whole table without INDEX is called Table-scan.
You will find exact information for Sql-Indexes in clear and reliable way:
follow these links:
For cocnept-wise understanding:
http://dotnetauthorities.blogspot.in/2013/12/Microsoft-SQL-Server-Training-Online-Learning-Classes-INDEX-Overview-and-Optimizations.html
For implementation-wise understanding:
http://dotnetauthorities.blogspot.in/2013/12/Microsoft-SQL-Server-Training-Online-Learning-Classes-INDEX-Creation-Deletetion-Optimizations.html
If you're using SQL Server, one of the best resources is its own Books Online that comes with the install! It's the 1st place I would refer to for ANY SQL Server related topics.
If it's practical "how should I do this?" kind of questions, then StackOverflow would be a better place to ask.
Also, I haven't been back for a while but sqlservercentral.com used to be one of the top SQL Server related sites out there.
An index is used for several different reasons. The main reason is to speed up querying so that you can get rows or sort rows faster. Another reason is to define a primary-key or unique index which will guarantee that no other columns have the same values.
So, How indexing actually works?
Well, first off, the database table does not reorder itself when we put index on a column to optimize the query performance.
An index is a data structure, (most commonly its B-tree {Its balanced tree, not binary tree}) that stores the value for a specific column in a table.
The major advantage of B-tree is that the data in it is sortable. Along with it, B-Tree data structure is time efficient and operations such as searching, insertion, deletion can be done in logarithmic time.
So the index would look like this -
Here for each column, it would be mapped with a database internal identifier (pointer) which points to the exact location of the row. And, now if we run the same query.
Visual Representation of the Query execution
So, indexing just cuts down the time complexity from o(n) to o(log n).
A detailed info - https://pankajtanwar.in/blog/what-is-the-sorting-algorithm-behind-order-by-query-in-mysql
INDEX is not part of SQL. INDEX creates a Balanced Tree on physical level to accelerate CRUD.
SQL is a language which describe the Conceptual Level Schema and External Level Schema. SQL doesn't describe Physical Level Schema.
The statement which creates an INDEX is defined by DBMS, not by SQL standard.
An index is an on-disk structure associated with a table or view that speeds retrieval of rows from the table or view. An index contains keys built from one or more columns in the table or view. These keys are stored in a structure (B-tree) that enables SQL Server to find the row or rows associated with the key values quickly and efficiently.
Indexes are automatically created when PRIMARY KEY and UNIQUE constraints are defined on table columns. For example, when you create a table with a UNIQUE constraint, Database Engine automatically creates a nonclustered index.
If you configure a PRIMARY KEY, Database Engine automatically creates a clustered index, unless a clustered index already exists. When you try to enforce a PRIMARY KEY constraint on an existing table and a clustered index already exists on that table, SQL Server enforces the primary key using a nonclustered index.
Please refer to this for more information about indexes (clustered and non clustered):
https://learn.microsoft.com/en-us/sql/relational-databases/indexes/clustered-and-nonclustered-indexes-described?view=sql-server-ver15
Hope this helps!