When I join 2 tables tbl1, tbl2 on column1, where column1 is primary key on tbl1. Assuming that column1 is not automatically indexed should I create an index on both tbl1.column1 and tbl2.column1 or just on tbl2.column1. Are the number of rows of each table affect that choice?
A primary key is automatically indexed. There is no way around that (this is how the "unique" part of the unique constraint is implemented). So, tbl1.column1 has an index. No other index is needed.
As for tbl2.column2, you should probably have an index on that. MySQL does create an index if you explicitly declare a foreign key relationship. So, with the explicit declaration, no other index is necessary. Note: this is not true of all databases.
The presence of indexes does not change the results of queries nor the number of rows in the table, so I don't understand your final question. Indexes implement relational integrity and improve (hopefully!) performance on certain types of queries
Generally yes, because often you'll want to do the reverse of the join at some point.
Related
Lets say you have Users table and Posts table.
Users
id
name
email
Posts
id
contents
user_id
If I add index to "user_id" in Posts table, and set it as NOT NULL, Can I expect same effect as Foreign Key?
I know that I can set user_id as any number, whereas foreign_key will force you set valid id. Let's assume that user_id is valid. Is there any performance benefit when we set foreign_key?
The main benefit of foreign keys is that they enforce data consistency, meaning that they keep the database clean in other words Keys are Indexes that have Integrity rules applied to prevent corruption of data.
Index is a data structure built on columns of a table to speed up search for indexed records based on values of indexed columns. In other words you gain search speed in exchange of insert/delete speed and storage.
Is there any performance benefit when we set foreign_key?
In performance terms, you will face no improvement.
Foreign keys will impact INSERT, UPDATE and DELETE statements because of the data checking rules , but keep in mind that your data will be consistet .
In MySQL, defining a foreign key constraint automatically creates an index, unless it can use an index that already exists. That is, if you create an index and subsequently add a foreign key on the same column(s), MySQL does not create an extra index just for the foreign key.
If you run a query that needs that index, it doesn't matter if you created the index yourself or if the index was created as a side-effect of adding the foreign key. Either way, the index can help the query. The performance benefit is the same.
If you run a query that does not need that index, then there's no benefit to having index either way.
You didn't describe any specific SQL query, so there's no way for us to guess whether the index is needed.
We are optimizing our company MySQL database. In many tables we find that there is:
primary key on id
unique key on id
"key" on id
Since a PRIMARY key is already unique, I understand that the UNIQUE key is redundant and not needed. I also understand that KEY in this sense means nothing more than "index" and so it is also redundant.
Is it true that we could delete take the UNIQUE and KEY and have the same speed provided by the primary key index?
Could it be somehow that having these three indexes causes SELECT queries to be slower?
Yes, you can drop the unique index and the key declaration. AFAIK, every SQL dbms implements a primary key constraint with a unique index. You can verify by running explain, dropping those two, and running explain again.
Redundant indexes might affect SELECT performance a little, because redundant indexes give the query planner more alternatives to consider. But I'd expect redundant indexes to have more effect on INSERT and DELETE statements, because those indexes have to be updated along with the table. (SQL UPDATE statements would require changes to the indexes only if the updated columns happen to be indexed.)
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mySQL's KEY keyword?
Like
PRIMARY KEY (ID),
KEY name (name),
KEY desc (desc),
etc.
what are they useful for?
Keys are used to enforce referential integrity in your database.
A primary key is, as its name suggests, the primary identification of a given row in your table. That is, each row's primary key will uniquely identify that row.
A unique key is a key that enforces uniqueness on that set of columns. It is similar to a primary key in that it will also uniquely identify a row in a table. However, there is the added benefit of allowing NULL in some of those combinations. There can only be 1 primary key, but you can have many unique keys.
A foreign key is used to enforce a relationship between 2 tables (think parent/child table). That way, a child table can not have a value of X in its parent column unless X actually appears in the parent table. This prevents orphaned records from appearing.
The primary key constraint ensures that the column(s) are:
not null
unique (unique sets if more than one column)
KEY is MySQL's terminology in CREATE TABLE statements for an index. Indexes are not ANSI currently, but all databases use indexes to speed up data retrieval (at the cost of insertion/update/deletion, because of maintenance to keep the index relevant).
There are other key constraints:
unique
foreign key (for referential integrity)
...but your question doesn't include examples of them.
keys are also called indexes. They are used for speeding up queries. Additionally keys can be constrains (unique key and foreign key). The primary key is also unique key and it identifies the records. The record can have other unique keys as well, that do not allow to duplicate a value in a given column. Foreign key enforces referential integrity (#Derek Kromm already wrote excellent description). The ordinary key is used only for speeding up queries. You need to index the columns used in the WHERE clause of the queries. If you have no index on the column, MySQL will need to read the whole table to find the records you need. When index is used, MySQL reads only the index (which is usually a B+ tree) and then read only those record from the table it found in the index.
Primary KEY is for creating unique/not null constraint for each row in the table. Also searching by this key is the fastest. You can create only one PK in the table.
Ordinary key/index is key for speeding your searching by this column, sorting, grouping and joining with other table by this key.
Indexes drawback:
Adding new indexes to table will influence on speed or running insert/update/delete statements. So you should select columns for indexing in your table very carefully.
Key are used for relation purposes between tables and you are able to create joins in order to select data from multiple tables
What, you didn't fine the wikipedia entry comprehensive? ;-)
So, a key, in a relational database (such as MySQL, PostgreSQL, Oracle, etc) is a data constraint on a column or set of columns. The most common keys are the Primary key and foreign keys and unique keys.
A foreign key specifically relates the data of one table to data in another table. You might see that a table blog_posts has a foreign key to users based on a user_id column. This means that every user_id in blog_posts will have a corresponding entry in the users column (this is a one-to-many relationship -- a topic for another time).
If a column (or group of columns) has a unique key, that means that there can only be one such incidence of the key in the table. Often you'll see things like email addresses be unique keys -- you only want one email address per user. I've also seen a combination of columns match to a unique key -- the five columns, first_name, last_name, address, city, and state, will often be a unique key -- realistically, there can only be one William Gates at 1835 73rd Ave NE, Medina, Washington. (I do realize that it is possible for a William Gates Jr. to be born, but the designers of that database didn't really care).
The primary key is the primary, unique identifier of a given table. By definition it is a unique key. It is something which cannot be null and must be unique. It holds a special place of prominence among the indexes of a given table.
I am new to indexing and gone through the basics of indexing. I can find a default clustered index for the primary key constraint within the indexes part of the corresponding table, but after creating a foreign key constraint i can't find any.
Now i've a requirement in which indexing should be implemented to improve performance. I've read about indexing the foreign key in order to improve the performance of a JOIN result.
Do i need to add the foreign key column to an additional non-clustered index or the foreign key has a default indexing?
How can i effectively implement indexing if my SQL table structure is as follows and i've a JOIN query with WHERE clause using t1_col3
table1 table2
------ ------
t1_col1(pk) t2_col1(pk)
t1_col2 t2_col2
t1_col3 t2_col3
t1_col4 t2_col4
t2_col1(FK)
Without a query plan, an actual query, or details about how your data are distributed it is not possible to make a concrete recommendation, but here are a few things to keep in mind.
Creating a foreign key does not automatically create an index on the foreign key column. It is simply a constraint.
Because joins between tables frequently involve foreign key columns, and because SQL Server will need to check the foreign key column regularly in order to enforce the constraint, it is usually a good idea to place an index on the foreign key column.
With what little you've described, your query has a criterion against the t1_col3 column - it will use that column to filter the values in table1; it might (it really depends on the query plan, and that's dependent on the distribution of data in the tables and other criteria and operations in the query) be a good idea to place an index on t1_col3.
For the third point, the usefulness of the index will depend a lot on the specific iterator/operator the query engine uses to implement a join between the two tables, and which table drives the join.
Any further information you can provide - the query, a query plan, number of rows - will help us provide a more specific answer.
I want to create a database with 3 tables. One for posts and one for tags and one that links posts to tags with the post_id and tag_id functioning as foreign key references.
Can you explain what an Index would be in this scenario and how it differs from a Foreign Key and how that impacts my database design?
an index on a table is a data structure that makes random access to the rows fast and efficient. It helps to optimize the internal organization of a table as well.
A foreign key is simply a pointer to a corresponding column in another table that forms a referential constraint between the two tables.
An index is added as a fast look up for data in the table.
An index can have constraints, in that the column or columns that are used to make the index might have to be unique (unique: only one row in the database is returned for that index, or non-unique: multiple rows can be returned). The primary key for the table is a unique index, and usually only has one column.
A foreign key is a value in a table that references a unique index in another table. It is used as a way to relate to tables together. For example, a child table can look up the one parent row via its column that is a unique index in the parent table.
You'll have foreign keys in the third table. Indexes are not necessary, you need them if you have lots of data where you want to find something by Id quickly. Maybe you'll want an index on posts primary key, but DBMS will probably create it automatically.
Index is a redundant data structure which speeds up some queries.
Foreign key, for practical matters, is a way to make sure that you have no invalid pointers between the rows in your tables (in your case, from the relationship table to posts and tags)
Question: Can you explain what an Index would be in this scenario and how it differs from a Foreign Key and how that impacts my database design?
Your foreign keys in this case are the two columns in your Posts_Tags table. With a foreign key, Each foreign key column must contain a value from the main table it is referencing. In this case, the Posts and Tags tables.
Posts_Tags->PostID must be a value contained in Posts->PostID
Posts_Tags->TagID must be a value contained in Tags->TagID
Think of an index as a column that has been given increased speed and efficiency for querying/searching values from it, at the cost of increased size of your database. Generally, primary keys are indexes, and other columns that require querying/searching on your website, in your case, probably the name of a post (Posts->PostName)
In your case, indexes will have little impact on your design (they are nice to have for speed and efficiency), but your foreign keys are very important to avoid data corruption (having values in them that don't match a post and/or tag).
You describe a very common database construct; it's called a "many-to-many relation".
Indexes shouldn't impact this schema at all. In fact, indexes shouldn't impact any schema. Indexes are a trade-off between space and time: indexes specify that you're willing to use extra storage space, in exchange for faster searches through the database.
Wikipedia has an excellent article about what database indexes are: Index (database)
To use foreign keys in mysql, you need to create indexes on both tables. For example, if you want the field a_id on table b to reference the id field on the table a, you have to create indexes on both a.id and b.a_id before you can create the reference.
Update: here you can read more about it: http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html