I created this table:
CREATE TABLE incident_originator (
id_incident INT (11) UNSIGNED NOT NULL,
id_user INT (11) NOT NULL,
PRIMARY KEY (
id_incident,
id_user
),
CONSTRAINT fk_incident_incident_originator FOREIGN KEY (id_incident) REFERENCES incident_table (id_incident) ON DELETE RESTRICT ON UPDATE CASCADE,
CONSTRAINT fk_user_incident_originator FOREIGN KEY (id_user) REFERENCES users (id_user) ON DELETE RESTRICT ON UPDATE CASCADE
) ENGINE = INNODB DEFAULT CHARSET = latin1;
Yet, the fk_user_incident_originator, is indexed, and the fk_incident_incident_originator is not. Why is that? Isn't InnoBD supposed to automatically index all foreign keys? The lack of an index in the id_incident would make joins slower, wouldn't it? The more I read, the less I understand...
Plus, when I add values to the table, they are ordered by the second column and it gets weird to read as a human being.
EDIT: When I do a SHOW INDEX FROM incident_originator; it returns this:
Non_unique Key_name Seq_in_index Column_name
0 PRIMARY 1 id_incident
0 PRIMARY 2 id_user
1 fk_user_incident_originator 1 id_user
fk_incident_incident_originator is not
Sure it is.
PRIMARY KEY (id_incident,id_user),
In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist.
https://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html
id_incident is the first (meaning left-most) column of the primary key... and the primary key is a perfectly good index for looking up values when enforcing the constraint. To add a second index would be redundant.
The same is true for joins (though joins are not the actual reason foreign keys are always indexed) -- any index that includes all of the joined columns anchored to the left side of the index is quite valid for a join.
they are ordered by the second column and it gets weird to read as a human being.
Tell any humans you know that the database doesn't order its output for the benefit of humans unless the humans use ORDER BY. Result sets without ORDER BY are unordered by definition. The fact that rows are often returned in primary key order is by coincidence, not by design or necessity. This behavior may shift when the table gets larger as the optimizer changes strategies when reading the table... but since the index on id_user is in fact a covering index (it contains all the columns in the table, because all indexes also contain a copy of the primary key... or, more precisely, it contains all the columns needed to satisfy this particular query -- those are sometimes two different things, and this is one of the best reasons not to use SELECT * in your actual code) so the optimizer happens to be selecting it as its source. It reads the rows in index order from whatever index it selects, and that order becomes the entirely coincidental ordering of the result.
Related
At work we have a big database with unique indexes instead of primary keys and all works fine.
I'm designing new database for a new project and I have a dilemma:
In DB theory, primary key is fundamental element, that's OK, but in REAL projects what are advantages and disadvantages of both?
What do you use in projects?
EDIT: ...and what about primary keys and replication on MS SQL server?
What is a unique index?
A unique index on a column is an index on that column that also enforces the constraint that you cannot have two equal values in that column in two different rows. Example:
CREATE TABLE table1 (foo int, bar int);
CREATE UNIQUE INDEX ux_table1_foo ON table1(foo); -- Create unique index on foo.
INSERT INTO table1 (foo, bar) VALUES (1, 2); -- OK
INSERT INTO table1 (foo, bar) VALUES (2, 2); -- OK
INSERT INTO table1 (foo, bar) VALUES (3, 1); -- OK
INSERT INTO table1 (foo, bar) VALUES (1, 4); -- Fails!
Duplicate entry '1' for key 'ux_table1_foo'
The last insert fails because it violates the unique index on column foo when it tries to insert the value 1 into this column for a second time.
In MySQL a unique constraint allows multiple NULLs.
It is possible to make a unique index on mutiple columns.
Primary key versus unique index
Things that are the same:
A primary key implies a unique index.
Things that are different:
A primary key also implies NOT NULL, but a unique index can be nullable.
There can be only one primary key, but there can be multiple unique indexes.
If there is no clustered index defined then the primary key will be the clustered index.
You can see it like this:
A Primary Key IS Unique
A Unique value doesn't have to be the Representaion of the Element
Meaning?; Well a primary key is used to identify the element, if you have a "Person" you would like to have a Personal Identification Number ( SSN or such ) which is Primary to your Person.
On the other hand, the person might have an e-mail which is unique, but doensn't identify the person.
I always have Primary Keys, even in relationship tables ( the mid-table / connection table ) I might have them. Why? Well I like to follow a standard when coding, if the "Person" has an identifier, the Car has an identifier, well, then the Person -> Car should have an identifier as well!
Foreign keys work with unique constraints as well as primary keys. From Books Online:
A FOREIGN KEY constraint does not have
to be linked only to a PRIMARY KEY
constraint in another table; it can
also be defined to reference the
columns of a UNIQUE constraint in
another table
For transactional replication, you need the primary key. From Books Online:
Tables published for transactional
replication must have a primary key.
If a table is in a transactional
replication publication, you cannot
disable any indexes that are
associated with primary key columns.
These indexes are required by
replication. To disable an index, you
must first drop the table from the
publication.
Both answers are for SQL Server 2005.
The choice of when to use a surrogate primary key as opposed to a natural key is tricky. Answers such as, always or never, are rarely useful. I find that it depends on the situation.
As an example, I have the following tables:
CREATE TABLE toll_booths (
id INTEGER NOT NULL PRIMARY KEY,
name VARCHAR(255) NOT NULL,
...
UNIQUE(name)
)
CREATE TABLE cars (
vin VARCHAR(17) NOT NULL PRIMARY KEY,
license_plate VARCHAR(10) NOT NULL,
...
UNIQUE(license_plate)
)
CREATE TABLE drive_through (
id INTEGER NOT NULL PRIMARY KEY,
toll_booth_id INTEGER NOT NULL REFERENCES toll_booths(id),
vin VARCHAR(17) NOT NULL REFERENCES cars(vin),
at TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
amount NUMERIC(10,4) NOT NULL,
...
UNIQUE(toll_booth_id, vin)
)
We have two entity tables (toll_booths and cars) and a transaction table (drive_through). The toll_booth table uses a surrogate key because it has no natural attribute that is not guaranteed to change (the name can easily be changed). The cars table uses a natural primary key because it has a non-changing unique identifier (vin). The drive_through transaction table uses a surrogate key for easy identification, but also has a unique constraint on the attributes that are guaranteed to be unique at the time the record is inserted.
http://database-programmer.blogspot.com has some great articles on this particular subject.
There are no disadvantages of primary keys.
To add just some information to #MrWiggles and #Peter Parker answers, when table doesn't have primary key for example you won't be able to edit data in some applications (they will end up saying sth like cannot edit / delete data without primary key). Postgresql allows multiple NULL values to be in UNIQUE column, PRIMARY KEY doesn't allow NULLs. Also some ORM that generate code may have some problems with tables without primary keys.
UPDATE:
As far as I know it is not possible to replicate tables without primary keys in MSSQL, at least without problems (details).
If something is a primary key, depending on your DB engine, the entire table gets sorted by the primary key. This means that lookups are much faster on the primary key because it doesn't have to do any dereferencing as it has to do with any other kind of index. Besides that, it's just theory.
In addition to what the other answers have said, some databases and systems may require a primary to be present. One situation comes to mind; when using enterprise replication with Informix a PK must be present for a table to participate in replication.
As long as you do not allow NULL for a value, they should be handled the same, but the value NULL is handled differently on databases(AFAIK MS-SQL do not allow more than one(1) NULL value, mySQL and Oracle allow this, if a column is UNIQUE)
So you must define this column NOT NULL UNIQUE INDEX
There is no such thing as a primary key in relational data theory, so your question has to be answered on the practical level.
Unique indexes are not part of the SQL standard. The particular implementation of a DBMS will determine what are the consequences of declaring a unique index.
In Oracle, declaring a primary key will result in a unique index being created on your behalf, so the question is almost moot. I can't tell you about other DBMS products.
I favor declaring a primary key. This has the effect of forbidding NULLs in the key column(s) as well as forbidding duplicates. I also favor declaring REFERENCES constraints to enforce entity integrity. In many cases, declaring an index on the coulmn(s) of a foreign key will speed up joins. This kind of index should in general not be unique.
There are some disadvantages of CLUSTERED INDEXES vs UNIQUE INDEXES.
As already stated, a CLUSTERED INDEX physically orders the data in the table.
This mean that when you have a lot if inserts or deletes on a table containing a clustered index, everytime (well, almost, depending on your fill factor) you change the data, the physical table needs to be updated to stay sorted.
In relative small tables, this is fine, but when getting to tables that have GB's worth of data, and insertrs/deletes affect the sorting, you will run into problems.
I almost never create a table without a numeric primary key. If there is also a natural key that should be unique, I also put a unique index on it. Joins are faster on integers than multicolumn natural keys, data only needs to change in one place (natural keys tend to need to be updated which is a bad thing when it is in primary key - foreign key relationships). If you are going to need replication use a GUID instead of an integer, but for the most part I prefer a key that is user readable especially if they need to see it to distinguish between John Smith and John Smith.
The few times I don't create a surrogate key are when I have a joining table that is involved in a many-to-many relationship. In this case I declare both fields as the primary key.
My understanding is that a primary key and a unique index with a not‑null constraint, are the same (*); and I suppose one choose one or the other depending on what the specification explicitly states or implies (a matter of what you want to express and explicitly enforce). If it requires uniqueness and not‑null, then make it a primary key. If it just happens all parts of a unique index are not‑null without any requirement for that, then just make it a unique index.
The sole remaining difference is, you may have multiple not‑null unique indexes, while you can't have multiple primary keys.
(*) Excepting a practical difference: a primary key can be the default unique key for some operations, like defining a foreign key. Ex. if one define a foreign key referencing a table and does not provide the column name, if the referenced table has a primary key, then the primary key will be the referenced column. Otherwise, the the referenced column will have to be named explicitly.
Others here have mentioned DB replication, but I don't know about it.
Unique Index can have one NULL value. It creates NON-CLUSTERED INDEX.
Primary Key cannot contain NULL value. It creates CLUSTERED INDEX.
In MSSQL, Primary keys should be monotonically increasing for best performance on the clustered index. Therefore an integer with identity insert is better than any natural key that might not be monotonically increasing.
If it were up to me...
You need to satisfy the requirements of the database and of your applications.
Adding an auto-incrementing integer or long id column to every table to serve as the primary key takes care of the database requirements.
You would then add at least one other unique index to the table for use by your application. This would be the index on employee_id, or account_id, or customer_id, etc. If possible, this index should not be a composite index.
I would favor indices on several fields individually over composite indices. The database will use the single field indices whenever the where clause includes those fields, but it will only use a composite when you provide the fields in exactly the correct order - meaning it can't use the second field in a composite index unless you provide both the first and second in your where clause.
I am all for using calculated or Function type indices - and would recommend using them over composite indices. It makes it very easy to use the function index by using the same function in your where clause.
This takes care of your application requirements.
It is highly likely that other non-primary indices are actually mappings of that indexes key value to a primary key value, not rowid()'s. This allows for physical sorting operations and deletes to occur without having to recreate these indices.
In MySQL, does following statement make sense?
CREATE TABLE `sku_classification` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`sku` int(10) unsigned NOT NULL,
`business_classification_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `IDX_SKU_BUSINESS_CLASSIFICATION` (`sku`,`business_classification_id`),
UNIQUE KEY `sku` (`sku`)
)
Is it an unnecessary overkill to add a unique key on a combination of fields (sku,business_classification_id), one of which (sku) already has unique index on it? Or is it not, and there is indeed some reason for such duplicate unique index?
Yes, you can. But it does not make sense. But, let's analyze what is going on.
An INDEX (UNIQUE or not) is a BTree that facilitates lookups in the table.
A UNIQUE index is both an index and a "constraint" saying that there shall not be any duplicates.
You have already said UNIQUE(sku). This provides both an index an a uniqueness constraint.
Adding UNIQUE(sku, x) in that order:
Does not provide any additional uniqueness constraint,
Does not provide any additional indexing capability, except...
Does provide a "covering" index that could be useful if the only two columns mentioned in a SELECT were sku and x. Even so, you may as make it an INDEX not a UNIQUE, because...
Every INSERT must do some extra effort to prevent "duplicate key". (OK, the INSERT code is not smart enough to see that you have UNIQUE(sku).)
If that is your complete table, there is no good reason to have the id AUTO_INCREMENT; you may as well promote sku to be the PRIMARY KEY. (A PK is a UNIQUE KEY.)
Furthermore... If, on the other hand, you were suggesting UNIQUE(x, sku), then there is one slight difference. This provides you a way to efficiently lookup by x -- a range of x, or x=constant AND sku BETWEEN ..., or certain other thing that are not provided by (sku, x). Order matters in an index. But, again, it may as well be INDEX(x, sku), not UNIQUE.
So, the optimal set of indexes for the table as presented is not 3 indexes, but 1:
PRIMARY KEY(sku)
One more note: With InnoDB, the PK is "clustered" in BTree with the data. That is, looking up by the PK is very efficient. When you need to go through a "secondary index", there are two steps: first drill down the secondary index's BTree to find the PK, then drill down the PK's BTree.
When should I use KEY, PRIMARY KEY, UNIQUE KEY and INDEX?
KEY and INDEX are synonyms in MySQL. They mean the same thing. In databases you would use indexes to improve the speed of data retrieval. An index is typically created on columns used in JOIN, WHERE, and ORDER BY clauses.
Imagine you have a table called users and you want to search for all the users which have the last name 'Smith'. Without an index, the database would have to go through all the records of the table: this is slow, because the more records you have in your database, the more work it has to do to find the result. On the other hand, an index will help the database skip quickly to the relevant pages where the 'Smith' records are held. This is very similar to how we, humans, go through a phone book directory to find someone by the last name: We don't start searching through the directory from cover to cover, as long we inserted the information in some order that we can use to skip quickly to the 'S' pages.
Primary keys and unique keys are similar. A primary key is a column, or a combination of columns, that can uniquely identify a row. It is a special case of unique key. A table can have at most one primary key, but more than one unique key. When you specify a unique key on a column, no two distinct rows in a table can have the same value.
Also note that columns defined as primary keys or unique keys are automatically indexed in MySQL.
KEY and INDEX are synonyms.
You should add an index when performance measurements and EXPLAIN shows you that the query is inefficient because of a missing index. Adding an index can improve the performance of queries (but it can slow down modifications to the table).
You should use UNIQUE when you want to contrain the values in that column (or columns) to be unique, so that attempts to insert duplicate values result in an error.
A PRIMARY KEY is both a unique constraint and it also implies that the column is NOT NULL. It is used to give an identity to each row. This can be useful for joining with another table via a foreign key constraint. While it is not required for a table to have a PRIMARY KEY it is usually a good idea.
Primary key does not allow NULL values, but unique key allows NULL values.
We can declare only one primary key in a table, but a table can have multiple unique keys (column assign).
PRIMARY KEY AND UNIQUE KEY are similar except it has different functions. Primary key makes the table row unique (i.e, there cannot be 2 row with the exact same key). You can only have 1 primary key in a database table.
Unique key makes the table column in a table row unique (i.e., no 2 table row may have the same exact value). You can have more than 1 unique key table column (unlike primary key which means only 1 table column in the table is unique).
INDEX also creates uniqueness. MySQL (example) will create a indexing table for the column that is indexed. This way, it's easier to retrieve the table row value when the query is queried on that indexed table column. The disadvantage is that if you do many updating/deleting/create, MySQL has to manage the indexing tables (and that can be a performance bottleneck).
Hope this helps.
Unique Keys: The columns in which no two rows are similar
Primary Key: Collection of minimum number of columns which can uniquely identify every row in a table (i.e. no two rows are similar in all the columns constituting primary key). There can be more than one primary key in a table. If there exists a unique-key then it is primary key (not "the" primary key) in the table. If there does not exist a unique key then more than one column values will be required to identify a row like (first_name, last_name, father_name, mother_name) can in some tables constitute primary key.
Index: used to optimize the queries. If you are going to search or sort the results on basis of some column many times (eg. mostly people are going to search the students by name and not by their roll no.) then it can be optimized if the column values are all "indexed" for example with a binary tree algorithm.
The primary key is used to work with different tables. This is the foundation of relational databases. If you have a book database it's better to create 2 tables - 1) books and 2) authors with INT primary key "id". Then you use id in books instead of authors name.
The unique key is used if you don't want to have repeated entries. For example you may have title in your book table and want to be sure there is only one entry for each title.
Primary key - we can put only one primary key on a table into a table and we can not left that column blank when we are entering the values into the table.
Unique Key - we can put more than one unique key on a table and we may left that column blank when we are entering the values into the table.
column take unique values (not same) when we applied primary & unique key.
Unique Key :
More than one value can be null.
No two tuples can have same values in unique key.
One or more unique keys can be combined to form a primary key, but not vice versa.
Primary Key
Can contain more than one unique keys.
Uniquely represents a tuple.
If I have a table that has a primary key and a foreign key, and searches are frequently done with queries that include both (...WHERE primary=n AND foreign=x), is there any performance benefit to making a multiple index in MySQL using the two keys?
I understand that they are both indexes already, but I am uncertain if the foreign key is still seen as an index when included in another table. For example, would MySQL go to the primary key, and then compare all values of the foreign key until the right one is found, or does it already know where it is because the foreign key is also an index?
Update: I am using InnoDB tables.
For equality comparisons, you cannot get an improvement over the primary key index (because at that point, there is at most just one row that can match).
The access path would be:
look at the primary key index for primary = n
get the single matching row from the table
check any other conditions using the row in the table
A composite index might make some sense if you have a range scan on the primary key and want to narrow that down by the other column.
When should I use KEY, PRIMARY KEY, UNIQUE KEY and INDEX?
KEY and INDEX are synonyms in MySQL. They mean the same thing. In databases you would use indexes to improve the speed of data retrieval. An index is typically created on columns used in JOIN, WHERE, and ORDER BY clauses.
Imagine you have a table called users and you want to search for all the users which have the last name 'Smith'. Without an index, the database would have to go through all the records of the table: this is slow, because the more records you have in your database, the more work it has to do to find the result. On the other hand, an index will help the database skip quickly to the relevant pages where the 'Smith' records are held. This is very similar to how we, humans, go through a phone book directory to find someone by the last name: We don't start searching through the directory from cover to cover, as long we inserted the information in some order that we can use to skip quickly to the 'S' pages.
Primary keys and unique keys are similar. A primary key is a column, or a combination of columns, that can uniquely identify a row. It is a special case of unique key. A table can have at most one primary key, but more than one unique key. When you specify a unique key on a column, no two distinct rows in a table can have the same value.
Also note that columns defined as primary keys or unique keys are automatically indexed in MySQL.
KEY and INDEX are synonyms.
You should add an index when performance measurements and EXPLAIN shows you that the query is inefficient because of a missing index. Adding an index can improve the performance of queries (but it can slow down modifications to the table).
You should use UNIQUE when you want to contrain the values in that column (or columns) to be unique, so that attempts to insert duplicate values result in an error.
A PRIMARY KEY is both a unique constraint and it also implies that the column is NOT NULL. It is used to give an identity to each row. This can be useful for joining with another table via a foreign key constraint. While it is not required for a table to have a PRIMARY KEY it is usually a good idea.
Primary key does not allow NULL values, but unique key allows NULL values.
We can declare only one primary key in a table, but a table can have multiple unique keys (column assign).
PRIMARY KEY AND UNIQUE KEY are similar except it has different functions. Primary key makes the table row unique (i.e, there cannot be 2 row with the exact same key). You can only have 1 primary key in a database table.
Unique key makes the table column in a table row unique (i.e., no 2 table row may have the same exact value). You can have more than 1 unique key table column (unlike primary key which means only 1 table column in the table is unique).
INDEX also creates uniqueness. MySQL (example) will create a indexing table for the column that is indexed. This way, it's easier to retrieve the table row value when the query is queried on that indexed table column. The disadvantage is that if you do many updating/deleting/create, MySQL has to manage the indexing tables (and that can be a performance bottleneck).
Hope this helps.
Unique Keys: The columns in which no two rows are similar
Primary Key: Collection of minimum number of columns which can uniquely identify every row in a table (i.e. no two rows are similar in all the columns constituting primary key). There can be more than one primary key in a table. If there exists a unique-key then it is primary key (not "the" primary key) in the table. If there does not exist a unique key then more than one column values will be required to identify a row like (first_name, last_name, father_name, mother_name) can in some tables constitute primary key.
Index: used to optimize the queries. If you are going to search or sort the results on basis of some column many times (eg. mostly people are going to search the students by name and not by their roll no.) then it can be optimized if the column values are all "indexed" for example with a binary tree algorithm.
The primary key is used to work with different tables. This is the foundation of relational databases. If you have a book database it's better to create 2 tables - 1) books and 2) authors with INT primary key "id". Then you use id in books instead of authors name.
The unique key is used if you don't want to have repeated entries. For example you may have title in your book table and want to be sure there is only one entry for each title.
Primary key - we can put only one primary key on a table into a table and we can not left that column blank when we are entering the values into the table.
Unique Key - we can put more than one unique key on a table and we may left that column blank when we are entering the values into the table.
column take unique values (not same) when we applied primary & unique key.
Unique Key :
More than one value can be null.
No two tuples can have same values in unique key.
One or more unique keys can be combined to form a primary key, but not vice versa.
Primary Key
Can contain more than one unique keys.
Uniquely represents a tuple.