If I add a foreign key between two tables, am I allowed to add orphan rows afterwards? Also when I'm creating a foreign key between two tables, is there any way to create it ignoring the orphan rows?
My next question is about the efficiency of foreign keys. I had always thought that they created an index between one key in a table and the corresponding key in another table which essentially made it a linear lookup when doing a join.
Is a foreign key much more efficient then simply having an index or are they the same?
Thanks.
foreign key relationship is often between a foreign key in one table and a primary key in another.
primary key do implicitly create an index.
Not foreign keys. Mostly it's good practice to add index on a foreign key column.
keys are constraints to guarantee data consistency and index can be used improve access performance on the data. So these are different things that you often combine in practices and thus are often confused.
Regarding orphan rows I think the concept of keys is to prevent this. But I'm not completely sure If I understand exactly what you are asking here. I think adding orphan rows is not possible and creating keys if orphan rows exist sounds kind of impossible.
see also other questions on SO dealing with foreign key / index topic.
Here some more good answers related to primary key / key / index
Related
I am studying database course now, and I know a foreign key is the primary key of other table. But I am curious about the presentation of relationship: if we don't use foreign key to link with other table, but use a function to map a primary key of a record from a table to another one, does it work? I think it works in theory, but I am not sure is good idea in reality. Is it reasonable?
P.S.
because I think a table is a set, a function is mapping method from a set to another.
Thanks in advance,
Sincerely.
You don't have to define foreign keys in a MySQL database. In a query, you can use joins to define how relations should be fed into your query's result set.
There are good reasons to define foreign keys, though. Some of these are:
Defining a foreign key creates an index on that column, influencing the way that the RDBMS stores its data, so that it can optimize queries for faster results;
If you set a foreign key in MySQL, you can also define what happens to child records when the table containing the primary key is changed. Child records may, for example, be deleted automatically (a "cascading delete"). This is helpful for database integrity, as this avoids orphaned records in the child table. These cascades may go through several relationships levels.
Is it possible to have a foreign key without an index in MySQL 5.6.34? I want that because I created a nullable column in 20M rows with a foreign key to another table. As this is a new feature, only the new rows MAY have this column filled with an actual value, and as you may expect, the cardinality of that index becomes horrible. So, for most of the time, using that index is actually a bad idea. The problem: I have tons of queries that shares this same restriction:
[...] from large_table where tenant_id = ? and nullable_foreign_key_with_index is null and [...]
The issue? MySQL thinks that it's a good idea to use an index_merge/intersect strategy for query resolution. In this case MySQL would do 2 queries in parallel: one with tenant_id (which uses a valid and good index) and another one with nullable_foreign_key_with_index which is bad, almost a "full table scan in parallel" given that the cardinality of this index is <1000 in a table with >20M rows. More details about this "problem" in here
So, what are the pssible solutions? Given that MySQL "forces" a foreign key to have an index attached:
Drop the foreign key and the index. This is bad, because in the case of a bug in the app we may compromise the referential integrity.
FOREIGN_KEY_CHECKS=0; Drop index; FOREIGN_KEY_CHECKS=1. This is bad, because even that the foreign key still exists, MySQL doesn't validade the column anymore to check if the value actually exists. Is that a bug?
Use query hints in all existing queries to make sure that we are only using the old and efficient "tenant_id_index". This is bad because I have to hunt down all existing queries and also remember to use it again when news queries are built.
So, how can I say: "MySQL, don't bother creating an index for this foreign key, but keep validating it's content in the related table, which is indexed by primary key anyway". Am I missing something? The best idea so far is to remove the foreign key and just believe that the app is working as expected, which probably it is, but this would start a classic discussion about having constraints in APP vs DATABASE. Any ideas?
For this query:
from large_table
where tenant_id = ? and
nullable_foreign_key_with_index is null and [...]
Just add the index large_table(tenant_id, nullable_foreign_key_with_index).
MySQL should use this index for the table.
I'm pretty sure you can do this backwards (I would be 100% sure if the comparison were to anything other than NULL, but I'm pretty sure MySQL does the right thing with NULL as well.)
large_table(nullable_foreign_key_with_index, tenant_id)
And MySQL will recognize that this index works for the foreign key and not create any other index.
Q: How can I say: "MySQL, don't bother creating an index for this foreign key, but keep validating it's content in the related table, which is indexed by primary key anyway"
A: No can do. InnoDB requires a suitable index to support the enforcement of foreign key constraint.
Consider the flip side of it... if we are going to DELETE a row in the parent table, then InnoDB needs to check the foreign key constraint.
That means InnoDB needs to check the contents of the child table, to find rows that have a specific value in foreign key column. Essentially equivalent to
SELECT ... FROM child_table c WHERE c.foreign_key_col = ?
And to do that, InnoDB requires that there be an index on child_table that has foreign_key_col as the leading column.
The options suggested in the question (disabling or dropping the foreign key) will work because then InnoDB isn't going enforce the foreign key. But as noted in the question, what this means is that the foreign key isn't enforced. Which defeats the purpose of the foreign key. The application code could be responsible for enforcing referential integrity, or we could write some ug-gghhh-ly triggers (no, we don't want to go there).
As Gordon already noted in his (as usual excellent) answer... the problem isn't really dropping the index on the foreign key column. The actual problem is the inefficient execution plan. And the most likely fix for that is to make sure a more suitable index is available.
Composite indexes are the way to go. An index like this:
... ON child_table (foreign_key_col,tenant_id,...)
would satisfy the requirement of the foreign key, an index with the foreign key column as a leading column. And drop the (now redundant) index on just the singleton foreign_key_col.
This index could also be used to satisfy the query that's using a horrible index merge access plan. (Verify with EXPLAIN.)
Also, consider adding columns (such as foreign_key_col) to the index that has tenant_id as the leading column
... ON child_table (tenant_id,...,foreign_key_col,...)
and drop the redundant index on the singleton tenant_id col.
Summary: Almost always it is better to have a composite index instead of depending on "index merge intersect".
If both columns are tested with = (or IS NULL), it does not matter which order the columns are in the index definition. That is, cardinality is irrelevant.
This question already has answers here:
SQL Primary Key - is it necessary?
(5 answers)
Closed 7 years ago.
In database systems, should every table have a primary key?
For example I have a table table1(foreignkey1,foreignkey2,attribute) like this.table1 does not have a primary key.
Should I define a primary key for this table like table1id?
This is a subjective question, so I hope you don't mind me answering with some opinion :)
In the vast majority of tables I've made – I'm talking 95%+ – I've added a primary key, and been glad I did. This is either the most critical unique field in my table (think "social security number") or, more often than not, just an auto-incrementing number that allows me to quickly and easily refer to a field when querying.
This latter use is the most common, and it even has its own name: a "surrogate" or "synthetic" key. This is a value auto-generated by the database and not derived from your application data. If you want to add relations between your tables, this surrogate key is immediately helpful as a foreign key. As someone else answered, these keys are so common that MySQL likes to add one even if you don't, so I'd suggest that means the consensus is very heavily biased towards adding primary keys.
One other thing I like about primary keys is that they help convey your intent to others reading your table schemata and also to your DMBS: "this bit is how I intend to identify my rows uniquely, don't let me try to break that rule!"
To answer your question specifically: no, a primary key is not necessary. But realistically if you intend to store data in the table for any period of time beyond a few minutes, I would very strongly recommend you add one.
No, it is not required for every table to have a primary key. Whether or not a table should have a primary key is based on requirements of your database.
Even though this is allowed it is bad practice because it allows for one to add duplicate rows further preventing the unique identification of rows. Which contradicts the underline purposes of having a database.
I am a strong fan of synthetic primary keys. These are auto-incremented columns that uniquely identify each row.
These provide functionality such as:
Ability to see the order of insertion of rows. Which were inserted most recently?
Ability to create a foreign key relationship to the table. You might not need one now, but it might be useful in the future.
Ability to rename "data" columns without affecting other tables.
Presumably, for your table, you can define a primary key on (foreignkey1, foreighkey2). Composite primary keys are also sensible, but they are cumbersome for foreign key relationships and joins. And, when there are foreign key relationships, they may cause additional storage, because the composite key ends up being stored across multiple tables.
It's a good practise to have a primary key/composite primary key for a table:
it helps to join tables,
clustered tables will need primary key.
Database design should have primary key for a table.
In MySQL storage engine always creates a PRIMARY KEY if you didn't specify it explicitly, thus making an extra column you don't have access to.
You can create Composite Primary key like:
CREATE TABLE table1(
FK1 INT,
FK2 INT,
ATTRIBUTE INT,
PRIMARY KEY (FK1, FK2)
)
or create a constraint on table1:
ALTER TABLE table_name
ADD CONSTRAINT pk_table1 PRIMARY KEY (FK1,FK2)
I have table department that has two column (dept_ID and dept_name)
my another table is login which i create a foreign key for the column dept_name referencing dept_name at table department. so i create an index named index_department, now in login table, i want to create another foreign key for the column eadd which will reference to DIFFERENT table named info_table.
should i create another index for the second foreign key??
another scenario, i want to create a dept_name column at info_table too. can i use the same index 'index_department'??
The general answer is, "It depends."
As #gordon-Linoff commented "You create indexes to meet performance requirements for queries."
Indexes take up space and and take processing time as the have to be maintained. So the case for any given index depends on the trade off between cost and usage. For example if you the data rarely changes, but you look it up a lot you will prefer to have more indexes.
My educated guess is that on the scale you are probably working you do want the indexes on all your foreign keys.
Specifically in mysql you seem to get the index is you formally add the FK constraint. It is discussed here does mysql index foreign key columns automatically
I say formally, because you can have implied foreign key relationships without actually declaring/enforcing the constraints. People sometimes do that to avoid even the cost of the checking/enforcing constraint. The risk is in updates that violate referential integrity. But I'm drifting onto a tangent.
As a side note, there is some pertinent discussion here does a foreign key automatically create an index
In MySQL (at least 5.6, which I am using), indices are automatically created for foreign keys.
I have to work with a database to do reporting
The DB is quite big : 416 055 104 rows
Each row is very light though, just booleans and int ids.
Each row is identify by 3 columns, but at my surprise, there is no Primary Key on it.
Only a Clustered Index with a unique constraint.
So Knowing that, I have 2 question.
Could there be ANY good reason for that?
Is there any way I can turn this into a primary key.
Regarding question 2
Creating a new primary key also creates a non-clustered index to associate with (there is already an existing clustered one).
This is not what I am looking for. I want to keep that same index, but also make it a primary key.
Is it possible?
Would that be faster that creating the whole index again? (I hope so)
What could be the consequences? (locks? crash? corrupted data?)
There is little or no difference between a PRIMARY KEY and a UNIQUE constraint on non-nullable columns. So if the columns in question are non-nullable then I suggest you do nothing. The main reason to make a candidate key into a primary key is if you have some software (such as a data modelling tool or other development tool) that expects the key to be identified with a PRIMARY KEY constraint.
Good question.
If you already have a unique index on non nullable columns then you have a candidate key. I'm not aware of any particular benefit of making this an "official" primary key. In fact I have a feeling that not making it a PK will give greater flexibility.
A unique index can allow null
values. A primary key can't.
I believe you can't "mark" an existing index as the primary key. You'd have to drop it and recreate. To avoid stuff, I'd say it'd be good to place a TABLOCKX, HOLDLOCK on the table before doing that.