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In my system I have temporary entities that are created based on rules stored in my database, and the entities are not persisted.
Now, I need is to store information about these entities, and because they are created based on rules and are not stored, they have no ID.
I came up with a formula to generate an ID for these temp entities based on the rule that was used to generate them: id = rule id + "-" + entity index in the rule. This formula generates unique strings of the form 164-3, 123-0, 432-2, etc...
My question is how should I build my table (regarding primary key and clustered index) when my keys have no relation or order? Keep in mind that I will only (99.9% of the time) query the table using the id mentioned above.
Options I thought about after much reading, but don't have the knowledge to determine which is better:
1) primary key on a varchar column with clustered index. -According to various sources, this would be bad because of fragmentation and the wideness of the key. Also their format is pretty weird for sorting.
2) primary key on varchar column without clustered index (heap table). -Also a bad idea according to various sources due to indexing and fragmentation issues.
3) identity int column with clustered index, and a varchar column as primary key with unique index. -Can't really see the benefit of the surogate key here since it would mainly help with range queries and ordering and I would never query the table based on this key because it would be unknown at all times.
4) 2 columns composite key: rule id + rule index columns.
-Now I don't have strings but I have two columns that will be copied to FKs and non clustered indexes. Also I'm not sure what indexes I would use in this case.
Can anybody shine a light here? Any help is appreciated.
--Edit
I will perform more selects than inserts;
I will perform more inserts than updates;
All selects will include at least rule id;
If I use a surogate primary key, and a unique index on (rule id, index), then I can use the surogate for subsequent operations after retrieving data by rule id, which would be faster. Also, inserts would be faster.
However, because the data will be stored according to the surogate key, I might have records that have the same rule id, but different index, stored quite far from each other on disk, which means even with an index on rule id, retrieving the data could be kinda slow.
If I use (rule id, index) as clustered primary key, rows with same rule id would be stored close to each other, and selecting data by rule id would be efficient enough. However, I suspect inserts would be slow.
Is the rationale above correct?
Using a heap is generally a bad idea unless proven otherwise. Even so, you will need a very solid reason for not having a clustered index (any one will make things better, even on identity column).
Storing this key in a single column is okay; if you want natural sorting, you can pad your numbers with zeroes, for example. However, this will widen the key.
Having a composite primary key (and, subsequently, foreign keys) is completely acceptable, especially when dealing with natural keys, like the one you have. This will give you the narrowest possible key - int + int or some such - while eliminating the sorting issue at the same time. I would recommend to make this PK clustered to reduce additional key lookups.
Fragmentation here will not be a big issue; at least, no bigger than with any other indexing decision. Any index built on such a key will be prone to fragmentation, clustered or no. In any case, your DBA should know how to keep an index such as this in top form.
Regarding the order of columns in the index, the following rules usually apply:
If partial key match will take place (filtering by one part of the key but not by the other) the one which is used most often should go first;
If No.1 isn't applicable and all parts of the key used in all queries, the column with the highest cardinality should go first.
The order of remaining columns (if there are more than 1) isn't of much importance because SQL Server only creates distribution statistics for the first column in a composite index. However, it is a good idea to list them in order of decreasing cardinality.
EDIT: Seeing your update with additional details, here are the most suitable options. Suppose your table looks like this:
-- Sample table
create table dbo.TempEntities (
RuleId int not null,
IndexId int not null,
-- Remaining columns listed here
EntityData xml not null
);
go
From here, the most straightforward way is to use the natural key as a clustered index:
-- Option 1 - natural clustered index
alter table dbo.TempEntities
add constraint PK_TempEntities primary key clustered (RuleId, IndexId);
go
However, if you have any child tables that would reference this one, it might not be the most convenient solution, because natural keys are prone to updates, which creates a mess where you could avoid it. Instead, a surrogate key can be introduced, like this:
-- Option 2 - surrogate clustered, natural nonclustered
alter table dbo.TempEntities add Id bigint identity(1,1) not null;
alter table dbo.TempEntities
add constraint PK_TempEntities primary key clustered (Id);
alter table dbo.TempEntities
add constraint UQ_TempEntities_RuleIdIndexId unique (RuleId, IndexId);
go
It makes sense to have the surrogate PK clustered, because it will result in much less page splits, making inserts faster (despite having one index more compared to Option 1). Without any intimate knowledge of your queries, this is probably the most balanced solution.
Shuffling the clustered attribute between surrogate and natural keys has mostly academic value and can only make difference on a high-load system with hundreds of inserts happening every second on 24*7 schedule. If your system is indeed as such, please seek a professional consultant who will analyse your queries and provide the solution tailored to your situation.
In SQL Server, I have a non nullable column with a unique clustered index on it.
If I make this column a Primary Key the exact same index is created automatically plus
the column is recognized as a Primary Key.
I understand the abstract/semantic difference.
(Primary Key identifies the entity, while any other column with this index may not.
For example, a Person can have Email field which is Unique,Non-nullable... but can be changed)
But what bothers me is the actual difference when it comes to the DB engine itself.
What will happen if I will just create an Id column, make it non-nullable, create a unique clustered index for it, make it Identity Increment, but without the Primary Key constraint?
In what scenarios the Primary Key constraint comes into play?
(I've looked at many related questions before asking this, but all the answers I saw ended up with an abstract/theoretical explanation).
Nothing will be different really. You specify PRIMARY KEY to relay your intentions, not so that the engine does anything differently. When constructing a query plan, the optimizer will still use the uniqueness for all of its properties, and will still use the clustered index for all of its properties, regardless of whether you technically created it as a PRIMARY KEY. When creating a FOREIGN KEY, you can still reference the column(s) specified as unique (clustered or not). The difference is solely in the metadata (sys.indexes.is_primary_key) and in SSMS' representation to you (oh and the fact that you can create a unique clustered index on a NULLable column, but you can't create a PRIMARY KEY on that column).
In fact there are many cases where you want to completely separate the clustered index from the PRIMARY KEY. If you have a table where the PK is a GUID, for example, and you are typically running date range queries against the table, you are probably better off having the PK be non-clustered and have a clustered index on a naturally increasing column (the datetime column) - both to minimize page splits on heavy insert activity and also to best assist date range queries. The non-clustered index will be perfectly fine for looking up individual GUIDs. (I wanted to mention that because a lot of people think the primary key has to be clustered. Not true.)
Also interesting to note that if you create a PRIMARY KEY constraint, then create a unique clustered index with the same name using DROP_EXISTING, the is_primary_key column will still be 1 and Object Explorer will still show the index name under Keys.
Here is one scenario - a lot of code to data mapping frameworks look at the database metadata (what are the primary keys, foreign keys, etc) to determine how code is executed. For example Hibernate requires a primary key.
A typical scenario might be generating a where clause for an update.
In tables where you need only 1 column as the key, and values in that column can be integers, when you shouldn't use an identity field?
To the contrary, in the same table and column, when would you generate manually its values and you wouldn't use an autogenerated value for each record?
I guess that it would be the case when there are lots of inserts and deletes to the table. Am I right? What other situations could be?
If you already settled on the surrogate side of the Great Primary Key Debacle then I can't find a single reason not use use identity keys. The usual alternatives are guids (they have many disadvatages, primarily from size and randomness) and application layer generated keys. But creating a surrogate key in the application layer is a little bit harder than it seems and also does not cover non-application related data access (ie. batch loads, imports, other apps etc). The one special case is distributed applications when guids and even sequential guids may offer a better alternative to site id + identity keys..
I suppose if you are creating a many-to-many linking table, where both fields are foreign keys, you don't need an identity field.
Nowadays I imagine that most ORMs expect there to be an identity field in every table. In general, it is a good practice to provide one.
I'm not sure I understand enough about your context, but I interpret your question to be:
"If I need the database to create a unique column (for whatever reason), when shouldn't it be a monotonically increasing integer (identity) column?"
In those cases, there's no reason to use anything other than the facility provided by the DBMS for the purpose; in your case (SQL Server?) that's an identity.
Except:
If you'll ever need to merge the table with data from another source, use a GUID, which will prevent duplicate keys from colliding.
If you need to merge databases it's a lot easier if you don't have to regenerate keys.
One case of not wanting an identity field would be in a one to one relationship. The secondary table would have as its primary key the same value as the primary table. The only reason to have an identity field in that situation would seem to be to satisfy an ORM.
You cannot (normally) specify values when inserting into identity columns, so for example if the column "id" was specified as an identify the following SQL would fail:
INSERT INTO MyTable (id, name) VALUES (1, 'Smith')
In order to perform this sort of insert you need to have IDENTITY_INSERT on for that table - this is not intended to be on normally and can only be on for a maximum of 1 tables in the database at any point in time.
If I need a surrogate, I would either use an IDENTITY column or a GUID column depending on the need for global uniqueness.
If there is a natural primary key, or the primary key is defined as a unique combination of other foreign keys, then I typically do not have an IDENTITY, nor do I use it as the primary key.
There is an exception, which is snapshot configuration tables which I am tracking with an audit trigger. In this case, there is usually a logical "primary key" (usually date of the snapshot and natural key of the row - like a cost center or gl account number for which the row is a configuration record), but instead of using the natural "primary key" as the primary key, I add an IDENTITY and make that the primary key and make a unique index or constraint on the date and natural key. Although theoretically the date and natural key shouldn't change, in these tables, if a user does that instead of adding a new row and deleting the old row, I want the audit (which reflects a change to a row identified by its primary key) to really reflect a change in the row - not the disappearance of a key and the appearance of a new one.
I recently implemented a Suffix Trie in C# that could index novels, and then allow searches to be done extremely fast, linear to the size of the search string. Part of the requirements (this was a homework assignment) was to use offline storage, so I used MS SQL, and needed a structure to represent a Node in a table.
I ended up with the following structure : NodeID Character ParentID, etc, where the NodeID was a primary key.
I didn't want this to be done as an autoincrementing identity for two main reasons.
How do I get the value of a NodeID after I add it to the database/data table?
I wanted more control when it came to generating my own IDs.
I am having a very small tables with at most 5 records that holds some labels. I am using Postgres.
The structure is as follows:
id - smallint
label - varchar(100)
The table will be used mainly to reference the rows from other tables. The question is if it's really necessary to have a primary key on id or to have just an index on the id or have them both?
I did read about indexes and primary keys and I understand that this depends quite a lot on what's the table going to be used for:
Tables with no Primary Key
Edit: I was going to ask about having a primary key or an index or have them both. I edited the question.
It is always good practice to have a primary key column. The typical scenario it is needed is when you want to update or delete a row, having a PK makes it much easier and safer.
Yes, a primary key is not only good practice -- it's crucial. A table that lacks a unique key fails to be in First Normal Form.
You must declare a PRIMARY KEY or UNIQUE constraint if you want other tables to reference this one with a foreign key.
In most RDBMS brands, both PRIMARY KEY and UNIQUE constraints implicitly create an index on the column(s). If it doesn't do this implicitly, you may be required to define the index yourself before you can declare the constraint.
Yes, you will need a primary key on the id field, since you do not want two labels that share the same id.
You also want an index, to speed up the search/lookup process in this table (although for small tables there is less performance gain). The sequence will just help you fill in the next ID; it does not prevent you from changing a previous value into one that already exists.
There are very little reasons for creating an index instead of a primary key. AS Bill Karwin said, you won't save resources at all. And, as you may have already guessed, there is no need at all to create a new index if you have the primary key.
In some cases it may be hard to find a key candidate. But it doesn't seems to be the case and it clearly goes against some good practices.
By the way. As your table is so small most queries will rather use a full table scan even if there is an index. Don't worry you see a full table scan.
From the developer's point of view, PRIMARY KEY is just a combination of NOT NULL and UNIQUE INDEX on one of the columns.
A UNIQUE INDEX is good if:
You need to enforce uniqueness. Using index is the most efficient way to do that.
You need to perform a SELECT, UPDATE or DELETE with a WHERE condition that is selective on the indexed field, that is number of rows affected by the query is much less that total number of the rows (say, 10 rows of 2,000,000).
A UNIQUE INDEX is bad if:
You don't need uniqueness on this field, of course :) But you'll better have one unique index in the table, for each record to be identifiable.
You need fast INSERTS
You need fast UPDATE's and DELETE's of the indexed value with a WHERE condition that is not selective on the indexed field, that is number of rows affected by the query is comparable to the total number of the rows (say, 1,500,000 rows of 2,000,000).
Given that you are going to have a little table, I'd advice you to create a PRIMARY KEY on it.
I'm currently designing a brand new database. In school, we always learned to put a primary key in each table.
I read a lot of articles/discussions/newsgroups posts saying that it's better to use unique constraint (aka unique index for some db) instead of PK.
What's your point of view?
A Primary Key is really just a candidate key that does not allow for NULL. As such, in SQL terms - it's no different than any other unique key.
However, for our non-theoretical RDBMS's, you should have a Primary Key - I've never heard it argued otherwise. If that Primary Key is a surrogate key, then you should also have unique constraints on the natural key(s).
The important bit to walk away with is that you should have unique constraints on all the candidate (whether natural or surrogate) keys. You should then pick the one that is easiest to reference in a Foreign Key to be your Primary Key*.
You should also have a clustered index*. this could be your Primary Key, or a natural key - but it's not required to be either. You should pick your clustered index based on query usage of the table. When in doubt, the Primary Key is not a bad first choice.
Though it's technically only required to refer to a unique key in a foreign key relationship, it's accepted standard practice to greatly favor the primary key. In fact, I wouldn't be surprised if some RDBMS only allow primary key references.
Edit: It's been pointed out that Oracle's term of "clustered table" and "clustered index" are different than Sql Server. The equivalent of what I'm speaking of in Oracle-ese is an Index Ordered Table and it is recommended for OLTP tables - which, I think, would be the main focus of SO questions. I assume if you're responsible for a large OLAP data warehouse, you should already have your own opinions on database design and optimization.
Can you provide references to these articles?
I see no reason to change the tried and true methods. After all, Primary Keys are a fundamental design feature of relational databases.
Using UNIQUE to serve the same purpose sounds really hackish to me. What is their rationale?
Edit: My attention just got drawn back to this old answer. Perhaps the discussion that you read regarding PK vs. UNIQUE dealt with people making something a PK for the sole purpose of enforcing uniqueness on it. The answer to this is, If it IS a key, then make it key, otherwise make it UNIQUE.
A primary key is just a candidate key (unique constraint) singled out for special treatment (automatic creation of indexes, etc).
I expect that the folks who argue against them see no reason to treat one key differently than another. That's where I stand.
[Edit] Apparently I can't comment even on my own answer without 50 points.
#chris: I don't think there's any harm. "Primary Key" is really just syntactic sugar. I use them all the time, but I certainly don't think they're required. A unique key is required, yes, but not necessarily a Primary Key.
It would be very rare denormalization that would make you want to have a table without a primary key. Primary keys have unique constraints automatically just by their nature as the PK.
A unique constraint would be used when you want to guarantee uniqueness in a column in ADDITION to the primary key.
The rule of always have a PK is a good one.
http://msdn.microsoft.com/en-us/library/ms191166.aspx
You should always have a primary key.
However I suspect your question is just worded bit misleading, and you actually mean to ask if the primary key should always be an automatically generated number (also known as surrogate key), or some unique field which is actual meaningful data (also known as natural key), like SSN for people, ISBN for books and so on.
This question is an age old religious war in the DB field.
My take is that natural keys are preferable if they indeed are unique and never change. However, you should be careful, even something seemingly stable like a persons SSN may change under certain circumstances.
Unless the table is a temporary table to stage the data while you work on it, you always want to put a primary key on the table and here's why:
1 - a unique constraint can allow nulls but a primary key never allows nulls. If you run a query with a join on columns with null values you eliminate those rows from the resulting data set because null is not equal to null. This is how even big companies can make accounting errors and have to restate their profits. Their queries didn't show certain rows that should have been included in the total because there were null values in some of the columns of their unique index. Shoulda used a primary key.
2 - a unique index will automatically be placed on the primary key, so you don't have to create one.
3 - most database engines will automatically put a clustered index on the primary key, making queries faster because the rows are stored contiguously in the data blocks. (This can be altered to place the clustered index on a different index if that would speed up the queries.) If a table doesn't have a clustered index, the rows won't be stored contiguously in the data blocks, making the queries slower because the read/write head has to travel all over the disk to pick up the data.
4 - many front end development environments require a primary key in order to update the table or make deletions.
Primary keys should be used in situations where you will be establishing relationships from this table to other tables that will reference this value. However, depending on the nature of the table and the data that you're thinking of applying the unique constraint to, you may be able to use that particular field as a natural primary key rather than having to establish a surrogate key. Of course, surrogate vs natural keys are a whole other discussion. :)
Unique keys can be used if there will be no relationship established between this table and other tables. For example, a table that contains a list of valid email addresses that will be compared against before inserting a new user record or some such. Or unique keys can be used when you have values in a table that has a primary key but must also be absolutely unique. For example, if you have a users table that has a user name. You wouldn't want to use the user name as the primary key, but it must also be unique in order for it to be used for log in purposes.
We need to make a distinction here between logical constructs and physical constructs, and similarly between theory and practice.
To begin with: from a theoretical perspective, if you don't have a primary key, you don't have a table. It's just that simple. So, your question isn't whether your table should have a primary key (of course it should) but how you label it within your RDBMS.
At the physical level, most RDBMSs implement the Primary Key constraint as a Unique Index. If your chosen RDBMS is one of these, there's probably not much practical difference, between designating a column as a Primary Key and simply putting a unique constraint on the column. However: one of these options captures your intent, and the other doesn't. So, the decision is a no-brainer.
Furthermore, some RDBMSs make additional features available if Primary Keys are properly labelled, such as diagramming, and semi-automated foreign-key-constraint support.
Anyone who tells you to use Unique Constraints instead of Primary Keys as a general rule should provide a pretty damned good reason.
the thing is that a primary key can be one or more columns which uniquely identify a single record of a table, where a Unique Constraint is just a constraint on a field which allows only a single instance of any given data element in a table.
PERSONALLY, I use either GUID or auto-incrementing BIGINTS (Identity Insert for SQL SERVER) for unique keys utilized for cross referencing amongst my tables. Then I'll use other data to allow the user to select specific records.
For example, I'll have a list of employees, and have a GUID attached to every record that I use behind the scenes, but when the user selects an employee, they're selecting them based off of the following fields: LastName + FirstName + EmployeeNumber.
My primary key in this scenario is LastName + FirstName + EmployeeNumber while unique key is the associated GUID.
posts saying that it's better to use unique constraint (aka unique index for some db) instead of PK
i guess that the only point here is the same old discussion "natural vs surrogate keys", because unique indexes and pk´s are the same thing.
translating:
posts saying that it's better to use natural key instead of surrogate key
I usually use both PK and UNIQUE KEY. Because even if you don't denote PK in your schema, one is always generated for you internally. It's true both for SQL Server 2005 and MySQL 5.
But I don't use the PK column in my SQLs. It is for management purposes like DELETEing some erroneous rows, finding out gaps between PK values if it's set to AUTO INCREMENT. And, it makes sense to have a PK as numbers, not a set of columns or char arrays.
I've written a lot on this subject: if you read anything of mine be clear that I was probably referring specifically to Jet a.k.a. MS Access.
In Jet, the tables are physically ordered on the PRIMARY KEY using a non-maintained clustered index (is clustered on compact). If the table has no PK but does have candidate keys defined using UNIQUE constraints on NOT NULL columns then the engine will pick one for the clustered index (if your table has no clustered index then it is called a heap, arguably not a table at all!) How does the engine pick a candidate key? Can it pick one which includes nullable columns? I really don't know. The point is that in Jet the only explicit way of specifying the clustered index to the engine is to use PRIMARY KEY. There are of course other uses for the PK in Jet e.g. it will be used as the key if one is omitted from a FOREIGN KEY declaration in SQL DDL but again why not be explicit.
The trouble with Jet is that most people who create tables are unaware of or unconcerned about clustered indexes. In fact, most users (I wager) put an autoincrement Autonumber column on every table and define the PRIMARY KEY solely on this column while failing to put any unique constraints on the natural key and candidate keys (whether an autoincrement column can actually be regarded as a key without exposing it to end users is another discussion in itself). I won't go into detail about clustered indexes here but suffice to say that IMO a sole autoincrement column is rarely to ideal choice.
Whatever you SQL engine, the choice of PRIMARY KEY is arbitrary and engine specific. Usually the engine will apply special meaning to the PK, therefore you should find out what it is and use it to your advantage. I encourage people to use NOT NULL UNIQUE constraints in the hope they will give greater consideration to all candidate keys, especially when they have chosen to use 'autonumber' columns which (should) have no meaning in the data model. But I'd rather folk choose one well considered key and used PRIMARY KEY rather than putting it on the autoincrement column out of habit.
Should all tables have a PK? I say yes because doing otherwise means at the very least you are missing out on a slight advantage the engine affords the PK and at worst you have no data integrity.
BTW Chris OC makes a good point here about temporal tables, which require sequenced primary keys (lowercase) which cannot be implemented via simple PRIMARY KEY constraints (SQL key words in uppercase).
PRIMARY KEY
1. Null
It doesn’t allow Null values. Because of this we refer PRIMARY KEY =
UNIQUE KEY + Not Null CONSTRAINT.
2. INDEX
By default it adds a clustered index.
3. LIMIT
A table can have only one PRIMARY KEY Column[s].
UNIQUE KEY
1. Null
Allows Null value. But only one Null value.
2. INDEX
By default it adds a UNIQUE non-clustered index.
3. LIMIT
A table can have more than one UNIQUE Key Column[s].
If you plan on using LINQ-to-SQL, your tables will require Primary Keys if you plan on performing updates, and they will require a timestamp column if you plan on working in a disconnected environment (such as passing an object through a WCF service application).
If you like .NET, PK's and FK's are your friends.
I submit that you may need both. Primary keys by nature need to be unique and not nullable. They are often surrogate keys as integers create faster joins than character fileds and especially than multiple field character joins. However, as these are often autogenerated, they do not guarantee uniqueness of the data record excluding the id itself. If your table has a natural key that should be unique, you should have a unique index on it to prevent data entry of duplicates. This is a basic data integrity requirement.
Edited to add: It is also a real problem that real world data often does not have a natural key that truly guarantees uniqueness in a normalized table structure, especially if the database is people centered. Names, even name, address and phone number combined (think father and son in the same medical practice) are not necessarily unique.
I was thinking of this problem my self. If you are using unique, you will hurt the 2. NF. According to this every non-pk-attribute has to be depending on the PK. The pair of attributes in this unique constraint are to be considered as part of the PK.
sorry for replying to this 7 years later but didn't want to start a new discussion.