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.
Related
I have unique contraint on 5 nullable columns that represent identifier of one row.
Is it okay to create unique key and create clustered index on it instead of primary key? I cannot use primary key on these columns because they are nullable, and i cannot create identity column because there are lot of deletes and inserts and it will make overflow on this identity column.
Yes, and there's an argument that this is actually "better" than a primary key, as the rule that a primary key column is non nullable is in many ways an artificial constraint.
If you make it a UNIQUE CLUSTERED INDEX then you get just about everything that a primary key brings to the table except the unwanted rule that the columns must be non nullable. However, they must still be unique, so you could only ever have one row where all five columns in your index are null for example.
So you can use your index when creating foreign key constraints, you will guarantee the order data is stored, and each row must be unique. However, the index probably won't be incredibly useful for querying, and, because it's going to be wide, and you said there's a lot of deletes/ inserts, it will have a tendency to fragment your data.
Personally, I would be tempted to make it a unique constraint, but not clustered. Then it will do the job of keeping non-unique data from being created.
You could then add a surrogate key and make this the primary key. I doubt you would ever "run out" (or "overflow"?) of numbers doing this.
So why would I use a surrogate key?
Your surrogate key will be much narrower, so less impact from fragmentation due to so many inserts/ updates/ deletes.
It's then useful if you need to extend your database. Say you only have one table, and this is always going to be the only table in the entire database. In this one scenario it would make sense to not bother with a surrogate key. It doesn't give you any value; it's just an unnecessary overhead.
However, let's assume that you have other tables hanging off your "main" table (the one with 5 columns forming a unique key). Adding a surrogate key here allows you to make any child tables with a single id that links back to the parent table. The alternative would be to enforce the addition of ALL five columns forming the unique (candidate) key every time you create a child table.
Now you have a narrow clustered index that actually serves a purpose, and the fragmentation will not be quite as bad as it would with five columns.
Suppose we'd have to define optimal indexing for Stackoverflow questions. But let's not take the schema of the actual Posts table, let's just include those columns that are actually relevant:
create table Posts (
Id int not null
identity,
PostTypeId tinyint not null,
LastActivityDate datetime not null
default getdate(),
Title nvarchar(500) null, -- answers don't have titles
Body nvarchar(max) not null,
...
)
I've added Id to be identity even though Data Stackexchange shows that none of the tables have a primary key constraint on them, nor identity columns. There are many just unique/non-unique clustered/non-clustered indices.
Usage scenarios
So basically two main scenarios for posts:
They're chronologically displayed in descending order by their LastActivityDate column (or maybe LastEditDate that I haven't included above as it's not so important)
They're individually displayed on question details
Answers are displayed on question details page in votes order (ScoreCount column not part of my upper code)
Indexing optimization
Which indices would be best created on above scenarios especially if we'd say that #1 is the most common scenario so it has to work really fast.
I'd say that one of the better possibilities would be to create these indices:
-- index 1
alter table Posts
add primary key nonclustered (Id);
-- index 2
create clustered index IX_Posts_LastActivityDate
on Posts(LastActivityDate desc);
-- index 3
create index IX_Posts_ParentId
on Posts(ParentId, PostTypeId)
include (ScoreCount);
This way we basically end up with three indices of which the second one is clustered.
So in order for #1 to work really fast I've set clustered index on LastActivityDate column, because clustered indices are especially great when we do range comparison on them. And we would be ordering questions chronologically newest to oldest hence I've set ordering direction and also included type on the clustered index.
So what did we solve with this?
scenario #1 is very efficiently covered by index 2 as it's clustered and fully covered; we can also easily and efficiently do result paging;
scenario #2 is somewhat covered with unique index 1 (to get the question) and non-unique index 3 to get all related answers (scenario #3) ordered by ScoreCount; and if we decide to chronologically order answers that's also covered with index 2;
Question 1
SQL internals are such that SQL implicitly adds clustered key to nonclustering index so it can locate records in the row store.
if clustering index is unique, than that's the key that will be added to nonclustering indices, and
if clustering index is non-unique, SQL supposedly generates its own UniqueId and uses that
Since I've also added a nonclustered primary key on the table (which must by design be unique), I would like to know whether SQL will still supply its own unique key on clustered non-unique index or will it use nonclustered primary key to uniquely identify each records instead?
Question 2
So if primary key isn't used to locate records on row store (clustered index) does it even make sense to actually create a PK? Would in this case be better to rather do this?
create unique index UX_Posts_Id
on Posts(Id);
-- include (Title, Body, ScoreCount);
It would be great to also include commented out columns, but then that would make this index inefficient as it will be worse in caching... Why I'm asking whether it would be better to create this index instead of a primary key constraint is because we can include additional non-key columns to this index while we can't do the same when we add a PK constraint that internally generates a unique index...
Question 3
I'm aware that LastActivityDate changes which isn't desired with clustered indices, but we have to consider the fact that this column is more likely to change for some time before it becomes more or less static, so it shouldn't cause too much index fragmentation as records will mostly be appended to the end whenever LastActivityDate changes. Index fragmentation on some arbitrary page should never happen because some new record would be inserted into some old(er) page as LastActivityDate will only increase. Hence most modifications will happen on the last page.
So the question is whether these changes can be harmful as LastActivityDate isn't the best candidate for clustering index key:
it's not unique - although one could argue about this, especially if we'd change datetime to datetime2 and use higher precision function sysdatetime()
and set index as unique
it's narrow - pretty much
it's not static - but I've explained how it changes
it's ever increasing
Since I've also added a nonclustered primary key on the table (which
must by design be unique), I would like to know whether SQL will still
supply its own unique key on clustered non-unique index or will it use
nonclustered primary key to uniquely identify each records instead?
SQL Server adds a 4-byte "uniqueifier" when a given non-unique clustered index key value isn't unique. All non-clustered index leaf nodes, including the primary key, will include LastActivityDate plus the uniqueifier (when present) as the row locator. The internal uniqueifier would be needed here only for posts with the same LastActivityDate so I'd expect relatively few rows would actually need a uniqueifier.
So if primary key isn't used to locate records on row store (clustered
index) does it even make sense to actually create a PK? Would in this
case be better to rather do this?
From a data modeling perspective, every relational table should have primary key. The implicitly created index can be declared as either clustered or non-clustered as needed to optimize performance. If LastActivity is a better choice for performance, then the primary key index must be non-clustered. This primary key index will provide the needed index to retrieve singleton posts.
Unfortunately, SQL Server doesn't provide a way to specify included columns on primary key and unique constraint definitions. This is a case where one can bend the rules and use a unique index instead of a declared primary key constraint in order to avoid the cost of redundant indexes and the benefits of included columns. The unique index is functionally identical to a primary key and can be referenced by foreign key constraints.
So the question is whether these changes can be harmful as
LastActivityDate isn't the best candidate for clustering index key
LastActivityDate alone can never be guaranteed to be unique regardless of the level of precision (barring single-threaded inserts or retry logic). One approach could be a composite primary key on LastActivityDate and Id. Individual posts would need to be retrieved using both values. That would eliminate the need for a separate unique index Id previously discussed.
My biggest concern about LastActivityDate as the leftmost clustered index key column is that it may change often for recent posts. This would require a lot of row movement to maintain the logical key order, may impact concurrency significantly compared to the current static Id key, and require updates to the non-clustered index row locator values upon each change. So even though this clustered index key may be optimal for many queries, the other costs on a highly transactional system may outweigh the benefits.
I have a series of questions about Keys, Indexes and Constraints in SQL, SQL 2005 in particular. I have been working with SQL for about 4 years but I have never been able to get definitive answers on this topic and there is always contradictory info on blog posts, etc. Most of the time tables I create and use just have an Identity column that is a Primary Key and other tables point to it via a Foreign Key.
With join tables I have no Identity and create a composite Primary Key over the Foreign Key columns. The following is a set of statements of my current beliefs, which may be wrong, please correct me if so, and other questions.
So here goes:
As I understand it the difference between a Clustered and Non Clustered Index (regardless of whether it is Unique or not) is that the Clustered Index affects the physical ordering of data in a table (hence you can only have one in a table), whereas a Non Clustered Index builds a tree data structure. When creating Indexes why should I care about Clustered vs Non Clustered? When should I use one or the other? I was told that inserting and deleting are slow with Non-Clustered indexes as the tree needs to be "rebuilt." I take it Clustered indexes do not affect performance this way?
I see that Primary Keys are actually just Clustered Indexes that are Unique (do they have to be clustered?). What is special about a Primary Key vs a Clustered Unique Index?
I have also seen Constraints, but I have never used them or really looked at them. I was told that the purpose of Constraints is that they are for enforcing data integrity, whereas Indexes are aimed at performance. I have also read that constraints are acually implemented as Indexes anyway so they are "the same." This doesnt sound right to me. How are constraints different to Indexes?
Clustered indexes are, as you put it correctly, the definition as to how data in a table is stored physically, i.e. you have a B-tree sorted using the clustering key and you have the data at the leaf level.
Non-clustered indexes on the other hand are separate tree structures which at the leaf level only have the clustering key (or a RID if the table is a heap), meaning that when you use a non-clustered index, you'll have to use the clustered index to get the other columns (unless your request is fully covered by the non-clustered index, which can happen if you request only the columns, which constitute the non-clustered index key columns).
When should you use one or the other ? Well, since you can have only one clustered index, define it on the columns which makes most sense, i.e. when you look up clients by ID most of the time, define a clustered index on the ID. Non-clustered indexes should be defined on columns which are used less often.
Regarding performance, inserts or updates that change the index key are always painfull, regardless of whether it is a clusted on non-clustered index, since page splits can happen, which forces data to be moved between pages (moving the pages of a clustered index hurts more, since you have more data in the leaf level). Thus the general rule is to avoid changing the index key and inserting new values so that they would be sequencial. Otherwise you'll encounter fragmentation and will have to rebuild your index on a regular basis.
Finally, regarding constraints, by definition, they have nothing to do with indexes, yet SQL server has chosen to implement them using indexes. E.g. currently, a unique constraint is implemented as an index, however this can change in a future version (though I doubt that will happen). The type of index (clustered or not) is up to you, just remember that you can have only one clustered index.
If you have more questions of this type, I highly recommend reading this book, which covers these topics in depth.
Your assumption about the clustered vs non-clustered is pretty good
It also seems that primary key enforces non null uniquenes, while the unique index does not enforce non null primary vs unique
The primary key is a logical concept in relational database theory - it's a key (and typically also an index) which is designed to uniquely identify any of your rows. Therefore it must be unique and it cannot be NULL.
The clustering key is a storage-physical concept of SQL Server specifically. It's a special index that isn't just used for lookups etc., but also defines the physical structure of your data in your table. In a printed phonebook in Western European culture (except maybe for Iceland ), the clustered index would be "LastName, FirstName".
Since the clustering index defines your physical data layout, you can only ever have one of those (or none - not recommended, though).
Requirements for a clustering key are:
must be unique (if not, SQL Server will add a 4-byte "uniqueifier")
should be stable (never changing)
should be as small as possible (INT is best)
should be ever-increasing (think: IDENTITY)
SQL Server makes your primary key the clustering key by default - but you can change that if you need to. Also, mind you: the columns that make up the clustering key will be added to each and every entry of each and every non-clustered index on your table - so you want to keep your clustering key as small as possible. This is because the clustering key will be used to do the "bookmark lookup" - if you found an entry in a non-clustered index (e.g. a person by their social security number) and now you need to grab the entire row of data to get more details, you need to do a lookup, and for this, the clustering key is used.
There's a great debate about what makes a good or useful clustering and/or primary key - here's a few excellent blog posts to read about this:
all of Kimberly Tripp's Indexing blog posts are a must-read
GUIDs as primary key and/or clustering key
The Clustered index debate continues....
Marc
You have several questions. I'll break some of them out:
When creating Indexes why should I care about Clustered vs Non Clustered?
Sometimes you do care how the rows are organized. It depends on your data and how you will use it. For example, if your primary key is a uniqueidentifier, you may not want it to be CLUSTERED, because GUID values are essentially random. This will cause SQL to insert rows randomly throughout the table, causing page splits which hurt performance. If your primary key value will always increment sequentially (int IDENTITY for example), then you probably want it to be CLUSTERED, so your table will always grow at the end.
A primary key is CLUSTERED by default, and most of the time you don't have to worry about it.
I was told that inserting and deleting are slow with Non-Clustered indexes as the tree needs to be "rebuilt." I take it Clustered indexes do not affect performance this way?
Actually, the opposite can be true. NONCLUSTERED indexes are kept as a separate data structure, but the structure is designed to allow some modification without needing to be "re-built". When the index is initially created, you can specify the FILLFACTOR, which specifies how much free space to leave on each page of the index. This allows the index to tolerate some modification before a page split is necessary. Even when a page split must occur, it only affects the neighboring pages, not the entire index.
The same behavior applies to CLUSTERED indexes, but since CLUSTERED indexes store the actual table data, page splitting operations on the index can be much more expensive because the whole row may need to be moved (versus just the key columns and the ROWID in a NONCLUSTERED index).
The following MSDN page talks about FILLFACTOR and page splits:
http://msdn.microsoft.com/en-us/library/aa933139(SQL.80).aspx
What is special about a Primary Key vs a Clustered Unique Index?
How are constraints different to Indexes?
For both of these I think it's more about declaring your intentions. When you call something a PRIMARY KEY you are declaring that it is the primary method for identifying a given row. Is a PRIMARY KEY physically different from a CLUSTERED UNIQUE INDEX? I'm not sure. The behavior is essentially the same, but your intentions may not be clear to someone working with your database.
Regarding constraints, there are many types of constraints. For a UNIQUE CONSTRAINT, there isn't really a difference between that and a UNIQUE INDEX, other than declaring your intention. There are other types of constraints that do not map directly to a type of index, such as CHECK constraints, DEFAULT constraints, and FOREIGN KEY constraints.
I don't have time to answer this in depth, so here is some info off the top of my head:
You're right about clustered indexes. They rearrange the physical data according to the sort order of the clustered index. You can use clustered indexes specifically for range-bound queries (e.g. between dates).
PKs are by default clustered, but they don't have to be. That's just a default setting. The PK is supposed to be a UID for the row.
Constraints can be implemented as indexes (for example, unique constraints), but can also be implemented as default values.
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.