The manual says "Indexes should not duplicate the columns of PRIMARY KEY, UNIQUE or FOREIGN key constraints as each of these constraints creates an index automatically." It is unclear to me whether that applies also to individual columns of a multi-column constraint. Say I have a unique constraint on columns (A,B) and I plan to do selects on B, do I need an index on B?
The quoted bit is from chapter 4. The answer can be found in chapter 2: "n HyperSQL 2.0, a multi-column index will speed up queries that contain joins or values on the first n columns of the index. You need NOT declare additional individual indexes on those columns unless you use queries that search only on a subset of the columns." So in my scenario I would need an extra index on B but I could obviate the need by making the uniqueness constraint be on (B,A) instead of (A,B).
Related
I want to search a DB with either the PK or a unique non null field that is indexed. Are there any performance differences between those? I am using Postgres as my DB. But a general DB-independent answer would be good too.
In postgreSQL, all indexes are secondary or unclustered indexes. That means the the index points to the heap, the data structure holding the actual column data. So, a primary key's index doesn't have any structural advantage over a UNIQUE index: SELECTs using the index for filtering must then bounce over to the heap for the data.
In fact, it might be the other way around, because postgreSQL indexes can have INCLUDES clauses.
For example consider a table with uniqueid, a, b, and c columns. If your workload is heavy with SELECT b FROM tbl WHERE uniqueid = something queries, you can declare this covering index.
CREATE UNIQUE INDEX uniq ON tbl(uniqueid) INCLUDE (b);
Your whole query can then be satisfied from the index. That saves the extra trip to the heap, and so saves IO and CPU time.
MySQL and SQL Server, on the other hand, use clustered indexes for their primary keys. That is, the table's data is stored in the primary key's index. So, the PK is, automatically, basically an index created like this.
CREATE UNIQUE INDEX pk ON tbl(uniqueid) INCLUDE (a, b, c);
In those databases the PK's index does have an advantage over a separate UNIQUE index, which necessarily is a secondary or unclustered index. (Note: MySQL's indexes don't have INCLUDE() clauses.)
In Teradata, I create table with unique primary key out of two varchar columns A and B. I will write queries that need to filter on one or both of these columns.
For best performance, should I submit a create index statement for each of the two columns (the table would have 3 indexes: the unique primary key(column A, B), non-unique column A, and non-unique column B)?
On this table, I only care about read performance and not insert/update performance.
In Teradata, if you specify a PRIMARY KEY clause when you create the table, then the table will automatically be created with a UNIQUE PRIMARY INDEX (UPI) on those PK columns. Although Teradata supports keys, it is more of an index-based DBMS.
In your case, you will have very, very fast reads (i.e. UPI access - single AMP, single row) only when you specify all of the fields in your PK. This applies to equality access as mentioned in the previous comments (thanks Dieter).
If you access the table on some but not ALL of the PK / UPI columns, then your query won't use the UPI access path. You'd need to define separate indexes or other optimization strategies, depending on your queries.
If you only care about read performance, then it makes sense to create secondary indexes on the separate columns. Just run the EXPLAIN on your query to make sure the indexes are actually being used by the Optimizer.
Another option is to ditch the PK specification altogether, especially if you never access the table on that group of columns. If there is one column you access more than the other, specify that one as your PRIMARY INDEX (non-unique) and create a secondary index on the other one. Something like:
CREATE TABLE mytable (
A INTEGER,
B INTEGER,
C VARCHAR(10)
)
PRIMARY INDEX(A) -- Non-unique primary index
;
CREATE INDEX (B) ON mytable; -- Create secondary index
You only need two indexes.
If you have a primary key on (A, B), then this also works for (A). If you want to filter on B, then you want an index on (B).
You might want to make it (B, A) so the index can handle cases such as:
where B = ? and A in (?, ?, ?)
I recently read about a way to ensure unique values in a column in SQL while allowing multiple NULLS.
This was done using filtered indexes:
CREATE UNIQUE INDEX indexName ON tableName(columns) INCLUDE includeColumns
WHERE columnName IS NOT NULL
Could someone explains how this actually works? Is the UNIQUE constraint created on the column or not ?
To answer your first question: When the index is filtered, anything that doesn't fix the criteria in the where clause is simply left out of the index.
If the index is unique, the uniqueness is enforced only on the data that fits the criteria in the where clause.
To answer your second question: In Sql server unique constraints are implemented by creating unique indexes under the hood, so there really is not much difference between them. In any case the uniqueness is enforced on an index and not directly on the table column.
Let say I have Item table, with composite unique constraint on company_id and code columns.
As documented, postgresql creates index on this constraint automatically. But if some queries
filter by company_id column but not code column, then to increase performance, should I define index on company_id column too?
It depends on how you defined your unique index (or constraint, which will automatically create a unique index with the affected columns, in that order).
A multicolumn B-tree index can be used with query conditions that involve any subset of the index's columns, but the index is most efficient when there are constraints on the leading (leftmost) columns. The exact rule is that equality constraints on leading columns, plus any inequality constraints on the first column that does not have an equality constraint, will be used to limit the portion of the index that is scanned ...
More in docs about Multicolumn Indexes
Why does INDEX creation statement have UNIQUE argument?
As I understand, the non-clustered index contains a bookmark, a pointer to a row, which should be unique to distinguish even non-unique rows,
so insuring non-clustered index to be unique ?
Correct?
So, do I understand that no-unique index can be only on clustered table? since
"A clustered index on a view must be unique" [1]
Since "The bottom, or leaf, level of the clustered index contains the actual data rows of the table" [1], do I understand correctly that the same effect as UNIUE on clustered index can be achieved by unique constraint on (possibly all or part of) columns of a table [2]?
Then, what does bring UNIQUE argument for index?
except confusion to basic concepts definitions [3]
Update:
This is again the same pitfall - explaining something already explained many times based on undefined terms converting all explanation to never-ending guessing game.
Please see my subquestion [4] which is really re-wording of this same question here.
Update2:
The problem is in ambiguous, lacking definitions or improper use of terms in improper contexts. If index is defined as structure serving to (find and) identify/point to real data, then non-unique or NULL indexes do not make any sense. Bye
Cited:
[1]
CREATE INDEX (Transact-SQL)
http://msdn.microsoft.com/en-us/library/ms188783.aspx
[2]
CREATE TABLE (Transact-SQL)
http://msdn.microsoft.com/en-us/library/ms174979.aspx
[3]
Unique index or unique key?
Unique index or unique key?
[4]
what is index and can non-clustered index be non-unique?
what is index and can non-clustered index be non-unique?
While a non-unique index is sufficient to distinguish between rows (as you said), the UNIQUE index serves as a constraint: it will prevent duplicates from being entered into the database - where "duplicates" are rows containing the same data in the indexed columns.
Example:
Firstname | Lastname | Login
================================
Joe | Smith | joes
Joe | Taylor | joet
Susan | Smith | susans
Let's assume that login names are by default generated from first name + first letter of last name.
What happens when we try to add Joe Sciavillo to the database? Normally, the system would happily generate loginname joes and insert (Joe,Sciavillo,joes). Now we'd have two users with the same username - probably a Bad Thing.
Now let's say we have a UNIQUE index on Login column - the database will check that no other row with the same data already exists, before it allows inserting the new row. In other words, the attempt to insert another joes will be rejected, because that data wouldn't be unique in that row any more.
Of course, you could have unique indexes on multiple columns, in which case the combination of data would have to be unique (e.g. a unique index on Firstname,Lastname will happily accept a row with (Joe,Badzhanov), as the combination is not in the table yet, but would reject a second row with (Joe,Smith))
The UNIQUE index clause is really just a quirk of syntax in SQL Server and some other DBMSs. In Standard SQL, uniqueness constraints are implemented through the use of the PRIMARY KEY and UNIQUE CONSTRAINT syntax, not through indexes (there are no indexes in standard SQL).
The mechanism SQL Server uses internally to implement uniqueness constraints is called a unique index. A unique index gets created automatically for you whenever you create a PRIMARY KEY or UNIQUE constraint. For reasons best known to the SQL Server development team they decided to expose the UNIQUE keyword as part of the CREATE INDEX syntax, even though the constraint syntax does the same job.
In the interests of clarity and standards support I would recommend you avoid creating UNIQUE indexes explicitly wherever possible. Use the PRIMARY KEY or UNQIUE constraint syntax instead.
The UNIQUE clause specifies that the values in the column(s) must be unique across the table, essentially adding a unique constraint. A clustered index on a table specifies that the ordering of the rows in the table will be the same as the index. A non-clustered index does not change the physical ordering, which is why it is OK to have multiple non-clustered but only one clustered index. You can have unique or non-unique clustered and non-clustered indexes on a table.
I think the underlying question is: what is the difference between unique and non-unique indexes?
The answer is that entries in unique indexes can each only point to a single row, while entries in non-unique indexes can point to many rows.
For example, consider an order item table:
ORDER_NO INTEGER
LINE_NO INTEGER
PRODUCT_NO INTEGER
QUANTITY DECIMAL
- with a unique index on ORDER_NO and LINE_NO, and a non-unique index on PRODUCT_NO.
For a single combination of ORDER_NO and LINE_NO there can only be one entry in the table, while for a single value of PRODUCT_NO there can be many entries in the table (because there will be many entries for that value in the index).