CODEWITHSUNDEEP
sqlmysqlprimary-keyindexing
David Jenings
David Jenings

Reputation: 653

Indexes and multi column primary keys

In a MySQL database I have a table with the following primary key

PRIMARY KEY id (invoice, item)

In my application I will also frequently be selecting on item by itself and less frequently on only invoice. I'm assuming I would benefit from indexes on these columns.

MySQL does not complain when I define the following:

INDEX (invoice),
INDEX (item),
PRIMARY KEY id (invoice, item)

But I don't see any evidence (using DESCRIBE -- the only way I know how to look) that separate indexes have been established for these two columns.

Are the columns that make up a primary key automatically indexed individually? Is there a better way than DESCRIBE to explore the structure of my table?

Upvotes: 58

Views: 36196

Answers (7)

Charles Bretana
Charles Bretana

Reputation: 146409

I'm not intimately familiar with the internals of indices on mySql, but on the two database vendor products that I am familiar with (MsSQL, Oracle) indices are Balanced-Tree (B-Tree) structures, whose nodes are organized as a sequenced tuple of the columns the index is defined on (In the Sequence Defined) See Note

So, unless mySql does it very differently, (probably not), any composite index (on more than one column) can be useable by any query that needs to filter or sort by a subset of the columns in the index, as long as the list of columns is compatible, i.e., if the columns, when sequenced the same as the sequenced list of columns in the complete index, is an ordered subset of the complete set of index columns, which starts at the beginning of the actual index sequence, with no gaps except at the end...

In other words, this means that if you have an index on (a,b,c,d) a query that filters on (a), (a,b), or (a,b,c) can also use the index, but a query that needs to filter on (b), or (c), (b,c) or (a, c) will not be able to use the index...

So in your case, if you often need to filter or sort on column item alone, you need to add another index on that column by itself...

NOTE: Normally, when using the term Balanced tree, it is implied that each node in the tree may have a maximum of two sub-nodes or branches. This is then called a Balanced Binary tree. Indices in SQL Server are just balanced non-binary B-Trees, where the nodes can have more than two sub-nodes or branches. The index optimizer calculates how many branches to allocate to each node based on optimizing performance. The fewer sub-nodes that exist on each node, the taller (height or depth) the tree will be. The optimum number of branches (that maximizes performance in locating a specific record), depends on the number of discreet values of the index attributes within the total data set domain.

Upvotes: 84

Syed Waqas Bukhary
Syed Waqas Bukhary

Reputation: 5340

If your query is using both columns in where clause then you don't need to create a separate index in a composite primary key.

EXPLAIN SELECT * FROM `table` WHERE invoice = 1 and item = 1

You are also fine if you want to query with first column only

EXPLAIN SELECT * FROM `table` WHERE invoice = 1

But if you want to query with subsequent columns col2, col3 in composite PK then you would need to create separate indexes on those columns. The following explain query shows the second column does not have a possible key detected by MySQL

EXPLAIN SELECT * FROM `table` WHERE item = 1

Upvotes: 0

valentine
valentine

Reputation: 457

Mysql auto create an index for composite keys. Depending on your queries, you may have to create separate index for individual column in the composite key.

If you are using mysql workbench, you can manually right click the schema and click on edit to see everything about the table

Upvotes: 0

cppcoder
cppcoder

Reputation: 23095

There is a difference between composite index and composite primary key. If you have defined a composite index like below

INDEX idx(invoice,item)

the index wont work if you query based on item and you need to add a separate index 

INDEX itemidx(item)

But, if you have defined a composite primary key like below 

PRIMARY KEY(invoice, item)

the index would work if you query based on item and no separate index is required.

Working example: 

mysql>create table test ( col1 int(20), col2 int(20) ) primary key(col1,col2);
mysql>explain select * from test where col2 = 1;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | index | NULL          | PRIMARY | 8       | NULL |   10 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+

Upvotes: 4

George Steel
George Steel

Reputation: 737

I personally use phpMyAdmin to view and edit the structure of MySQL databases. It is a web application but it runs well enough on a local web server (I run an instance of apache on my machine for this and phpPgAdmin).

As for the composite key of (invoice, item), it acts like an index for (invoice, item) and for invoice. If you want to index by just item you have to add that index yourself. Your PK will be sorted by invoice and then by item where invoice is the same in multiple records. While the order in a composite PK does not matter for uniqueness enforcement, it does matter for access.

On your table I would use:

PRIMARY KEY id (invoice, item), INDEX (item)

Upvotes: 22

Allan
Allan

Reputation: 17429

I'm not that familiar with MySQL, but generally an multiple-column index is equally useful on the first column in the index as an index on that column alone. The multiple-column index becomes less useful for querying against a single column the further the column appears into the index.

This makes some sense if you think of the multi-column index as a hierarchy. The first column in the index is the root of the hierarchy, so searching it is just a matter of scanning that first level. However, in order to scan the second column, the database has to look up the tree for each unique value found in the first column. This can be costly enough that most optimizers won't bother to look deeply into a multi-column index, instead opting to full-table-scan.

For example, if you have a table as follows:

Col1 |Col2 |Col3
----------------
   A |   1 |   Z
   A |   2 |   Y
   A |   2 |   X
   B |   1 |   Z
   B |   2 |   X

Assuming you have an index on all three columns, in order, the tree will look something like this:

A
+-1
  +-Z
+-2
  +-X
  +-Y
B
+-1
  +-Z
+-2
  +-X

Looking for Col1='A' is easy: you only have to look at 2 ordered values. However, to resolve col3='X', you have to look at all of the values in the 4 bigger buckets, each of which is ordered individually.

Upvotes: 5

Mike
Mike

Reputation: 21659

To return table index information, you can use:

SHOW INDEX FROM <table>;

See: http://dev.mysql.com/doc/refman/5.0/en/show-index.html

To view table information:

SHOW CREATE TABLE <table>;

See: http://dev.mysql.com/doc/refman/5.0/en/show-create-table.html

Primary keys are indexes, so there's no need to create additional indexes. You can find out more information about them under the CREATE TABLE syntax (there's too much to insert here):

http://dev.mysql.com/doc/refman/5.0/en/create-table.html

Upvotes: 4

Related Questions