Sql – How do composite indexes work

compositeindexingsql

I've created composite indexes (indices for you mathematical folk) on tables before with an assumption of how they worked. I was just curious if my assumption is correct or not.

I assume that when you list the order of columns for the index, you are also specifying how the indexes will be grouped. For instance, if you have columns a, b, and c, and you specify the index in that same order a ASC, b ASC, and c ASC then the resultant index will essentially be many indexes for each "group" in a.

Is this correct? If not, what will the resultant index actually look like?

Best Answer

Composite indexes work just like regular indexes, except they have multi-values keys.

If you define an index on the fields (a,b,c) , the records are sorted first on a, then b, then c.

Example:

| A | B | C |
-------------
| 1 | 2 | 3 |
| 1 | 4 | 2 |
| 1 | 4 | 4 |
| 2 | 3 | 5 |
| 2 | 4 | 4 |
| 2 | 4 | 5 |

Related Solutions

Php – How to prevent SQL injection in PHP

The correct way to avoid SQL injection attacks, no matter which database you use, is to separate the data from SQL, so that data stays data and will never be interpreted as commands by the SQL parser. It is possible to create SQL statement with correctly formatted data parts, but if you don't fully understand the details, you should always use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.

You basically have two options to achieve this:

Using PDO (for any supported database driver):

 $stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');

 $stmt->execute([ 'name' => $name ]);

 foreach ($stmt as $row) {
     // Do something with $row
 }

Using MySQLi (for MySQL):

 $stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
 $stmt->bind_param('s', $name); // 's' specifies the variable type => 'string'

 $stmt->execute();

 $result = $stmt->get_result();
 while ($row = $result->fetch_assoc()) {
     // Do something with $row
 }

If you're connecting to a database other than MySQL, there is a driver-specific second option that you can refer to (for example, pg_prepare() and pg_execute() for PostgreSQL). PDO is the universal option.

Correctly setting up the connection

Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:

$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8', 'user', 'password');

$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

In the above example the error mode isn't strictly necessary, but it is advised to add it. This way the script will not stop with a Fatal Error when something goes wrong. And it gives the developer the chance to catch any error(s) which are thrown as PDOExceptions.

What is mandatory, however, is the first setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it to the MySQL server (giving a possible attacker no chance to inject malicious SQL).

Although you can set the charset in the options of the constructor, it's important to note that 'older' versions of PHP (before 5.3.6) silently ignored the charset parameter in the DSN.

Explanation

The SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute, the prepared statement is combined with the parameter values you specify.

The important thing here is that the parameter values are combined with the compiled statement, not an SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters, you limit the risk of ending up with something you didn't intend.

Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains 'Sarah'; DELETE FROM employees the result would simply be a search for the string "'Sarah'; DELETE FROM employees", and you will not end up with an empty table.

Another benefit of using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.

Oh, and since you asked about how to do it for an insert, here's an example (using PDO):

$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');

$preparedStatement->execute([ 'column' => $unsafeValue ]);

Can prepared statements be used for dynamic queries?

While you can still use prepared statements for the query parameters, the structure of the dynamic query itself cannot be parametrized and certain query features cannot be parametrized.

For these specific scenarios, the best thing to do is use a whitelist filter that restricts the possible values.

// Value whitelist
// $dir can only be 'DESC', otherwise it will be 'ASC'
if (empty($dir) || $dir !== 'DESC') {
   $dir = 'ASC';
}

Sql-server – Multiple Indexes vs Multi-Column Indexes

I agree with Cade Roux.

This article should get you on the right track:

One thing to note, clustered indexes should have a unique key (an identity column I would recommend) as the first column. Basically it helps your data insert at the end of the index and not cause lots of disk IO and Page splits.

Secondly, if you are creating other indexes on your data and they are constructed cleverly they will be reused.

e.g. imagine you search a table on three columns

state, county, zip.

you sometimes search by state only.
you sometimes search by state and county.
you frequently search by state, county, zip.

Then an index with state, county, zip. will be used in all three of these searches.

If you search by zip alone quite a lot then the above index will not be used (by SQL Server anyway) as zip is the third part of that index and the query optimiser will not see that index as helpful.

You could then create an index on Zip alone that would be used in this instance.

By the way We can take advantage of the fact that with Multi-Column indexing the first index column is always usable for searching and when you search only by 'state' it is efficient but yet not as efficient as Single-Column index on 'state'

I guess the answer you are looking for is that it depends on your where clauses of your frequently used queries and also your group by's.

The article will help a lot. :-)