Multiple-Column Indexes and Hashing: The Ultimate Guide to Boosting Database Performance
Tags: mysql, tutorial, programming
Published: 2023-10-05 06:58:51

Greetings, tech aficionados! Today, we're diving deep into the realms of multiple-column indexes and hashing—critical yet often overlooked aspects in the database world. As a Software Architect and a database enthusiast, I can tell you that understanding these concepts can make a world of difference in your application's performance. So grab your coffee, and let's get started!

## What are Multiple-Column Indexes?

In database management systems like MySQL, PostgreSQL, or SQL Server, you can create indexes not just on a single column, but on multiple columns. These are known as composite or multi-column indexes.

```sql
CREATE INDEX composite_index
ON table_name (column1, column2);
```

## Why Hash a Composite Index?

Hashing a composite index is essentially a technique that involves converting a large composite key into a smaller, fixed-size value. Why do we do this? Simple: Performance. Hashing can substantially reduce the time it takes to search through a large set of data. When you hash a composite key, you're essentially creating a unique identifier for a record, which allows the database engine to retrieve the data in the blink of an eye. It's like having a VIP pass to skip the line at a concert—swift, efficient, and straight to the point.

## How to Generate a Hashing

Generating a hash involves using a hash function that takes in an input (or 'message') and returns a fixed-size string of bytes. The output is typically a 'digest' that represents the data. Creating a hash isn't merely a call to a function; it's the transformation of data into a unique, fixed-size digest. This transformation is deterministic, meaning the same input will yield the same output, but even a minor change in the input creates a drastically different output.

```sql
-- MySQL example using MD5
UPDATE table_name 
SET hash_column = MD5(CONCAT(column1, column2));
```

## Types of Hashing

### MD5

- **Use Case**: Quick hash generation but not recommended for security-critical applications.
- MD5, or Message Digest Algorithm 5, is a widely used cryptographic hash function that produces a 32-character hexadecimal number. Although it's blazing fast, it has vulnerabilities and is susceptible to collision attacks. Therefore, it's often relegated to non-critical applications where speed is a priority over security.

### SHA-1

- **Use Case**: Deprecated for most cryptographic uses but still acceptable for checksums.
- SHA-1 or Secure Hash Algorithm 1, creates a 40-character digest and was once considered stronger than MD5. However, it's now considered broken and unsuitable for further use in security certificates, although it's still used in some checksum applications.

### SHA-2

- **Use Case**: Stronger than SHA-1 and MD5, suitable for most security-sensitive applications.
- The SHA-2 (Secure Hash Algorithm 2) family consists of hash functions like SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256. It provides better security than its predecessors and is recommended for most security-sensitive operations.

### CRC32

- **Use Case**: Not cryptographic. Fast and suitable for checksums.
- CRC32 or Cyclic Redundancy Check is a non-cryptographic hash function mainly used to check for accidental changes in raw data. It's blazingly fast but should not be used for any cryptographic applications.

## Using CONCAT for Hashing

Concatenating fields is a common method for creating composite keys. The \`CONCAT\` function stitches together the values from multiple columns into a single string. This concatenated string serves as the input to your hash function. It's a straightforward yet powerful approach that helps you generate unique hashes even when individual column values are not unique.

```sql
-- Combining three columns
UPDATE table_name 
SET hash_column = MD5(CONCAT(column1, column2, column3));
```

## The Problem with CONCAT and NULL

The \`CONCAT()\` function usually returns NULL if any of the inputs is NULL. When dealing with \`NULL\` values, \`CONCAT\` can be tricky. A single \`NULL\` value in the concatenation process will yield a \`NULL\` output. This is problematic because you end up losing the other non-NULL values, rendering the hash useless.

## Using CONCAT_WS

To tackle this, you can use \`CONCAT_WS\`, which ignores NULL values. \`CONCAT_WS\` stands for CONCAT With Separator. It allows you to define a separator that will be inserted between the strings to be concatenated. More importantly, it skips over any \`NULL\` values, ensuring that your hash remains effective.

```sql
SELECT CONCAT_WS('|', 'Hello', NULL, 'World');  -- Returns 'Hello|World'
```

## What is "Generated Always As"?

The "Generated Always As" clause can be used to create a computed column. The "Generated Always As" clause is like having a virtual assistant who calculates the hash for you every time a new record is inserted or updated. It ensures that the hash column is always in sync with the columns it depends upon.

```sql
ALTER TABLE table_name
ADD hash_column CHAR(32) GENERATED ALWAYS AS (MD5(CONCAT(column1, column2))) STORED;
```

## Choosing the Cache Type

For MD5, you'll most likely want to use \`CHAR(32)\`. Choosing the correct data type for storing your hash can be pivotal. For MD5 hashes, \`CHAR(32)\` is generally recommended over \`VARCHAR(32)\` because the size is fixed, which allows MySQL to optimize space allocation.

## Optimizing with UNHEX and Binary Types

To further optimize space, you can use the \`UNHEX\` function and change the column type to \`BINARY(16)\`. Converting the hash to a binary format via \`UNHEX\` can further optimize storage. This conversion slashes the storage size in half, making your database lighter and faster.

```sql
ALTER TABLE table_name
ADD hash_column BINARY(16) GENERATED ALWAYS AS (UNHEX(MD5(CONCAT(column1, column2)))) STORED;
```

## Querying the Index

```sql
SELECT * FROM table_name WHERE hash_column = UNHEX(MD5('value1value2'));
```

When querying, using \`UNHEX\` can help you directly compare binary hash values, making queries faster. It's the final piece in a well-optimized puzzle, allowing you to reap the benefits of all previous optimizations.

## Speed Comparisons

You'll find that MD5 and SHA-1 are generally faster than SHA-2. CRC32 is the fastest but least secure. Conducting speed tests on different hash algorithms with varying numbers of rows can be eye-opening. Generally, you'll find that as the number of rows scales up, hash algorithms like MD5 and SHA-1 maintain consistent speed, while SHA-2 tends to slow down a bit.