Are there disadvantages to using a generic varchar(255) for all text-based fields?

sql

database-optimization

data-integrity

validation

byAnton Shumikhin·Oct 22, 2024

Perfunctory use of VARCHAR(255) across the board is a resource leech: it commandeers storage, derails indexing speed and leads to unnecessary I/O operations. Think of data: the column length should match the expected input, e.g., VARCHAR(20) for names or VARCHAR(15) for phone numbers. It's a swift way to marshal resources and uphold data quality.

-- SAY NO to `VARCHAR(255)` Everywhere
CREATE TABLE users (
    name VARCHAR(20),           -- Names longer than this might be spam 🤖
    phone_number VARCHAR(15)    -- Even aliens don't have such long phone numbers 👽
);

The impact of size and performance

Consider this: SQL engines tend to allocate memory anticipating the max possible data length for each VARCHAR field. Meaning, if you're mostly dealing with short strings, there's lots of redundant memory reservation. Do you see the problem while sorting large tables? It's like handling a supertanker for a yacht's job!

Now think about indexing. Larger VARCHAR fields make your server sweat more with lengthier indexes. But, setting just-right column lengths gives your indexes a speed boost, and the ripple effect means a healthier, faster system.

Don't forget character encoding - think utf8mb4 that allows up to four bytes per character. This could mean serious memory and disk space use even for small strings. Choose your encoding and length in tandem to supercharge your queries and save precious space.

A byte of disk space usage

Disk space is not just physical storage; it affects your database's very breath— yes, its performance too! Mostly short entries scribbled in a VARCHAR(255) field? You're wasting disk space—a key concern in large databases or in systems where disk resources are sparse.

An entry in VARCHAR(255) combined with utf8mb4 could take up to 1020 bytes. Imagine this stretching across millions of rows. It's such a disk-hogging monstrosity your backups would run for coffee breaks! So, using well-sized fields helps cut down both storage and backup sizes, also speeding up restore operations.

Watch out for application defaults

Some application frameworks like Ruby on Rails take the liberty of setting 'String' as VARCHAR(255). You don't want such hefty defaults thrown around carelessly, do you?

Armed with awareness, you can override these defaults during migrations or schema definitions and shun database inefficiencies.

# Fight the `VARCHAR(255)` defaults in Rails
class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name, limit: 20   -- "Tony Stark" fits, and so does "T'Challa"
      t.string :phone_number, limit: 15 -- The longest a number can get without turning into Pi (3.14.....) 😉
    end
  end
end

Putting all your bets on maximum field lengths? It may backfire without solid validation or constraints. Pair optimal lengths with stringent validation to ensure wonderful data quality.

Thinking of future requirements

Entropy increases, data needs evolve. Tweaking column sizes with up-to-date data needs is easier than dealing with oversized columns from the onset. It's wise to review your schemas periodically. A quick data usage analysis helps the cause and keeps your schema definitions updated.

Remember, performance metrics are your torchbearers. They guide iterative optimization and enable a process aligned to ongoing usage and data growth.

Data integrity with checks and validations

Ensure sturdy data integrity using well-defined column sizes. Lay the frontier with CHECK constraints that enforce proper data boundaries. Pair these constraints with application-level validations to defy data violations at the input stage itself.

-- SQL Server with check constraints: Because not all superheroes wear capes!
CREATE TABLE users (
  name VARCHAR(20),
  phone_number VARCHAR(15),
  CHECK (LEN(name) > 0 AND LEN(name) <= 20), -- Precaution against nameless entities 👤
  CHECK (LEN(phone_number) > 0 AND LEN(phone_number) <= 15) -- Because numbers are more than just 0 and 1
);