Sql Indexes

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In my last post, I mentioned that unique is a reserved word in SQL because of indexes. But to be totally honest, that was the first time I’d even heard of indexes. So I did some more research…and they’re actually pretty cool.

Basically, an index is a special lookup for tables in a database that are used to speed up data retrieval, aka quicker queries using WHERE and SELECT clauses. In even simpler terms, it’s a pointer to specific data. Only downside is that it slows down data inputs with UPDATE and INSERT clauses. Also, indexes can be created or dropped easily, and will have no effect on the code.

So first things first, you want to create an index. This process is incredibly simple:

  CREATE INDEX index_name
  on table_name (column_name);

For example, I have a recipes table, and that table has the columns name, ingredients, and preparations. And say you want to be able to easily query for recipes with a specific ingredient, this would be a great time to use an index.

  CREATE INDEX recipe_ingredients
  on recipes (ingredients);

Super easy.

Now, there are a few different kinds of indexes:

1. Unique Index: allows no duplicate values to be inserted in the selected column in your table.

  CREATE UNIQUE INDEX recipe_ingredients
  on recipes (ingredients);

It’s simple to create a unique index, but there is an appropriate moment to use it. The above example is the proper way to create a unique index, but not it’s proper use. You wouldn’t be able to include butter in more than one recipe. Or salt. Or hot sauce. Not ok.

2. Composite Index: used on two or more columns in a table.

CREATE INDEX recipes
on recipes (ingredients, preparations);

This index will allow speedy retrieval of data in both the ingredients column and the preparations column.

3. Implicit Index: this is an implied index, that is automatically created with the formation of a PRIMARY_KEY column, or UNIQUE_KEY column. Any time your table has an ID column, you have an implicit index on that column.

Obviously, you need to know how to get rid of an index. It’s also really simple and will not affect your data in any way.

  DROP INDEX index_name;
  DROP INDEX recipes;
  DROP INDEX recipe_ingredients;

Now we got rid of all the indexes (except the implicit index) on my recipes table.

So of course Rails has built in methods with ActiveRecord to deal with indexes. The declaration for indexes in rails is included in your migrations, but the methods must become class methods….

add_index :table_name, :column_names, options

Your options include :name, :unique, and :order

So to take my same example of my recipes table from before and the migration AddIndexesToRecipes:

  def self.up
    add_index :recipes, ingredients, :name => 'recipe_ingredients'
  end  

In this example, I created an index on my recipes table, on the ingredients column, and I named the index “recipe_ingredients”. If I had used :unique, I would have created the same problem as before, where my cookies could have butter, but my cake couldn’t. No thanks.

Rails also makes it really simple to remove indexes, and there are two ways to do so:

  1. Remove an index by column name: remove_index (table_name, column: column_name) remove_index (recipes, column: ingredients)

  2. Remove an index by index name: remove_index (table_name, name: index_name) remove_index (recipes, name: recipe_ingredients)

    Helpful Resources:
  1. http://www.tutorialspoint.com/sql/sql-indexes.htm
  2. http://api.rubyonrails.org/classes/ActiveRecord/Migration.html

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