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What is the difference between
t.boolean :is_live, index: true and
add_index :table_name, :is_live If there is no difference, how come only the add_index is reflected in schema.rb. When I use index: true, I can't actually see the index in schema.rb. Should I only use the add_index method.
When use the add_index method, I can see this in my schema.rb
add_index "table_name", ["is_live"], name: "index_table_name_on_is_live", using: :blahblah 
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An index is used to speed up the performance of queries on a database. Rails allows us to create index on a database column by means of a migration. By default, the sort order for the index is ascending. But consider the case where we are fetching reports from the database.
index: true adds a database index to the referenced column. For example, if creating a :products table: create_table :products do |t| t.references :user, index: true end. That will create a non-unique index on the user_id column in the products table named index_products_on_user_id .
Since this is something we would be querying quite frequently in our database, it is best to use the add_index to our name data. This allows for faster lookup by only grabbing all of the names in the name column in our database, then comparing it to our search query of the name we are looking for.
A Rails migration is a tool for changing an application's database schema. Instead of managing SQL scripts, you define database changes in a domain-specific language (DSL). The code is database-independent, so you can easily move your app to a new platform.
Just found out that the :index option is for references only (t.references or t.belongs_to). For 'ordinary' column types this option is ignored (that's why indices are not reflected in your schema.db when you used the :index option).
For less verbose syntax there is an index type:
t.index :column_name # extra options may be provided also In short: both do the same job. ìndex: true` just saves you an additional line. Look here https://github.com/rails/rails/pull/5262#issuecomment-4329571
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