#Fast Change Table

Use fast_change_table instead of change_table in your migrations on large tables of data. Uses a duplication pattern to speed things up.

uses ordinary change_table syntax but adds two options

• "replace_keys" to remove all indexes; new indexes will be specified

• "disable_keys" to remove indexes and apply them after data load; this is a tremendous performance enhancement for a dbms with fast index creation; it is active by default. set it to false to prevent its use

Example:

fast_change_table :table_name, :disable_keys => true do |t|
  t.change :old_string, :string, :limit => 64
  t.rename :old_string, :new_string
  t.integer :an_integer
end

other methods:

create_table_like(orignal_table, table_to_copy_to) creates a table with the same structure

disable_indexes(table) removes all indexes from a table, returns a list of index objects removed

enable_indexes(table, list_of_indexes) restores a list of indexes to a table

fast_add_indexes(table, &block) allows you to pass a block to add indexes. For mysql creates specified indexes in one statement; allows the data to be scanned once.

Example:

fast_add_indexes :sometable do |t|
 t.index :some_column
 t.index [:some_other_column, :column_three], :name => "a_multicolumn_index"
end

copy_table(from_table, to_table, remaps = [])

• copies rows from one table into another. this probably only works with Mysql. by default copies data from column of from_table to to_table of same name. will not copy data where there is no corresponding column. the remaps argument can be supplied to tell copy table how to handle unmatched columns or override this behavior

Examples:

copy_table(old_users_without_email_hash, new_table, ['MD5(email)', 'email_hash'])

copy_table(old_users_without_total, new_table, ['sum(payments)', 'total_payments'])