Add Expression Indexes and Operator Classes support for PostgreSQL

[kamipo]

Example:

create_table :users do |t|
  t.string :name
  t.index 'lower(name) varchar_pattern_ops'
end

Fixes #19090.
Fixes #21765.
Fixes #21819.
Fixes #24359.

[rails-bot]

r? @kaspth

(@rails-bot has picked a reviewer for you, use r? to override)

[kamipo]

r? @rafaelfranca

[kamipo]

Added opclasses support.

[jeremy]

Using a proc suggests that we expect a dynamic result, rather than using the special value to indicate an expression index.

How would this compare to using an new keyword argument, like

create_table :users do |t|
  t.string :name
  t.index expression: 'lower(name)'
  t.index opclass: 'name varchar_pattern_ops'
end

[kamipo]

Expressions and opclasses can be specified at the same time.

http://www.postgresql.org/docs/9.5/static/sql-createindex.html

CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] name ] ON table_name [ USING method ]
    ( { column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
    [ WITH ( storage_parameter = value [, ... ] ) ]
    [ TABLESPACE tablespace_name ]
    [ WHERE predicate ]

create_table :users do |t|
  t.string :first_name
  t.string :last_name
  t.index -> { <<-EXPRESSIONS }
    first_name || last_name,
    upper(first_name) varchar_pattern_ops,
    lower(last_name) varchar_pattern_ops
  EXPRESSIONS
end

[jeremy]

Gotcha. In that case, compare with e.g.

create_table :users do |t|
  t.string :first_name
  t.string :last_name
  t.index expression: <<-EXPRESSIONS
    first_name || last_name,
    upper(first_name) varchar_pattern_ops,
    lower(last_name) varchar_pattern_ops
  EXPRESSIONS
end

[kamipo]

We already have using procs syntax for schema and attribute default expression.
#20005
https://github.com/rails/rails/blob/v5.0.0.beta3/activerecord/lib/active_record/attributes.rb#L83

Do we prefer new :expression option for indexes?

[jeremy]

Good point.

Generally prefer keyword arguments for new constant-value arguments. Using a proc has worked well to bypass quoting or indicate SQL literals. So it makes sense to stick with them. This usage looks a lot more like passing a block to an index method, though. The other cases are keyword arg values, clear that they're 'special' values vs passing a proc/block.

[kamipo]

I changed implementation in This PR. This does not introduce new syntax now.
If column name that pass to add_index includes non word charactor, handles as raw string (regard as expressions).

create_table :users do |t|
  t.string :first_name
  t.string :last_name
  t.index <<-EXPRESSIONS
    first_name || last_name,
    upper(first_name) varchar_pattern_ops,
    lower(last_name) varchar_pattern_ops
  EXPRESSIONS
end

How about it?

[jeremy]

Merged edc2b77 😊