This obsoletes the ruby based implementations.

The type map was introduced in aafee233, but wasn't properly filled.

This mainly adjusts many locations, that expected strings instead of
integers or boolean.

add_pg_decoders is moved after setup of the StatementPool, because
execute_and_clear could potentially make use of it.

Ruby-pg's default way to serialize values for transmission to the database is to
call #to_s . This however creates a temporary String object for each value.
Setting a class based type map avoids the allocation of this additional String.

The performance benefit is measurable in microbenchmarks, but not with
the overhead of activerecord. However it's free to use and has no drawback.

As described here https://github.com/rails/rails/issues/19420. When
using the Postgres BigInt[] field type the big int value was not being
translated into schema.rb. This caused the field to become just a
regular integer field when building off of schema.rb. This fix will
address this by delegating the limit from the subtype to the Array type.

https://github.com/rails/rails/issues/19420

Fixes #19389.

to support MySQL 5.7.6 `optimizer_switch='derived_merge=on'`

Versions of the pg gem earlier than 0.18.0 cannot be used safely with Ruby 2.2.
Specifically, pg 0.17 when used with Ruby 2.2 has a known bug that causes
random bits to be added to the end of strings. Further explanation here:
https://bitbucket.org/ged/ruby-pg/issue/210/crazy-bytes-being-added-to-record

This change was prompted by 598b8418.

There was a typo in the `:requires_new` option. This led
to `#<ArgumentError: unknown keyword: require_new>` leaving all the
triggers in a disabled state.

- Add example to column_exists?
- Add example to index_exists?
- Add document for foreign_key
- Add document for foreign_key_exists?

This reverts commit a38732c8.

Since the mutation logic was reverted in
07278519, we must bring the reader
method back as well, since the implementation relies on it.

This reverts commit 393e65b4 and
ec51c3fe

We don't want the records to hold hard references to transactions
because they point at records that have callbacks.

this lets us keep singleton instances of "state" values and precalculate
return values of things like `finalized?` and `completed?`.

this way we don't have to mutate a state object, we can just change the
state of the txn

As far as I can tell nobody is setting this variable.

MySQL unicode support is not only `utf8mb4`.
Then, The index length problem is not only `utf8mb4`.

http://dev.mysql.com/doc/refman/5.6/en/charset-unicode.html

    SELECT * FROM information_schema.character_sets WHERE maxlen > 3;
    +--------------------+----------------------+------------------+--------+
    | CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION      | MAXLEN |
    +--------------------+----------------------+------------------+--------+
    | utf8mb4            | utf8mb4_general_ci   | UTF-8 Unicode    |      4 |
    | utf16              | utf16_general_ci     | UTF-16 Unicode   |      4 |
    | utf16le            | utf16le_general_ci   | UTF-16LE Unicode |      4 |
    | utf32              | utf32_general_ci     | UTF-32 Unicode   |      4 |
    +--------------------+----------------------+------------------+--------+

we do this in other adapters, and it's a nice speed improvement

[fixes #18903]

[Toby Ovod-Everett & Andrey Nering & Yves Senn]

Closes #17726.
Closes #10939.

This patch makes three distinct modifications:

1. no longer fall back to disabling user triggers if system triggers can't be disabled
2. warn the user when referential integrity can't be disabled
3. restore aborted transactions when referential integrity can't be disabled

The motivation behind these changes is to make the behavior of Rails
transparent and less error-prone. To require superuser privileges is not optimal
but it's what Rails currently needs. Users who absolutely rely on disabling user triggers
can patch `disable_referential_integrity`.

We should investigate `SET CONSTRAINTS` as a possible solution which does not require
superuser privileges.

/cc @matthewd

Only `primary_key` should be extracted by d47357e2 in #19030, but
`new_coclumn_definition` was also extracted because #17631 is merged
previously, then #19030 is auto merged without conflicts.

This commit is for move back `new_column_definition` into
`TableDefinition`.

Example:

    create_table :foos, id: :primary_key, limit: 8 do |t|
    end

    # or

    create_table :foos, id: false do |t|
      t.column :id, limit: 8
    end