postgresql_adapter.rb

require 'active_record/connection_adapters/abstract_adapter'
require 'active_support/core_ext/kernel/requires'
require 'active_support/core_ext/object/blank'
require 'pg'

module ActiveRecord
  class Base
    # Establishes a connection to the database that's used by all Active Record objects
    def self.postgresql_connection(config) # :nodoc:
      config = config.symbolize_keys
      host     = config[:host]
      port     = config[:port] || 5432
      username = config[:username].to_s if config[:username]
      password = config[:password].to_s if config[:password]

      if config.key?(:database)
        database = config[:database]
      else
        raise ArgumentError, "No database specified. Missing argument: database."
      end

      # The postgres drivers don't allow the creation of an unconnected PGconn object,
      # so just pass a nil connection object for the time being.
      ConnectionAdapters::PostgreSQLAdapter.new(nil, logger, [host, port, nil, nil, database, username, password], config)
    end
  end

  module ConnectionAdapters
    # PostgreSQL-specific extensions to column definitions in a table.
    class PostgreSQLColumn < Column #:nodoc:
      # Instantiates a new PostgreSQL column definition in a table.
      def initialize(name, default, sql_type = nil, null = true)
        super(name, self.class.extract_value_from_default(default), sql_type, null)
      end

      # :stopdoc:
      class << self
        attr_accessor :money_precision
      end
      # :startdoc:

      private
        def extract_limit(sql_type)
          case sql_type
          when /^bigint/i;    8
          when /^smallint/i;  2
          else super
          end
        end

        # Extracts the scale from PostgreSQL-specific data types.
        def extract_scale(sql_type)
          # Money type has a fixed scale of 2.
          sql_type =~ /^money/ ? 2 : super
        end

        # Extracts the precision from PostgreSQL-specific data types.
        def extract_precision(sql_type)
          if sql_type == 'money'
            self.class.money_precision
          else
            super
          end
        end

        # Maps PostgreSQL-specific data types to logical Rails types.
        def simplified_type(field_type)
          case field_type
            # Numeric and monetary types
            when /^(?:real|double precision)$/
              :float
            # Monetary types
            when 'money'
              :decimal
            # Character types
            when /^(?:character varying|bpchar)(?:\(\d+\))?$/
              :string
            # Binary data types
            when 'bytea'
              :binary
            # Date/time types
            when /^timestamp with(?:out)? time zone$/
              :datetime
            when 'interval'
              :string
            # Geometric types
            when /^(?:point|line|lseg|box|"?path"?|polygon|circle)$/
              :string
            # Network address types
            when /^(?:cidr|inet|macaddr)$/
              :string
            # Bit strings
            when /^bit(?: varying)?(?:\(\d+\))?$/
              :string
            # XML type
            when 'xml'
              :xml
            # Arrays
            when /^\D+\[\]$/
              :string
            # Object identifier types
            when 'oid'
              :integer
            # UUID type
            when 'uuid'
              :string
            # Small and big integer types
            when /^(?:small|big)int$/
              :integer
            # Pass through all types that are not specific to PostgreSQL.
            else
              super
          end
        end

        # Extracts the value from a PostgreSQL column default definition.
        def self.extract_value_from_default(default)
          case default
            # Numeric types
            when /\A\(?(-?\d+(\.\d*)?\)?)\z/
              $1
            # Character types
            when /\A'(.*)'::(?:character varying|bpchar|text)\z/m
              $1
            # Character types (8.1 formatting)
            when /\AE'(.*)'::(?:character varying|bpchar|text)\z/m
              $1.gsub(/\\(\d\d\d)/) { $1.oct.chr }
            # Binary data types
            when /\A'(.*)'::bytea\z/m
              $1
            # Date/time types
            when /\A'(.+)'::(?:time(?:stamp)? with(?:out)? time zone|date)\z/
              $1
            when /\A'(.*)'::interval\z/
              $1
            # Boolean type
            when 'true'
              true
            when 'false'
              false
            # Geometric types
            when /\A'(.*)'::(?:point|line|lseg|box|"?path"?|polygon|circle)\z/
              $1
            # Network address types
            when /\A'(.*)'::(?:cidr|inet|macaddr)\z/
              $1
            # Bit string types
            when /\AB'(.*)'::"?bit(?: varying)?"?\z/
              $1
            # XML type
            when /\A'(.*)'::xml\z/m
              $1
            # Arrays
            when /\A'(.*)'::"?\D+"?\[\]\z/
              $1
            # Object identifier types
            when /\A-?\d+\z/
              $1
            else
              # Anything else is blank, some user type, or some function
              # and we can't know the value of that, so return nil.
              nil
          end
        end
    end

    # The PostgreSQL adapter works both with the native C (http://ruby.scripting.ca/postgres/) and the pure
    # Ruby (available both as gem and from http://rubyforge.org/frs/?group_id=234&release_id=1944) drivers.
    #
    # Options:
    #
    # * <tt>:host</tt> - Defaults to "localhost".
    # * <tt>:port</tt> - Defaults to 5432.
    # * <tt>:username</tt> - Defaults to nothing.
    # * <tt>:password</tt> - Defaults to nothing.
    # * <tt>:database</tt> - The name of the database. No default, must be provided.
    # * <tt>:schema_search_path</tt> - An optional schema search path for the connection given
    #   as a string of comma-separated schema names.  This is backward-compatible with the <tt>:schema_order</tt> option.
    # * <tt>:encoding</tt> - An optional client encoding that is used in a <tt>SET client_encoding TO
    #   <encoding></tt> call on the connection.
    # * <tt>:min_messages</tt> - An optional client min messages that is used in a
    #   <tt>SET client_min_messages TO <min_messages></tt> call on the connection.
    class PostgreSQLAdapter < AbstractAdapter
      class TableDefinition < ActiveRecord::ConnectionAdapters::TableDefinition
        def xml(*args)
          options = args.extract_options!
          column(args[0], 'xml', options)
        end
      end

      ADAPTER_NAME = 'PostgreSQL'

      NATIVE_DATABASE_TYPES = {
        :primary_key => "serial primary key",
        :string      => { :name => "character varying", :limit => 255 },
        :text        => { :name => "text" },
        :integer     => { :name => "integer" },
        :float       => { :name => "float" },
        :decimal     => { :name => "decimal" },
        :datetime    => { :name => "timestamp" },
        :timestamp   => { :name => "timestamp" },
        :time        => { :name => "time" },
        :date        => { :name => "date" },
        :binary      => { :name => "bytea" },
        :boolean     => { :name => "boolean" },
        :xml         => { :name => "xml" }
      }

      # Returns 'PostgreSQL' as adapter name for identification purposes.
      def adapter_name
        ADAPTER_NAME
      end

      # Returns +true+, since this connection adapter supports prepared statement
      # caching.
      def supports_statement_cache?
        true
      end

      # Initializes and connects a PostgreSQL adapter.
      def initialize(connection, logger, connection_parameters, config)
        super(connection, logger)
        @connection_parameters, @config = connection_parameters, config

        # @local_tz is initialized as nil to avoid warnings when connect tries to use it
        @local_tz = nil
        @table_alias_length = nil
        @postgresql_version = nil
        @statements = {}

        connect

        if postgresql_version < 80200
          raise "Your version of PostgreSQL (#{postgresql_version}) is too old, please upgrade!"
        end

        @local_tz = execute('SHOW TIME ZONE', 'SCHEMA').first["TimeZone"]
      end

      # Clears the prepared statements cache.
      def clear_cache!
        @statements.each_value do |value|
          @connection.query "DEALLOCATE #{value}"
        end
        @statements.clear
      end

      # Is this connection alive and ready for queries?
      def active?
        if @connection.respond_to?(:status)
          @connection.status == PGconn::CONNECTION_OK
        else
          # We're asking the driver, not Active Record, so use @connection.query instead of #query
          @connection.query 'SELECT 1'
          true
        end
      # postgres-pr raises a NoMethodError when querying if no connection is available.
      rescue PGError, NoMethodError
        false
      end

      # Close then reopen the connection.
      def reconnect!
        if @connection.respond_to?(:reset)
          clear_cache!
          @connection.reset
          configure_connection
        else
          disconnect!
          connect
        end
      end

      def reset!
        clear_cache!
        super
      end

      # Disconnects from the database if already connected. Otherwise, this
      # method does nothing.
      def disconnect!
        clear_cache!
        @connection.close rescue nil
      end

      def native_database_types #:nodoc:
        NATIVE_DATABASE_TYPES
      end

      # Does PostgreSQL support migrations?
      def supports_migrations?
        true
      end

      # Does PostgreSQL support finding primary key on non-Active Record tables?
      def supports_primary_key? #:nodoc:
        true
      end

      # Enable standard-conforming strings if available.
      def set_standard_conforming_strings
        old, self.client_min_messages = client_min_messages, 'panic'
        execute('SET standard_conforming_strings = on', 'SCHEMA') rescue nil
      ensure
        self.client_min_messages = old
      end

      def supports_insert_with_returning?
        true
      end

      def supports_ddl_transactions?
        true
      end

      def supports_savepoints?
        true
      end

      # Returns the configured supported identifier length supported by PostgreSQL
      def table_alias_length
        @table_alias_length ||= query('SHOW max_identifier_length')[0][0].to_i
      end

      # QUOTING ==================================================

      # Escapes binary strings for bytea input to the database.
      def escape_bytea(value)
        @connection.escape_bytea(value) if value
      end

      # Unescapes bytea output from a database to the binary string it represents.
      # NOTE: This is NOT an inverse of escape_bytea! This is only to be used
      #       on escaped binary output from database drive.
      def unescape_bytea(value)
        @connection.unescape_bytea(value) if value
      end

      # Quotes PostgreSQL-specific data types for SQL input.
      def quote(value, column = nil) #:nodoc:
        return super unless column

        case value
        when Numeric
          return super unless column.sql_type == 'money'
          # Not truly string input, so doesn't require (or allow) escape string syntax.
          "'#{value}'"
        when String
          case column.sql_type
          when 'bytea' then "'#{escape_bytea(value)}'"
          when 'xml'   then "xml '#{quote_string(value)}'"
          when /^bit/
            case value
            when /^[01]*$/      then "B'#{value}'" # Bit-string notation
            when /^[0-9A-F]*$/i then "X'#{value}'" # Hexadecimal notation
            end
          else
            super
          end
        else
          super
        end
      end

      def type_cast(value, column)
        return super unless column

        case value
        when String
          return super unless 'bytea' == column.sql_type
          { :value => value, :format => 1 }
        else
          super
        end
      end

      # Quotes strings for use in SQL input.
      def quote_string(s) #:nodoc:
        @connection.escape(s)
      end

      # Checks the following cases:
      #
      # - table_name
      # - "table.name"
      # - schema_name.table_name
      # - schema_name."table.name"
      # - "schema.name".table_name
      # - "schema.name"."table.name"
      def quote_table_name(name)
        schema, name_part = extract_pg_identifier_from_name(name.to_s)

        unless name_part
          quote_column_name(schema)
        else
          table_name, name_part = extract_pg_identifier_from_name(name_part)
          "#{quote_column_name(schema)}.#{quote_column_name(table_name)}"
        end
      end

      # Quotes column names for use in SQL queries.
      def quote_column_name(name) #:nodoc:
        PGconn.quote_ident(name.to_s)
      end

      # Quote date/time values for use in SQL input. Includes microseconds
      # if the value is a Time responding to usec.
      def quoted_date(value) #:nodoc:
        if value.acts_like?(:time) && value.respond_to?(:usec)
          "#{super}.#{sprintf("%06d", value.usec)}"
        else
          super
        end
      end

      # Set the authorized user for this session
      def session_auth=(user)
        clear_cache!
        exec_query "SET SESSION AUTHORIZATION #{user}"
      end

      # REFERENTIAL INTEGRITY ====================================

      def supports_disable_referential_integrity?() #:nodoc:
        true
      end

      def disable_referential_integrity #:nodoc:
        if supports_disable_referential_integrity?() then
          execute(tables.collect { |name| "ALTER TABLE #{quote_table_name(name)} DISABLE TRIGGER ALL" }.join(";"))
        end
        yield
      ensure
        if supports_disable_referential_integrity?() then
          execute(tables.collect { |name| "ALTER TABLE #{quote_table_name(name)} ENABLE TRIGGER ALL" }.join(";"))
        end
      end

      # DATABASE STATEMENTS ======================================

      # Executes a SELECT query and returns an array of rows. Each row is an
      # array of field values.
      def select_rows(sql, name = nil)
        select_raw(sql, name).last
      end

      # Executes an INSERT query and returns the new record's ID
      def insert_sql(sql, name = nil, pk = nil, id_value = nil, sequence_name = nil)
        # Extract the table from the insert sql. Yuck.
        _, table = extract_schema_and_table(sql.split(" ", 4)[2])

        pk ||= primary_key(table)

        if pk
          select_value("#{sql} RETURNING #{quote_column_name(pk)}")
        else
          super
        end
      end
      alias :create :insert

      # create a 2D array representing the result set
      def result_as_array(res) #:nodoc:
        # check if we have any binary column and if they need escaping
        unescape_col = []
        res.nfields.times do |j|
          unescape_col << res.ftype(j)
        end

        ary = []
        res.ntuples.times do |i|
          ary << []
          res.nfields.times do |j|
            data = res.getvalue(i,j)
            case unescape_col[j]

            # unescape string passed BYTEA field (OID == 17)
            when BYTEA_COLUMN_TYPE_OID
              data = unescape_bytea(data) if String === data

            # If this is a money type column and there are any currency symbols,
            # then strip them off. Indeed it would be prettier to do this in
            # PostgreSQLColumn.string_to_decimal but would break form input
            # fields that call value_before_type_cast.
            when MONEY_COLUMN_TYPE_OID
              # Because money output is formatted according to the locale, there are two
              # cases to consider (note the decimal separators):
              #  (1) $12,345,678.12
              #  (2) $12.345.678,12
              case data
              when /^-?\D+[\d,]+\.\d{2}$/  # (1)
                data.gsub!(/[^-\d.]/, '')
              when /^-?\D+[\d.]+,\d{2}$/  # (2)
                data.gsub!(/[^-\d,]/, '').sub!(/,/, '.')
              end
            end
            ary[i] << data
          end
        end
        return ary
      end


      # Queries the database and returns the results in an Array-like object
      def query(sql, name = nil) #:nodoc:
        log(sql, name) do
          if @async
            res = @connection.async_exec(sql)
          else
            res = @connection.exec(sql)
          end
          return result_as_array(res)
        end
      end

      # Executes an SQL statement, returning a PGresult object on success
      # or raising a PGError exception otherwise.
      def execute(sql, name = nil)
        log(sql, name) do
          if @async
            @connection.async_exec(sql)
          else
            @connection.exec(sql)
          end
        end
      end

      def substitute_at(column, index)
        Arel.sql("$#{index + 1}")
      end

      def exec_query(sql, name = 'SQL', binds = [])
        return exec_no_cache(sql, name) if binds.empty?

        log(sql, name, binds) do
          unless @statements.key? sql
            nextkey = "a#{@statements.length + 1}"
            @connection.prepare nextkey, sql
            @statements[sql] = nextkey
          end

          key = @statements[sql]

          # Clear the queue
          @connection.get_last_result
          @connection.send_query_prepared(key, binds.map { |col, val|
            type_cast(val, col)
          })
          @connection.block
          result = @connection.get_last_result
          ret = ActiveRecord::Result.new(result.fields, result_as_array(result))
          result.clear
          return ret
        end
      end

      def exec_insert(sql, name, binds)
        exec_query(sql, name, binds)
      end

      def sql_for_insert(sql, pk, id_value, sequence_name, binds)
        unless pk
          _, table = extract_schema_and_table(sql.split(" ", 4)[2])

          pk = primary_key(table)
        end

        sql = "#{sql} RETURNING #{quote_column_name(pk)}" if pk

        [sql, binds]
      end

      # Executes an UPDATE query and returns the number of affected tuples.
      def update_sql(sql, name = nil)
        super.cmd_tuples
      end

      # Begins a transaction.
      def begin_db_transaction
        execute "BEGIN"
      end

      # Commits a transaction.
      def commit_db_transaction
        execute "COMMIT"
      end

      # Aborts a transaction.
      def rollback_db_transaction
        execute "ROLLBACK"
      end

      def outside_transaction?
        @connection.transaction_status == PGconn::PQTRANS_IDLE
      end

      def create_savepoint
        execute("SAVEPOINT #{current_savepoint_name}")
      end

      def rollback_to_savepoint
        execute("ROLLBACK TO SAVEPOINT #{current_savepoint_name}")
      end

      def release_savepoint
        execute("RELEASE SAVEPOINT #{current_savepoint_name}")
      end

      # SCHEMA STATEMENTS ========================================

      def recreate_database(name) #:nodoc:
        drop_database(name)
        create_database(name)
      end

      # Create a new PostgreSQL database.  Options include <tt>:owner</tt>, <tt>:template</tt>,
      # <tt>:encoding</tt>, <tt>:tablespace</tt>, and <tt>:connection_limit</tt> (note that MySQL uses
      # <tt>:charset</tt> while PostgreSQL uses <tt>:encoding</tt>).
      #
      # Example:
      #   create_database config[:database], config
      #   create_database 'foo_development', :encoding => 'unicode'
      def create_database(name, options = {})
        options = options.reverse_merge(:encoding => "utf8")

        option_string = options.symbolize_keys.sum do |key, value|
          case key
          when :owner
            " OWNER = \"#{value}\""
          when :template
            " TEMPLATE = \"#{value}\""
          when :encoding
            " ENCODING = '#{value}'"
          when :tablespace
            " TABLESPACE = \"#{value}\""
          when :connection_limit
            " CONNECTION LIMIT = #{value}"
          else
            ""
          end
        end

        execute "CREATE DATABASE #{quote_table_name(name)}#{option_string}"
      end

      # Drops a PostgreSQL database.
      #
      # Example:
      #   drop_database 'matt_development'
      def drop_database(name) #:nodoc:
        execute "DROP DATABASE IF EXISTS #{quote_table_name(name)}"
      end

      # Returns the list of all tables in the schema search path or a specified schema.
      def tables(name = nil)
        query(<<-SQL, 'SCHEMA').map { |row| row[0] }
          SELECT tablename
          FROM pg_tables
          WHERE schemaname = ANY (current_schemas(false))
        SQL
      end

      def table_exists?(name)
        schema, table = extract_schema_and_table(name.to_s)

        binds = [[nil, table.gsub(/(^"|"$)/,'')]]
        binds << [nil, schema] if schema

        exec_query(<<-SQL, 'SCHEMA', binds).rows.first[0].to_i > 0
            SELECT COUNT(*)
            FROM pg_tables
            WHERE tablename = $1
            #{schema ? "AND schemaname = $2" : ''}
        SQL
      end

      # Extracts the table and schema name from +name+
      def extract_schema_and_table(name)
        schema, table = name.split('.', 2)

        unless table # A table was provided without a schema
          table  = schema
          schema = nil
        end

        if name =~ /^"/ # Handle quoted table names
          table  = name
          schema = nil
        end
        [schema, table]
      end

      # Returns the list of all indexes for a table.
      def indexes(table_name, name = nil)
         schemas = schema_search_path.split(/,/).map { |p| quote(p) }.join(',')
         result = query(<<-SQL, name)
           SELECT distinct i.relname, d.indisunique, d.indkey, t.oid
             FROM pg_class t, pg_class i, pg_index d
           WHERE i.relkind = 'i'
             AND d.indexrelid = i.oid
             AND d.indisprimary = 'f'
             AND t.oid = d.indrelid
             AND t.relname = '#{table_name}'
             AND i.relnamespace IN (SELECT oid FROM pg_namespace WHERE nspname IN (#{schemas}) )
          ORDER BY i.relname
        SQL


        result.map do |row|
          index_name = row[0]
          unique = row[1] == 't'
          indkey = row[2].split(" ")
          oid = row[3]

          columns = Hash[query(<<-SQL, "Columns for index #{row[0]} on #{table_name}")]
          SELECT a.attnum, a.attname
          FROM pg_attribute a
          WHERE a.attrelid = #{oid}
          AND a.attnum IN (#{indkey.join(",")})
          SQL

          column_names = columns.values_at(*indkey).compact
          column_names.empty? ? nil : IndexDefinition.new(table_name, index_name, unique, column_names)
        end.compact
      end

      # Returns the list of all column definitions for a table.
      def columns(table_name, name = nil)
        # Limit, precision, and scale are all handled by the superclass.
        column_definitions(table_name).collect do |column_name, type, default, notnull|
          PostgreSQLColumn.new(column_name, default, type, notnull == 'f')
        end
      end

      # Returns the current database name.
      def current_database
        query('select current_database()')[0][0]
      end

      # Returns the current database encoding format.
      def encoding
        query(<<-end_sql)[0][0]
          SELECT pg_encoding_to_char(pg_database.encoding) FROM pg_database
          WHERE pg_database.datname LIKE '#{current_database}'
        end_sql
      end

      # Sets the schema search path to a string of comma-separated schema names.
      # Names beginning with $ have to be quoted (e.g. $user => '$user').
      # See: http://www.postgresql.org/docs/current/static/ddl-schemas.html
      #
      # This should be not be called manually but set in database.yml.
      def schema_search_path=(schema_csv)
        if schema_csv
          execute "SET search_path TO #{schema_csv}"
          @schema_search_path = schema_csv
        end
      end

      # Returns the active schema search path.
      def schema_search_path
        @schema_search_path ||= query('SHOW search_path')[0][0]
      end

      # Returns the current client message level.
      def client_min_messages
        query('SHOW client_min_messages', 'SCHEMA')[0][0]
      end

      # Set the client message level.
      def client_min_messages=(level)
        execute("SET client_min_messages TO '#{level}'", 'SCHEMA')
      end

      # Returns the sequence name for a table's primary key or some other specified key.
      def default_sequence_name(table_name, pk = nil) #:nodoc:
        serial_sequence(table_name, pk || 'id').split('.').last
      rescue ActiveRecord::StatementInvalid
        "#{table_name}_#{pk || 'id'}_seq"
      end

      def serial_sequence(table, column)
        result = exec_query(<<-eosql, 'SCHEMA', [[nil, table], [nil, column]])
          SELECT pg_get_serial_sequence($1, $2)
        eosql
        result.rows.first.first
      end

      # Resets the sequence of a table's primary key to the maximum value.
      def reset_pk_sequence!(table, pk = nil, sequence = nil) #:nodoc:
        unless pk and sequence
          default_pk, default_sequence = pk_and_sequence_for(table)

          pk ||= default_pk
          sequence ||= default_sequence
        end

        if @logger && pk && !sequence
          @logger.warn "#{table} has primary key #{pk} with no default sequence"
        end

        if pk && sequence
          quoted_sequence = quote_column_name(sequence)

          select_value <<-end_sql, 'Reset sequence'
            SELECT setval('#{quoted_sequence}', (SELECT COALESCE(MAX(#{quote_column_name pk})+(SELECT increment_by FROM #{quoted_sequence}), (SELECT min_value FROM #{quoted_sequence})) FROM #{quote_table_name(table)}), false)
          end_sql
        end
      end

      # Returns a table's primary key and belonging sequence.
      def pk_and_sequence_for(table) #:nodoc:
        # First try looking for a sequence with a dependency on the
        # given table's primary key.
        result = exec_query(<<-end_sql, 'SCHEMA').rows.first
          SELECT attr.attname, seq.relname
          FROM pg_class      seq,
               pg_attribute  attr,
               pg_depend     dep,
               pg_namespace  name,
               pg_constraint cons
          WHERE seq.oid           = dep.objid
            AND seq.relkind       = 'S'
            AND attr.attrelid     = dep.refobjid
            AND attr.attnum       = dep.refobjsubid
            AND attr.attrelid     = cons.conrelid
            AND attr.attnum       = cons.conkey[1]
            AND cons.contype      = 'p'
            AND dep.refobjid      = '#{quote_table_name(table)}'::regclass
        end_sql

        # [primary_key, sequence]
        [result.first, result.last]
      rescue
        nil
      end

      # Returns just a table's primary key
      def primary_key(table)
        row = exec_query(<<-end_sql, 'SCHEMA', [[nil, table]]).rows.first
          SELECT DISTINCT(attr.attname)
          FROM pg_attribute  attr,
               pg_depend     dep,
               pg_namespace  name,
               pg_constraint cons
          WHERE attr.attrelid     = dep.refobjid
            AND attr.attnum       = dep.refobjsubid
            AND attr.attrelid     = cons.conrelid
            AND attr.attnum       = cons.conkey[1]
            AND cons.contype      = 'p'
            AND dep.refobjid      = $1::regclass
        end_sql

        row && row.first
      end

      # Renames a table.
      def rename_table(name, new_name)
        execute "ALTER TABLE #{quote_table_name(name)} RENAME TO #{quote_table_name(new_name)}"
      end

      # Adds a new column to the named table.
      # See TableDefinition#column for details of the options you can use.
      def add_column(table_name, column_name, type, options = {})
        add_column_sql = "ALTER TABLE #{quote_table_name(table_name)} ADD COLUMN #{quote_column_name(column_name)} #{type_to_sql(type, options[:limit], options[:precision], options[:scale])}"
        add_column_options!(add_column_sql, options)

        execute add_column_sql
      end

      # Changes the column of a table.
      def change_column(table_name, column_name, type, options = {})
        quoted_table_name = quote_table_name(table_name)

        execute "ALTER TABLE #{quoted_table_name} ALTER COLUMN #{quote_column_name(column_name)} TYPE #{type_to_sql(type, options[:limit], options[:precision], options[:scale])}"

        change_column_default(table_name, column_name, options[:default]) if options_include_default?(options)
        change_column_null(table_name, column_name, options[:null], options[:default]) if options.key?(:null)
      end

      # Changes the default value of a table column.
      def change_column_default(table_name, column_name, default)
        execute "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} SET DEFAULT #{quote(default)}"
      end

      def change_column_null(table_name, column_name, null, default = nil)
        unless null || default.nil?
          execute("UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote(default)} WHERE #{quote_column_name(column_name)} IS NULL")
        end
        execute("ALTER TABLE #{quote_table_name(table_name)} ALTER #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL")
      end

      # Renames a column in a table.
      def rename_column(table_name, column_name, new_column_name)
        execute "ALTER TABLE #{quote_table_name(table_name)} RENAME COLUMN #{quote_column_name(column_name)} TO #{quote_column_name(new_column_name)}"
      end

      def remove_index!(table_name, index_name) #:nodoc:
        execute "DROP INDEX #{quote_table_name(index_name)}"
      end

      def rename_index(table_name, old_name, new_name)
        execute "ALTER INDEX #{quote_column_name(old_name)} RENAME TO #{quote_table_name(new_name)}"
      end

      def index_name_length
        63
      end

      # Maps logical Rails types to PostgreSQL-specific data types.
      def type_to_sql(type, limit = nil, precision = nil, scale = nil)
        return super unless type.to_s == 'integer'
        return 'integer' unless limit

        case limit
          when 1, 2; 'smallint'
          when 3, 4; 'integer'
          when 5..8; 'bigint'
          else raise(ActiveRecordError, "No integer type has byte size #{limit}. Use a numeric with precision 0 instead.")
        end
      end

      # Returns a SELECT DISTINCT clause for a given set of columns and a given ORDER BY clause.
      #
      # PostgreSQL requires the ORDER BY columns in the select list for distinct queries, and
      # requires that the ORDER BY include the distinct column.
      #
      #   distinct("posts.id", "posts.created_at desc")
      def distinct(columns, orders) #:nodoc:
        return "DISTINCT #{columns}" if orders.empty?

        # Construct a clean list of column names from the ORDER BY clause, removing
        # any ASC/DESC modifiers
        order_columns = orders.collect { |s| s =~ /^(.+)\s+(ASC|DESC)\s*$/i ? $1 : s }
        order_columns.delete_if { |c| c.blank? }
        order_columns = order_columns.zip((0...order_columns.size).to_a).map { |s,i| "#{s} AS alias_#{i}" }

        # Return a DISTINCT clause that's distinct on the columns we want but includes
        # all the required columns for the ORDER BY to work properly.
        sql = "DISTINCT ON (#{columns}) #{columns}, "
        sql << order_columns * ', '
      end

      protected
        # Returns the version of the connected PostgreSQL version.
        def postgresql_version
          @postgresql_version ||=
            if @connection.respond_to?(:server_version)
              @connection.server_version
            else
              # Mimic PGconn.server_version behavior
              begin
                if query('SELECT version()')[0][0] =~ /PostgreSQL ([0-9.]+)/
                  major, minor, tiny = $1.split(".")
                  (major.to_i * 10000) + (minor.to_i * 100) + tiny.to_i
                else
                  0
                end
              rescue
                0
              end
            end
        end

        def translate_exception(exception, message)
          case exception.message
          when /duplicate key value violates unique constraint/
            RecordNotUnique.new(message, exception)
          when /violates foreign key constraint/
            InvalidForeignKey.new(message, exception)
          else
            super
          end
        end

      private
      def exec_no_cache(sql, name)
        log(sql, name) do
          result = @connection.async_exec(sql)
          ret = ActiveRecord::Result.new(result.fields, result_as_array(result))
          result.clear
          ret
        end
      end

        # The internal PostgreSQL identifier of the money data type.
        MONEY_COLUMN_TYPE_OID = 790 #:nodoc:
        # The internal PostgreSQL identifier of the BYTEA data type.
        BYTEA_COLUMN_TYPE_OID = 17 #:nodoc:

        # Connects to a PostgreSQL server and sets up the adapter depending on the
        # connected server's characteristics.
        def connect
          @connection = PGconn.connect(*@connection_parameters)
          PGconn.translate_results = false if PGconn.respond_to?(:translate_results=)

          # Ignore async_exec and async_query when using postgres-pr.
          @async = @connection.respond_to?(:async_exec)

          # Money type has a fixed precision of 10 in PostgreSQL 8.2 and below, and as of
          # PostgreSQL 8.3 it has a fixed precision of 19. PostgreSQLColumn.extract_precision
          # should know about this but can't detect it there, so deal with it here.
          PostgreSQLColumn.money_precision = (postgresql_version >= 80300) ? 19 : 10

          configure_connection
        end

        # Configures the encoding, verbosity, schema search path, and time zone of the connection.
        # This is called by #connect and should not be called manually.
        def configure_connection
          if @config[:encoding]
            if @connection.respond_to?(:set_client_encoding)
              @connection.set_client_encoding(@config[:encoding])
            else
              execute("SET client_encoding TO '#{@config[:encoding]}'")
            end
          end
          self.client_min_messages = @config[:min_messages] if @config[:min_messages]
          self.schema_search_path = @config[:schema_search_path] || @config[:schema_order]

          # Use standard-conforming strings if available so we don't have to do the E'...' dance.
          set_standard_conforming_strings

          # If using Active Record's time zone support configure the connection to return
          # TIMESTAMP WITH ZONE types in UTC.
          if ActiveRecord::Base.default_timezone == :utc
            execute("SET time zone 'UTC'", 'SCHEMA')
          elsif @local_tz
            execute("SET time zone '#{@local_tz}'", 'SCHEMA')
          end
        end

        # Returns the current ID of a table's sequence.
        def last_insert_id(sequence_name) #:nodoc:
          r = exec_query("SELECT currval($1)", 'SQL', [[nil, sequence_name]])
          Integer(r.rows.first.first)
        end

        # Executes a SELECT query and returns the results, performing any data type
        # conversions that are required to be performed here instead of in PostgreSQLColumn.
        def select(sql, name = nil, binds = [])
          exec_query(sql, name, binds).to_a
        end

        def select_raw(sql, name = nil)
          res = execute(sql, name)
          results = result_as_array(res)
          fields = res.fields
          res.clear
          return fields, results
        end

        # Returns the list of a table's column names, data types, and default values.
        #
        # The underlying query is roughly:
        #  SELECT column.name, column.type, default.value
        #    FROM column LEFT JOIN default
        #      ON column.table_id = default.table_id
        #     AND column.num = default.column_num
        #   WHERE column.table_id = get_table_id('table_name')
        #     AND column.num > 0
        #     AND NOT column.is_dropped
        #   ORDER BY column.num
        #
        # If the table name is not prefixed with a schema, the database will
        # take the first match from the schema search path.
        #
        # Query implementation notes:
        #  - format_type includes the column size constraint, e.g. varchar(50)
        #  - ::regclass is a function that gives the id for a table name
        def column_definitions(table_name) #:nodoc:
          exec_query(<<-end_sql, 'SCHEMA').rows
            SELECT a.attname, format_type(a.atttypid, a.atttypmod), d.adsrc, a.attnotnull
              FROM pg_attribute a LEFT JOIN pg_attrdef d
                ON a.attrelid = d.adrelid AND a.attnum = d.adnum
             WHERE a.attrelid = '#{quote_table_name(table_name)}'::regclass
               AND a.attnum > 0 AND NOT a.attisdropped
             ORDER BY a.attnum
          end_sql
        end

        def extract_pg_identifier_from_name(name)
          match_data = name.start_with?('"') ? name.match(/\"([^\"]+)\"/) : name.match(/([^\.]+)/)

          if match_data
            rest = name[match_data[0].length, name.length]
            rest = rest[1, rest.length] if rest.start_with? "."
            [match_data[1], (rest.length > 0 ? rest : nil)]
          end
        end

      def table_definition
        TableDefinition.new(self)
      end
    end
  end
end