# oracle/base.py
# Copyright (C) 2005-2015 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""
.. dialect:: oracle
    :name: Oracle

    Oracle version 8 through current (11g at the time of this writing) are
    supported.

Connect Arguments
-----------------

The dialect supports several :func:`~sqlalchemy.create_engine()` arguments
which affect the behavior of the dialect regardless of driver in use.

* ``use_ansi`` - Use ANSI JOIN constructs (see the section on Oracle 8).
  Defaults to ``True``.  If ``False``, Oracle-8 compatible constructs are used
  for joins.

* ``optimize_limits`` - defaults to ``False``. see the section on
  LIMIT/OFFSET.

* ``use_binds_for_limits`` - defaults to ``True``.  see the section on
  LIMIT/OFFSET.

Auto Increment Behavior
-----------------------

SQLAlchemy Table objects which include integer primary keys are usually
assumed to have "autoincrementing" behavior, meaning they can generate their
own primary key values upon INSERT.  Since Oracle has no "autoincrement"
feature, SQLAlchemy relies upon sequences to produce these values.   With the
Oracle dialect, *a sequence must always be explicitly specified to enable
autoincrement*.  This is divergent with the majority of documentation
examples which assume the usage of an autoincrement-capable database.   To
specify sequences, use the sqlalchemy.schema.Sequence object which is passed
to a Column construct::

  t = Table('mytable', metadata,
        Column('id', Integer, Sequence('id_seq'), primary_key=True),
        Column(...), ...
  )

This step is also required when using table reflection, i.e. autoload=True::

  t = Table('mytable', metadata,
        Column('id', Integer, Sequence('id_seq'), primary_key=True),
        autoload=True
  )

Identifier Casing
-----------------

In Oracle, the data dictionary represents all case insensitive identifier
names using UPPERCASE text.   SQLAlchemy on the other hand considers an
all-lower case identifier name to be case insensitive.   The Oracle dialect
converts all case insensitive identifiers to and from those two formats during
schema level communication, such as reflection of tables and indexes.   Using
an UPPERCASE name on the SQLAlchemy side indicates a case sensitive
identifier, and SQLAlchemy will quote the name - this will cause mismatches
against data dictionary data received from Oracle, so unless identifier names
have been truly created as case sensitive (i.e. using quoted names), all
lowercase names should be used on the SQLAlchemy side.


LIMIT/OFFSET Support
--------------------

Oracle has no support for the LIMIT or OFFSET keywords.  SQLAlchemy uses
a wrapped subquery approach in conjunction with ROWNUM.  The exact methodology
is taken from
http://www.oracle.com/technology/oramag/oracle/06-sep/o56asktom.html .

There are two options which affect its behavior:

* the "FIRST ROWS()" optimization keyword is not used by default.  To enable
  the usage of this optimization directive, specify ``optimize_limits=True``
  to :func:`.create_engine`.
* the values passed for the limit/offset are sent as bound parameters.   Some
  users have observed that Oracle produces a poor query plan when the values
  are sent as binds and not rendered literally.   To render the limit/offset
  values literally within the SQL statement, specify
  ``use_binds_for_limits=False`` to :func:`.create_engine`.

Some users have reported better performance when the entirely different
approach of a window query is used, i.e. ROW_NUMBER() OVER (ORDER BY), to
provide LIMIT/OFFSET (note that the majority of users don't observe this).
To suit this case the method used for LIMIT/OFFSET can be replaced entirely.
See the recipe at
http://www.sqlalchemy.org/trac/wiki/UsageRecipes/WindowFunctionsByDefault
which installs a select compiler that overrides the generation of limit/offset
with a window function.

.. _oracle_returning:

RETURNING Support
-----------------

The Oracle database supports a limited form of RETURNING, in order to retrieve
result sets of matched rows from INSERT, UPDATE and DELETE statements.
Oracle's RETURNING..INTO syntax only supports one row being returned, as it
relies upon OUT parameters in order to function.  In addition, supported
DBAPIs have further limitations (see :ref:`cx_oracle_returning`).

SQLAlchemy's "implicit returning" feature, which employs RETURNING within an
INSERT and sometimes an UPDATE statement in order to fetch newly generated
primary key values and other SQL defaults and expressions, is normally enabled
on the Oracle backend.  By default, "implicit returning" typically only
fetches the value of a single ``nextval(some_seq)`` expression embedded into
an INSERT in order to increment a sequence within an INSERT statement and get
the value back at the same time. To disable this feature across the board,
specify ``implicit_returning=False`` to :func:`.create_engine`::

    engine = create_engine("oracle://scott:tiger@dsn",
                           implicit_returning=False)

Implicit returning can also be disabled on a table-by-table basis as a table
option::

    # Core Table
    my_table = Table("my_table", metadata, ..., implicit_returning=False)


    # declarative
    class MyClass(Base):
        __tablename__ = 'my_table'
        __table_args__ = {"implicit_returning": False}

.. seealso::

    :ref:`cx_oracle_returning` - additional cx_oracle-specific restrictions on
    implicit returning.

ON UPDATE CASCADE
-----------------

Oracle doesn't have native ON UPDATE CASCADE functionality.  A trigger based
solution is available at
http://asktom.oracle.com/tkyte/update_cascade/index.html .

When using the SQLAlchemy ORM, the ORM has limited ability to manually issue
cascading updates - specify ForeignKey objects using the
"deferrable=True, initially='deferred'" keyword arguments,
and specify "passive_updates=False" on each relationship().

Oracle 8 Compatibility
----------------------

When Oracle 8 is detected, the dialect internally configures itself to the
following behaviors:

* the use_ansi flag is set to False.  This has the effect of converting all
  JOIN phrases into the WHERE clause, and in the case of LEFT OUTER JOIN
  makes use of Oracle's (+) operator.

* the NVARCHAR2 and NCLOB datatypes are no longer generated as DDL when
  the :class:`~sqlalchemy.types.Unicode` is used - VARCHAR2 and CLOB are
  issued instead.   This because these types don't seem to work correctly on
  Oracle 8 even though they are available.  The
  :class:`~sqlalchemy.types.NVARCHAR` and
  :class:`~sqlalchemy.dialects.oracle.NCLOB` types will always generate
  NVARCHAR2 and NCLOB.

* the "native unicode" mode is disabled when using cx_oracle, i.e. SQLAlchemy
  encodes all Python unicode objects to "string" before passing in as bind
  parameters.

Synonym/DBLINK Reflection
-------------------------

When using reflection with Table objects, the dialect can optionally search
for tables indicated by synonyms, either in local or remote schemas or
accessed over DBLINK, by passing the flag ``oracle_resolve_synonyms=True`` as
a keyword argument to the :class:`.Table` construct::

    some_table = Table('some_table', autoload=True,
                                autoload_with=some_engine,
                                oracle_resolve_synonyms=True)

When this flag is set, the given name (such as ``some_table`` above) will
be searched not just in the ``ALL_TABLES`` view, but also within the
``ALL_SYNONYMS`` view to see if this name is actually a synonym to another
name.  If the synonym is located and refers to a DBLINK, the oracle dialect
knows how to locate the table's information using DBLINK syntax(e.g.
``@dblink``).

``oracle_resolve_synonyms`` is accepted wherever reflection arguments are
accepted, including methods such as :meth:`.MetaData.reflect` and
:meth:`.Inspector.get_columns`.

If synonyms are not in use, this flag should be left disabled.

DateTime Compatibility
----------------------

Oracle has no datatype known as ``DATETIME``, it instead has only ``DATE``,
which can actually store a date and time value.  For this reason, the Oracle
dialect provides a type :class:`.oracle.DATE` which is a subclass of
:class:`.DateTime`.   This type has no special behavior, and is only
present as a "marker" for this type; additionally, when a database column
is reflected and the type is reported as ``DATE``, the time-supporting
:class:`.oracle.DATE` type is used.

.. versionchanged:: 0.9.4 Added :class:`.oracle.DATE` to subclass
   :class:`.DateTime`.  This is a change as previous versions
   would reflect a ``DATE`` column as :class:`.types.DATE`, which subclasses
   :class:`.Date`.   The only significance here is for schemes that are
   examining the type of column for use in special Python translations or
   for migrating schemas to other database backends.

.. _oracle_table_options:

Oracle Table Options
-------------------------

The CREATE TABLE phrase supports the following options with Oracle
in conjunction with the :class:`.Table` construct:


* ``ON COMMIT``::

    Table(
        "some_table", metadata, ...,
        prefixes=['GLOBAL TEMPORARY'], oracle_on_commit='PRESERVE ROWS')

.. versionadded:: 1.0.0

* ``COMPRESS``::

    Table('mytable', metadata, Column('data', String(32)),
        oracle_compress=True)

    Table('mytable', metadata, Column('data', String(32)),
        oracle_compress=6)

   The ``oracle_compress`` parameter accepts either an integer compression
   level, or ``True`` to use the default compression level.

.. versionadded:: 1.0.0

.. _oracle_index_options:

Oracle Specific Index Options
-----------------------------

Bitmap Indexes
~~~~~~~~~~~~~~

You can specify the ``oracle_bitmap`` parameter to create a bitmap index
instead of a B-tree index::

    Index('my_index', my_table.c.data, oracle_bitmap=True)

Bitmap indexes cannot be unique and cannot be compressed. SQLAlchemy will not
check for such limitations, only the database will.

.. versionadded:: 1.0.0

Index compression
~~~~~~~~~~~~~~~~~

Oracle has a more efficient storage mode for indexes containing lots of
repeated values. Use the ``oracle_compress`` parameter to turn on key c
ompression::

    Index('my_index', my_table.c.data, oracle_compress=True)

    Index('my_index', my_table.c.data1, my_table.c.data2, unique=True,
           oracle_compress=1)

The ``oracle_compress`` parameter accepts either an integer specifying the
number of prefix columns to compress, or ``True`` to use the default (all
columns for non-unique indexes, all but the last column for unique indexes).

.. versionadded:: 1.0.0

"""

import re

from sqlalchemy import util, sql
from sqlalchemy.engine import default, reflection
from sqlalchemy.sql import compiler, visitors, expression
from sqlalchemy.sql import operators as sql_operators
from sqlalchemy import types as sqltypes, schema as sa_schema
from sqlalchemy.types import VARCHAR, NVARCHAR, CHAR, \
    BLOB, CLOB, TIMESTAMP, FLOAT

RESERVED_WORDS = \
    set('SHARE RAW DROP BETWEEN FROM DESC OPTION PRIOR LONG THEN '
        'DEFAULT ALTER IS INTO MINUS INTEGER NUMBER GRANT IDENTIFIED '
        'ALL TO ORDER ON FLOAT DATE HAVING CLUSTER NOWAIT RESOURCE '
        'ANY TABLE INDEX FOR UPDATE WHERE CHECK SMALLINT WITH DELETE '
        'BY ASC REVOKE LIKE SIZE RENAME NOCOMPRESS NULL GROUP VALUES '
        'AS IN VIEW EXCLUSIVE COMPRESS SYNONYM SELECT INSERT EXISTS '
        'NOT TRIGGER ELSE CREATE INTERSECT PCTFREE DISTINCT USER '
        'CONNECT SET MODE OF UNIQUE VARCHAR2 VARCHAR LOCK OR CHAR '
        'DECIMAL UNION PUBLIC AND START UID COMMENT CURRENT LEVEL'.split())

NO_ARG_FNS = set('UID CURRENT_DATE SYSDATE USER '
                 'CURRENT_TIME CURRENT_TIMESTAMP'.split())


class RAW(sqltypes._Binary):
    __visit_name__ = 'RAW'
OracleRaw = RAW


class NCLOB(sqltypes.Text):
    __visit_name__ = 'NCLOB'


class VARCHAR2(VARCHAR):
    __visit_name__ = 'VARCHAR2'

NVARCHAR2 = NVARCHAR


class NUMBER(sqltypes.Numeric, sqltypes.Integer):
    __visit_name__ = 'NUMBER'

    def __init__(self, precision=None, scale=None, asdecimal=None):
        if asdecimal is None:
            asdecimal = bool(scale and scale > 0)

        super(NUMBER, self).__init__(
            precision=precision, scale=scale, asdecimal=asdecimal)

    def adapt(self, impltype):
        ret = super(NUMBER, self).adapt(impltype)
        # leave a hint for the DBAPI handler
        ret._is_oracle_number = True
        return ret

    @property
    def _type_affinity(self):
        if bool(self.scale and self.scale > 0):
            return sqltypes.Numeric
        else:
            return sqltypes.Integer


class DOUBLE_PRECISION(sqltypes.Numeric):
    __visit_name__ = 'DOUBLE_PRECISION'

    def __init__(self, precision=None, scale=None, asdecimal=None):
        if asdecimal is None:
            asdecimal = False

        super(DOUBLE_PRECISION, self).__init__(
            precision=precision, scale=scale, asdecimal=asdecimal)


class BFILE(sqltypes.LargeBinary):
    __visit_name__ = 'BFILE'


class LONG(sqltypes.Text):
    __visit_name__ = 'LONG'


class DATE(sqltypes.DateTime):
    """Provide the oracle DATE type.

    This type has no special Python behavior, except that it subclasses
    :class:`.types.DateTime`; this is to suit the fact that the Oracle
    ``DATE`` type supports a time value.

    .. versionadded:: 0.9.4

    """
    __visit_name__ = 'DATE'

    def _compare_type_affinity(self, other):
        return other._type_affinity in (sqltypes.DateTime, sqltypes.Date)


class INTERVAL(sqltypes.TypeEngine):
    __visit_name__ = 'INTERVAL'

    def __init__(self,
                 day_precision=None,
                 second_precision=None):
        """Construct an INTERVAL.

        Note that only DAY TO SECOND intervals are currently supported.
        This is due to a lack of support for YEAR TO MONTH intervals
        within available DBAPIs (cx_oracle and zxjdbc).

        :param day_precision: the day precision value.  this is the number of
          digits to store for the day field.  Defaults to "2"
        :param second_precision: the second precision value.  this is the
          number of digits to store for the fractional seconds field.
          Defaults to "6".

        """
        self.day_precision = day_precision
        self.second_precision = second_precision

    @classmethod
    def _adapt_from_generic_interval(cls, interval):
        return INTERVAL(day_precision=interval.day_precision,
                        second_precision=interval.second_precision)

    @property
    def _type_affinity(self):
        return sqltypes.Interval


class ROWID(sqltypes.TypeEngine):
    """Oracle ROWID type.

    When used in a cast() or similar, generates ROWID.

    """
    __visit_name__ = 'ROWID'


class _OracleBoolean(sqltypes.Boolean):
    def get_dbapi_type(self, dbapi):
        return dbapi.NUMBER

colspecs = {
    sqltypes.Boolean: _OracleBoolean,
    sqltypes.Interval: INTERVAL,
    sqltypes.DateTime: DATE
}

ischema_names = {
    'VARCHAR2': VARCHAR,
    'NVARCHAR2': NVARCHAR,
    'CHAR': CHAR,
    'DATE': DATE,
    'NUMBER': NUMBER,
    'BLOB': BLOB,
    'BFILE': BFILE,
    'CLOB': CLOB,
    'NCLOB': NCLOB,
    'TIMESTAMP': TIMESTAMP,
    'TIMESTAMP WITH TIME ZONE': TIMESTAMP,
    'INTERVAL DAY TO SECOND': INTERVAL,
    'RAW': RAW,
    'FLOAT': FLOAT,
    'DOUBLE PRECISION': DOUBLE_PRECISION,
    'LONG': LONG,
}


class OracleTypeCompiler(compiler.GenericTypeCompiler):
    # Note:
    # Oracle DATE == DATETIME
    # Oracle does not allow milliseconds in DATE
    # Oracle does not support TIME columns

    def visit_datetime(self, type_, **kw):
        return self.visit_DATE(type_, **kw)

    def visit_float(self, type_, **kw):
        return self.visit_FLOAT(type_, **kw)

    def visit_unicode(self, type_, **kw):
        if self.dialect._supports_nchar:
            return self.visit_NVARCHAR2(type_, **kw)
        else:
            return self.visit_VARCHAR2(type_, **kw)

    def visit_INTERVAL(self, type_, **kw):
        return "INTERVAL DAY%s TO SECOND%s" % (
            type_.day_precision is not None and
            "(%d)" % type_.day_precision or
            "",
            type_.second_precision is not None and
            "(%d)" % type_.second_precision or
            "",
        )

    def visit_LONG(self, type_, **kw):
        return "LONG"

    def visit_TIMESTAMP(self, type_, **kw):
        if type_.timezone:
            return "TIMESTAMP WITH TIME ZONE"
        else:
            return "TIMESTAMP"

    def visit_DOUBLE_PRECISION(self, type_, **kw):
        return self._generate_numeric(type_, "DOUBLE PRECISION", **kw)

    def visit_NUMBER(self, type_, **kw):
        return self._generate_numeric(type_, "NUMBER", **kw)

    def _generate_numeric(self, type_, name, precision=None, scale=None, **kw):
        if precision is None:
            precision = type_.precision

        if scale is None:
            scale = getattr(type_, 'scale', None)

        if precision is None:
            return name
        elif scale is None:
            n = "%(name)s(%(precision)s)"
            return n % {'name': name, 'precision': precision}
        else:
            n = "%(name)s(%(precision)s, %(scale)s)"
            return n % {'name': name, 'precision': precision, 'scale': scale}

    def visit_string(self, type_, **kw):
        return self.visit_VARCHAR2(type_, **kw)

    def visit_VARCHAR2(self, type_, **kw):
        return self._visit_varchar(type_, '', '2')

    def visit_NVARCHAR2(self, type_, **kw):
        return self._visit_varchar(type_, 'N', '2')
    visit_NVARCHAR = visit_NVARCHAR2

    def visit_VARCHAR(self, type_, **kw):
        return self._visit_varchar(type_, '', '')

    def _visit_varchar(self, type_, n, num):
        if not type_.length:
            return "%(n)sVARCHAR%(two)s" % {'two': num, 'n': n}
        elif not n and self.dialect._supports_char_length:
            varchar = "VARCHAR%(two)s(%(length)s CHAR)"
            return varchar % {'length': type_.length, 'two': num}
        else:
            varchar = "%(n)sVARCHAR%(two)s(%(length)s)"
            return varchar % {'length': type_.length, 'two': num, 'n': n}

    def visit_text(self, type_, **kw):
        return self.visit_CLOB(type_, **kw)

    def visit_unicode_text(self, type_, **kw):
        if self.dialect._supports_nchar:
            return self.visit_NCLOB(type_, **kw)
        else:
            return self.visit_CLOB(type_, **kw)

    def visit_large_binary(self, type_, **kw):
        return self.visit_BLOB(type_, **kw)

    def visit_big_integer(self, type_, **kw):
        return self.visit_NUMBER(type_, precision=19, **kw)

    def visit_boolean(self, type_, **kw):
        return self.visit_SMALLINT(type_, **kw)

    def visit_RAW(self, type_, **kw):
        if type_.length:
            return "RAW(%(length)s)" % {'length': type_.length}
        else:
            return "RAW"

    def visit_ROWID(self, type_, **kw):
        return "ROWID"


class OracleCompiler(compiler.SQLCompiler):
    """Oracle compiler modifies the lexical structure of Select
    statements to work under non-ANSI configured Oracle databases, if
    the use_ansi flag is False.
    """

    compound_keywords = util.update_copy(
        compiler.SQLCompiler.compound_keywords,
        {
            expression.CompoundSelect.EXCEPT: 'MINUS'
        }
    )

    def __init__(self, *args, **kwargs):
        self.__wheres = {}
        self._quoted_bind_names = {}
        super(OracleCompiler, self).__init__(*args, **kwargs)

    def visit_mod_binary(self, binary, operator, **kw):
        return "mod(%s, %s)" % (self.process(binary.left, **kw),
                                self.process(binary.right, **kw))

    def visit_now_func(self, fn, **kw):
        return "CURRENT_TIMESTAMP"

    def visit_char_length_func(self, fn, **kw):
        return "LENGTH" + self.function_argspec(fn, **kw)

    def visit_match_op_binary(self, binary, operator, **kw):
        return "CONTAINS (%s, %s)" % (self.process(binary.left),
                                      self.process(binary.right))

    def visit_true(self, expr, **kw):
        return '1'

    def visit_false(self, expr, **kw):
        return '0'

    def get_cte_preamble(self, recursive):
        return "WITH"

    def get_select_hint_text(self, byfroms):
        return " ".join(
            "/*+ %s */" % text for table, text in byfroms.items()
        )

    def function_argspec(self, fn, **kw):
        if len(fn.clauses) > 0 or fn.name.upper() not in NO_ARG_FNS:
            return compiler.SQLCompiler.function_argspec(self, fn, **kw)
        else:
            return ""

    def default_from(self):
        """Called when a ``SELECT`` statement has no froms,
        and no ``FROM`` clause is to be appended.

        The Oracle compiler tacks a "FROM DUAL" to the statement.
        """

        return " FROM DUAL"

    def visit_join(self, join, **kwargs):
        if self.dialect.use_ansi:
            return compiler.SQLCompiler.visit_join(self, join, **kwargs)
        else:
            kwargs['asfrom'] = True
            if isinstance(join.right, expression.FromGrouping):
                right = join.right.element
            else:
                right = join.right
            return self.process(join.left, **kwargs) + \
                ", " + self.process(right, **kwargs)

    def _get_nonansi_join_whereclause(self, froms):
        clauses = []

        def visit_join(join):
            if join.isouter:
                def visit_binary(binary):
                    if binary.operator == sql_operators.eq:
                        if join.right.is_derived_from(binary.left.table):
                            binary.left = _OuterJoinColumn(binary.left)
                        elif join.right.is_derived_from(binary.right.table):
                            binary.right = _OuterJoinColumn(binary.right)
                clauses.append(visitors.cloned_traverse(
                    join.onclause, {}, {'binary': visit_binary}))
            else:
                clauses.append(join.onclause)

            for j in join.left, join.right:
                if isinstance(j, expression.Join):
                    visit_join(j)
                elif isinstance(j, expression.FromGrouping):
                    visit_join(j.element)

        for f in froms:
            if isinstance(f, expression.Join):
                visit_join(f)

        if not clauses:
            return None
        else:
            return sql.and_(*clauses)

    def visit_outer_join_column(self, vc, **kw):
        return self.process(vc.column, **kw) + "(+)"

    def visit_sequence(self, seq):
        return (self.dialect.identifier_preparer.format_sequence(seq) +
                ".nextval")

    def get_render_as_alias_suffix(self, alias_name_text):
        """Oracle doesn't like ``FROM table AS alias``"""

        return " " + alias_name_text

    def returning_clause(self, stmt, returning_cols):
        columns = []
        binds = []
        for i, column in enumerate(
                expression._select_iterables(returning_cols)):
            if column.type._has_column_expression:
                col_expr = column.type.column_expression(column)
            else:
                col_expr = column
            outparam = sql.outparam("ret_%d" % i, type_=column.type)
            self.binds[outparam.key] = outparam
            binds.append(
                self.bindparam_string(self._truncate_bindparam(outparam)))
            columns.append(
                self.process(col_expr, within_columns_clause=False))

            self._add_to_result_map(
                outparam.key, outparam.key,
                (column, getattr(column, 'name', None),
                 getattr(column, 'key', None)),
                column.type
            )

        return 'RETURNING ' + ', '.join(columns) + " INTO " + ", ".join(binds)

    def _TODO_visit_compound_select(self, select):
        """Need to determine how to get ``LIMIT``/``OFFSET`` into a
        ``UNION`` for Oracle.
        """
        pass

    def visit_select(self, select, **kwargs):
        """Look for ``LIMIT`` and OFFSET in a select statement, and if
        so tries to wrap it in a subquery with ``rownum`` criterion.
        """

        if not getattr(select, '_oracle_visit', None):
            if not self.dialect.use_ansi:
                froms = self._display_froms_for_select(
                    select, kwargs.get('asfrom', False))
                whereclause = self._get_nonansi_join_whereclause(froms)
                if whereclause is not None:
                    select = select.where(whereclause)
                    select._oracle_visit = True

            limit_clause = select._limit_clause
            offset_clause = select._offset_clause
            if limit_clause is not None or offset_clause is not None:
                # See http://www.oracle.com/technology/oramag/oracle/06-sep/\
                # o56asktom.html
                #
                # Generalized form of an Oracle pagination query:
                #   select ... from (
                #     select /*+ FIRST_ROWS(N) */ ...., rownum as ora_rn from
                #       (  select distinct ... where ... order by ...
                #     ) where ROWNUM <= :limit+:offset
                #   ) where ora_rn > :offset
                # Outer select and "ROWNUM as ora_rn" can be dropped if
                # limit=0

                kwargs['select_wraps_for'] = select
                select = select._generate()
                select._oracle_visit = True

                # Wrap the middle select and add the hint
                limitselect = sql.select([c for c in select.c])
                if limit_clause is not None and \
                    self.dialect.optimize_limits and \
                        select._simple_int_limit:
                    limitselect = limitselect.prefix_with(
                        "/*+ FIRST_ROWS(%d) */" %
                        select._limit)

                limitselect._oracle_visit = True
                limitselect._is_wrapper = True

                # If needed, add the limiting clause
                if limit_clause is not None:
                    if not self.dialect.use_binds_for_limits:
                        # use simple int limits, will raise an exception
                        # if the limit isn't specified this way
                        max_row = select._limit

                        if offset_clause is not None:
                            max_row += select._offset
                        max_row = sql.literal_column("%d" % max_row)
                    else:
                        max_row = limit_clause
                        if offset_clause is not None:
                            max_row = max_row + offset_clause
                    limitselect.append_whereclause(
                        sql.literal_column("ROWNUM") <= max_row)

                # If needed, add the ora_rn, and wrap again with offset.
                if offset_clause is None:
                    limitselect._for_update_arg = select._for_update_arg
                    select = limitselect
                else:
                    limitselect = limitselect.column(
                        sql.literal_column("ROWNUM").label("ora_rn"))
                    limitselect._oracle_visit = True
                    limitselect._is_wrapper = True

                    offsetselect = sql.select(
                        [c for c in limitselect.c if c.key != 'ora_rn'])
                    offsetselect._oracle_visit = True
                    offsetselect._is_wrapper = True

                    if not self.dialect.use_binds_for_limits:
                        offset_clause = sql.literal_column(
                            "%d" % select._offset)
                    offsetselect.append_whereclause(
                        sql.literal_column("ora_rn") > offset_clause)

                    offsetselect._for_update_arg = select._for_update_arg
                    select = offsetselect

        return compiler.SQLCompiler.visit_select(self, select, **kwargs)

    def limit_clause(self, select, **kw):
        return ""

    def for_update_clause(self, select, **kw):
        if self.is_subquery():
            return ""

        tmp = ' FOR UPDATE'

        if select._for_update_arg.of:
            tmp += ' OF ' + ', '.join(
                self.process(elem, **kw) for elem in
                select._for_update_arg.of
            )

        if select._for_update_arg.nowait:
            tmp += " NOWAIT"

        return tmp


class OracleDDLCompiler(compiler.DDLCompiler):

    def define_constraint_cascades(self, constraint):
        text = ""
        if constraint.ondelete is not None:
            text += " ON DELETE %s" % constraint.ondelete

        # oracle has no ON UPDATE CASCADE -
        # its only available via triggers
        # http://asktom.oracle.com/tkyte/update_cascade/index.html
        if constraint.onupdate is not None:
            util.warn(
                "Oracle does not contain native UPDATE CASCADE "
                "functionality - onupdates will not be rendered for foreign "
                "keys.  Consider using deferrable=True, initially='deferred' "
                "or triggers.")

        return text

    def visit_create_index(self, create):
        index = create.element
        self._verify_index_table(index)
        preparer = self.preparer
        text = "CREATE "
        if index.unique:
            text += "UNIQUE "
        if index.dialect_options['oracle']['bitmap']:
            text += "BITMAP "
        text += "INDEX %s ON %s (%s)" % (
            self._prepared_index_name(index, include_schema=True),
            preparer.format_table(index.table, use_schema=True),
            ', '.join(
                self.sql_compiler.process(
                    expr,
                    include_table=False, literal_binds=True)
                for expr in index.expressions)
        )
        if index.dialect_options['oracle']['compress'] is not False:
            if index.dialect_options['oracle']['compress'] is True:
                text += " COMPRESS"
            else:
                text += " COMPRESS %d" % (
                    index.dialect_options['oracle']['compress']
                )
        return text

    def post_create_table(self, table):
        table_opts = []
        opts = table.dialect_options['oracle']

        if opts['on_commit']:
            on_commit_options = opts['on_commit'].replace("_", " ").upper()
            table_opts.append('\n ON COMMIT %s' % on_commit_options)

        if opts['compress']:
            if opts['compress'] is True:
                table_opts.append("\n COMPRESS")
            else:
                table_opts.append("\n COMPRESS FOR %s" % (
                    opts['compress']
                ))

        return ''.join(table_opts)


class OracleIdentifierPreparer(compiler.IdentifierPreparer):

    reserved_words = set([x.lower() for x in RESERVED_WORDS])
    illegal_initial_characters = set(
        (str(dig) for dig in range(0, 10))).union(["_", "$"])

    def _bindparam_requires_quotes(self, value):
        """Return True if the given identifier requires quoting."""
        lc_value = value.lower()
        return (lc_value in self.reserved_words
                or value[0] in self.illegal_initial_characters
                or not self.legal_characters.match(util.text_type(value))
                )

    def format_savepoint(self, savepoint):
        name = re.sub(r'^_+', '', savepoint.ident)
        return super(
            OracleIdentifierPreparer, self).format_savepoint(savepoint, name)


class OracleExecutionContext(default.DefaultExecutionContext):
    def fire_sequence(self, seq, type_):
        return self._execute_scalar(
            "SELECT " +
            self.dialect.identifier_preparer.format_sequence(seq) +
            ".nextval FROM DUAL", type_)


class OracleDialect(default.DefaultDialect):
    name = 'oracle'
    supports_alter = True
    supports_unicode_statements = False
    supports_unicode_binds = False
    max_identifier_length = 30
    supports_sane_rowcount = True
    supports_sane_multi_rowcount = False

    supports_sequences = True
    sequences_optional = False
    postfetch_lastrowid = False

    default_paramstyle = 'named'
    colspecs = colspecs
    ischema_names = ischema_names
    requires_name_normalize = True

    supports_default_values = False
    supports_empty_insert = False

    statement_compiler = OracleCompiler
    ddl_compiler = OracleDDLCompiler
    type_compiler = OracleTypeCompiler
    preparer = OracleIdentifierPreparer
    execution_ctx_cls = OracleExecutionContext

    reflection_options = ('oracle_resolve_synonyms', )

    construct_arguments = [
        (sa_schema.Table, {
            "resolve_synonyms": False,
            "on_commit": None,
            "compress": False
        }),
        (sa_schema.Index, {
            "bitmap": False,
            "compress": False
        })
    ]

    def __init__(self,
                 use_ansi=True,
                 optimize_limits=False,
                 use_binds_for_limits=True,
                 **kwargs):
        default.DefaultDialect.__init__(self, **kwargs)
        self.use_ansi = use_ansi
        self.optimize_limits = optimize_limits
        self.use_binds_for_limits = use_binds_for_limits

    def initialize(self, connection):
        super(OracleDialect, self).initialize(connection)
        self.implicit_returning = self.__dict__.get(
            'implicit_returning',
            self.server_version_info > (10, )
        )

        if self._is_oracle_8:
            self.colspecs = self.colspecs.copy()
            self.colspecs.pop(sqltypes.Interval)
            self.use_ansi = False

    @property
    def _is_oracle_8(self):
        return self.server_version_info and \
            self.server_version_info < (9, )

    @property
    def _supports_table_compression(self):
        return self.server_version_info and \
            self.server_version_info >= (9, 2, )

    @property
    def _supports_table_compress_for(self):
        return self.server_version_info and \
            self.server_version_info >= (11, )

    @property
    def _supports_char_length(self):
        return not self._is_oracle_8

    @property
    def _supports_nchar(self):
        return not self._is_oracle_8

    def do_release_savepoint(self, connection, name):
        # Oracle does not support RELEASE SAVEPOINT
        pass

    def has_table(self, connection, table_name, schema=None):
        if not schema:
            schema = self.default_schema_name
        cursor = connection.execute(
            sql.text("SELECT table_name FROM all_tables "
                     "WHERE table_name = :name AND owner = :schema_name"),
            name=self.denormalize_name(table_name),
            schema_name=self.denormalize_name(schema))
        return cursor.first() is not None

    def has_sequence(self, connection, sequence_name, schema=None):
        if not schema:
            schema = self.default_schema_name
        cursor = connection.execute(
            sql.text("SELECT sequence_name FROM all_sequences "
                     "WHERE sequence_name = :name AND "
                     "sequence_owner = :schema_name"),
            name=self.denormalize_name(sequence_name),
            schema_name=self.denormalize_name(schema))
        return cursor.first() is not None

    def normalize_name(self, name):
        if name is None:
            return None
        if util.py2k:
            if isinstance(name, str):
                name = name.decode(self.encoding)
        if name.upper() == name and not \
                self.identifier_preparer._requires_quotes(name.lower()):
            return name.lower()
        else:
            return name

    def denormalize_name(self, name):
        if name is None:
            return None
        elif name.lower() == name and not \
                self.identifier_preparer._requires_quotes(name.lower()):
            name = name.upper()
        if util.py2k:
            if not self.supports_unicode_binds:
                name = name.encode(self.encoding)
            else:
                name = unicode(name)
        return name

    def _get_default_schema_name(self, connection):
        return self.normalize_name(
            connection.execute('SELECT USER FROM DUAL').scalar())

    def _resolve_synonym(self, connection, desired_owner=None,
                         desired_synonym=None, desired_table=None):
        """search for a local synonym matching the given desired owner/name.

        if desired_owner is None, attempts to locate a distinct owner.

        returns the actual name, owner, dblink name, and synonym name if
        found.
        """

        q = "SELECT owner, table_owner, table_name, db_link, "\
            "synonym_name FROM all_synonyms WHERE "
        clauses = []
        params = {}
        if desired_synonym:
            clauses.append("synonym_name = :synonym_name")
            params['synonym_name'] = desired_synonym
        if desired_owner:
            clauses.append("owner = :desired_owner")
            params['desired_owner'] = desired_owner
        if desired_table:
            clauses.append("table_name = :tname")
            params['tname'] = desired_table

        q += " AND ".join(clauses)

        result = connection.execute(sql.text(q), **params)
        if desired_owner:
            row = result.first()
            if row:
                return (row['table_name'], row['table_owner'],
                        row['db_link'], row['synonym_name'])
            else:
                return None, None, None, None
        else:
            rows = result.fetchall()
            if len(rows) > 1:
                raise AssertionError(
                    "There are multiple tables visible to the schema, you "
                    "must specify owner")
            elif len(rows) == 1:
                row = rows[0]
                return (row['table_name'], row['table_owner'],
                        row['db_link'], row['synonym_name'])
            else:
                return None, None, None, None

    @reflection.cache
    def _prepare_reflection_args(self, connection, table_name, schema=None,
                                 resolve_synonyms=False, dblink='', **kw):

        if resolve_synonyms:
            actual_name, owner, dblink, synonym = self._resolve_synonym(
                connection,
                desired_owner=self.denormalize_name(schema),
                desired_synonym=self.denormalize_name(table_name)
            )
        else:
            actual_name, owner, dblink, synonym = None, None, None, None
        if not actual_name:
            actual_name = self.denormalize_name(table_name)

        if dblink:
            # using user_db_links here since all_db_links appears
            # to have more restricted permissions.
            # http://docs.oracle.com/cd/B28359_01/server.111/b28310/ds_admin005.htm
            # will need to hear from more users if we are doing
            # the right thing here.  See [ticket:2619]
            owner = connection.scalar(
                sql.text("SELECT username FROM user_db_links "
                         "WHERE db_link=:link"), link=dblink)
            dblink = "@" + dblink
        elif not owner:
            owner = self.denormalize_name(schema or self.default_schema_name)

        return (actual_name, owner, dblink or '', synonym)

    @reflection.cache
    def get_schema_names(self, connection, **kw):
        s = "SELECT username FROM all_users ORDER BY username"
        cursor = connection.execute(s,)
        return [self.normalize_name(row[0]) for row in cursor]

    @reflection.cache
    def get_table_names(self, connection, schema=None, **kw):
        schema = self.denormalize_name(schema or self.default_schema_name)

        # note that table_names() isn't loading DBLINKed or synonym'ed tables
        if schema is None:
            schema = self.default_schema_name
        s = sql.text(
            "SELECT table_name FROM all_tables "
            "WHERE nvl(tablespace_name, 'no tablespace') NOT IN "
            "('SYSTEM', 'SYSAUX') "
            "AND OWNER = :owner "
            "AND IOT_NAME IS NULL "
            "AND DURATION IS NULL")
        cursor = connection.execute(s, owner=schema)
        return [self.normalize_name(row[0]) for row in cursor]

    @reflection.cache
    def get_temp_table_names(self, connection, **kw):
        schema = self.denormalize_name(self.default_schema_name)
        s = sql.text(
            "SELECT table_name FROM all_tables "
            "WHERE nvl(tablespace_name, 'no tablespace') NOT IN "
            "('SYSTEM', 'SYSAUX') "
            "AND OWNER = :owner "
            "AND IOT_NAME IS NULL "
            "AND DURATION IS NOT NULL")
        cursor = connection.execute(s, owner=schema)
        return [self.normalize_name(row[0]) for row in cursor]

    @reflection.cache
    def get_view_names(self, connection, schema=None, **kw):
        schema = self.denormalize_name(schema or self.default_schema_name)
        s = sql.text("SELECT view_name FROM all_views WHERE owner = :owner")
        cursor = connection.execute(s, owner=self.denormalize_name(schema))
        return [self.normalize_name(row[0]) for row in cursor]

    @reflection.cache
    def get_table_options(self, connection, table_name, schema=None, **kw):
        options = {}

        resolve_synonyms = kw.get('oracle_resolve_synonyms', False)
        dblink = kw.get('dblink', '')
        info_cache = kw.get('info_cache')

        (table_name, schema, dblink, synonym) = \
            self._prepare_reflection_args(connection, table_name, schema,
                                          resolve_synonyms, dblink,
                                          info_cache=info_cache)

        params = {"table_name": table_name}

        columns = ["table_name"]
        if self._supports_table_compression:
            columns.append("compression")
        if self._supports_table_compress_for:
            columns.append("compress_for")

        text = "SELECT %(columns)s "\
            "FROM ALL_TABLES%(dblink)s "\
            "WHERE table_name = :table_name"

        if schema is not None:
            params['owner'] = schema
            text += " AND owner = :owner "
        text = text % {'dblink': dblink, 'columns': ", ".join(columns)}

        result = connection.execute(sql.text(text), **params)

        enabled = dict(DISABLED=False, ENABLED=True)

        row = result.first()
        if row:
            if "compression" in row and enabled.get(row.compression, False):
                if "compress_for" in row:
                    options['oracle_compress'] = row.compress_for
                else:
                    options['oracle_compress'] = True

        return options

    @reflection.cache
    def get_columns(self, connection, table_name, schema=None, **kw):
        """

        kw arguments can be:

            oracle_resolve_synonyms

            dblink

        """

        resolve_synonyms = kw.get('oracle_resolve_synonyms', False)
        dblink = kw.get('dblink', '')
        info_cache = kw.get('info_cache')

        (table_name, schema, dblink, synonym) = \
            self._prepare_reflection_args(connection, table_name, schema,
                                          resolve_synonyms, dblink,
                                          info_cache=info_cache)
        columns = []
        if self._supports_char_length:
            char_length_col = 'char_length'
        else:
            char_length_col = 'data_length'

        params = {"table_name": table_name}
        text = "SELECT column_name, data_type, %(char_length_col)s, "\
            "data_precision, data_scale, "\
            "nullable, data_default FROM ALL_TAB_COLUMNS%(dblink)s "\
            "WHERE table_name = :table_name"
        if schema is not None:
            params['owner'] = schema
            text += " AND owner = :owner "
        text += " ORDER BY column_id"
        text = text % {'dblink': dblink, 'char_length_col': char_length_col}

        c = connection.execute(sql.text(text), **params)

        for row in c:
            (colname, orig_colname, coltype, length, precision, scale, nullable, default) = \
                (self.normalize_name(row[0]), row[0], row[1], row[
                 2], row[3], row[4], row[5] == 'Y', row[6])

            if coltype == 'NUMBER':
                coltype = NUMBER(precision, scale)
            elif coltype in ('VARCHAR2', 'NVARCHAR2', 'CHAR'):
                coltype = self.ischema_names.get(coltype)(length)
            elif 'WITH TIME ZONE' in coltype:
                coltype = TIMESTAMP(timezone=True)
            else:
                coltype = re.sub(r'\(\d+\)', '', coltype)
                try:
                    coltype = self.ischema_names[coltype]
                except KeyError:
                    util.warn("Did not recognize type '%s' of column '%s'" %
                              (coltype, colname))
                    coltype = sqltypes.NULLTYPE

            cdict = {
                'name': colname,
                'type': coltype,
                'nullable': nullable,
                'default': default,
                'autoincrement': default is None
            }
            if orig_colname.lower() == orig_colname:
                cdict['quote'] = True

            columns.append(cdict)
        return columns

    @reflection.cache
    def get_indexes(self, connection, table_name, schema=None,
                    resolve_synonyms=False, dblink='', **kw):

        info_cache = kw.get('info_cache')
        (table_name, schema, dblink, synonym) = \
            self._prepare_reflection_args(connection, table_name, schema,
                                          resolve_synonyms, dblink,
                                          info_cache=info_cache)
        indexes = []

        params = {'table_name': table_name}
        text = \
            "SELECT a.index_name, a.column_name, "\
            "\nb.index_type, b.uniqueness, b.compression, b.prefix_length "\
            "\nFROM ALL_IND_COLUMNS%(dblink)s a, "\
            "\nALL_INDEXES%(dblink)s b "\
            "\nWHERE "\
            "\na.index_name = b.index_name "\
            "\nAND a.table_owner = b.table_owner "\
            "\nAND a.table_name = b.table_name "\
            "\nAND a.table_name = :table_name "

        if schema is not None:
            params['schema'] = schema
            text += "AND a.table_owner = :schema "

        text += "ORDER BY a.index_name, a.column_position"

        text = text % {'dblink': dblink}

        q = sql.text(text)
        rp = connection.execute(q, **params)
        indexes = []
        last_index_name = None
        pk_constraint = self.get_pk_constraint(
            connection, table_name, schema, resolve_synonyms=resolve_synonyms,
            dblink=dblink, info_cache=kw.get('info_cache'))
        pkeys = pk_constraint['constrained_columns']
        uniqueness = dict(NONUNIQUE=False, UNIQUE=True)
        enabled = dict(DISABLED=False, ENABLED=True)

        oracle_sys_col = re.compile(r'SYS_NC\d+\$', re.IGNORECASE)

        def upper_name_set(names):
            return set([i.upper() for i in names])

        pk_names = upper_name_set(pkeys)

        def remove_if_primary_key(index):
            # don't include the primary key index
            if index is not None and \
               upper_name_set(index['column_names']) == pk_names:
                indexes.pop()

        index = None
        for rset in rp:
            if rset.index_name != last_index_name:
                remove_if_primary_key(index)
                index = dict(name=self.normalize_name(rset.index_name),
                             column_names=[], dialect_options={})
                indexes.append(index)
            index['unique'] = uniqueness.get(rset.uniqueness, False)

            if rset.index_type in ('BITMAP', 'FUNCTION-BASED BITMAP'):
                index['dialect_options']['oracle_bitmap'] = True
            if enabled.get(rset.compression, False):
                index['dialect_options']['oracle_compress'] = rset.prefix_length

            # filter out Oracle SYS_NC names.  could also do an outer join
            # to the all_tab_columns table and check for real col names there.
            if not oracle_sys_col.match(rset.column_name):
                index['column_names'].append(
                    self.normalize_name(rset.column_name))
            last_index_name = rset.index_name
        remove_if_primary_key(index)
        return indexes

    @reflection.cache
    def _get_constraint_data(self, connection, table_name, schema=None,
                             dblink='', **kw):

        params = {'table_name': table_name}

        text = \
            "SELECT"\
            "\nac.constraint_name,"\
            "\nac.constraint_type,"\
            "\nloc.column_name AS local_column,"\
            "\nrem.table_name AS remote_table,"\
            "\nrem.column_name AS remote_column,"\
            "\nrem.owner AS remote_owner,"\
            "\nloc.position as loc_pos,"\
            "\nrem.position as rem_pos"\
            "\nFROM all_constraints%(dblink)s ac,"\
            "\nall_cons_columns%(dblink)s loc,"\
            "\nall_cons_columns%(dblink)s rem"\
            "\nWHERE ac.table_name = :table_name"\
            "\nAND ac.constraint_type IN ('R','P')"

        if schema is not None:
            params['owner'] = schema
            text += "\nAND ac.owner = :owner"

        text += \
            "\nAND ac.owner = loc.owner"\
            "\nAND ac.constraint_name = loc.constraint_name"\
            "\nAND ac.r_owner = rem.owner(+)"\
            "\nAND ac.r_constraint_name = rem.constraint_name(+)"\
            "\nAND (rem.position IS NULL or loc.position=rem.position)"\
            "\nORDER BY ac.constraint_name, loc.position"

        text = text % {'dblink': dblink}
        rp = connection.execute(sql.text(text), **params)
        constraint_data = rp.fetchall()
        return constraint_data

    @reflection.cache
    def get_pk_constraint(self, connection, table_name, schema=None, **kw):
        resolve_synonyms = kw.get('oracle_resolve_synonyms', False)
        dblink = kw.get('dblink', '')
        info_cache = kw.get('info_cache')

        (table_name, schema, dblink, synonym) = \
            self._prepare_reflection_args(connection, table_name, schema,
                                          resolve_synonyms, dblink,
                                          info_cache=info_cache)
        pkeys = []
        constraint_name = None
        constraint_data = self._get_constraint_data(
            connection, table_name, schema, dblink,
            info_cache=kw.get('info_cache'))

        for row in constraint_data:
            (cons_name, cons_type, local_column, remote_table, remote_column, remote_owner) = \
                row[0:2] + tuple([self.normalize_name(x) for x in row[2:6]])
            if cons_type == 'P':
                if constraint_name is None:
                    constraint_name = self.normalize_name(cons_name)
                pkeys.append(local_column)
        return {'constrained_columns': pkeys, 'name': constraint_name}

    @reflection.cache
    def get_foreign_keys(self, connection, table_name, schema=None, **kw):
        """

        kw arguments can be:

            oracle_resolve_synonyms

            dblink

        """

        requested_schema = schema  # to check later on
        resolve_synonyms = kw.get('oracle_resolve_synonyms', False)
        dblink = kw.get('dblink', '')
        info_cache = kw.get('info_cache')

        (table_name, schema, dblink, synonym) = \
            self._prepare_reflection_args(connection, table_name, schema,
                                          resolve_synonyms, dblink,
                                          info_cache=info_cache)

        constraint_data = self._get_constraint_data(
            connection, table_name, schema, dblink,
            info_cache=kw.get('info_cache'))

        def fkey_rec():
            return {
                'name': None,
                'constrained_columns': [],
                'referred_schema': None,
                'referred_table': None,
                'referred_columns': []
            }

        fkeys = util.defaultdict(fkey_rec)

        for row in constraint_data:
            (cons_name, cons_type, local_column, remote_table, remote_column, remote_owner) = \
                row[0:2] + tuple([self.normalize_name(x) for x in row[2:6]])

            if cons_type == 'R':
                if remote_table is None:
                    # ticket 363
                    util.warn(
                        ("Got 'None' querying 'table_name' from "
                         "all_cons_columns%(dblink)s - does the user have "
                         "proper rights to the table?") % {'dblink': dblink})
                    continue

                rec = fkeys[cons_name]
                rec['name'] = cons_name
                local_cols, remote_cols = rec[
                    'constrained_columns'], rec['referred_columns']

                if not rec['referred_table']:
                    if resolve_synonyms:
                        ref_remote_name, ref_remote_owner, ref_dblink, ref_synonym = \
                            self._resolve_synonym(
                                connection,
                                desired_owner=self.denormalize_name(
                                    remote_owner),
                                desired_table=self.denormalize_name(
                                    remote_table)
                            )
                        if ref_synonym:
                            remote_table = self.normalize_name(ref_synonym)
                            remote_owner = self.normalize_name(
                                ref_remote_owner)

                    rec['referred_table'] = remote_table

                    if requested_schema is not None or \
                       self.denormalize_name(remote_owner) != schema:
                        rec['referred_schema'] = remote_owner

                local_cols.append(local_column)
                remote_cols.append(remote_column)

        return list(fkeys.values())

    @reflection.cache
    def get_view_definition(self, connection, view_name, schema=None,
                            resolve_synonyms=False, dblink='', **kw):
        info_cache = kw.get('info_cache')
        (view_name, schema, dblink, synonym) = \
            self._prepare_reflection_args(connection, view_name, schema,
                                          resolve_synonyms, dblink,
                                          info_cache=info_cache)

        params = {'view_name': view_name}
        text = "SELECT text FROM all_views WHERE view_name=:view_name"

        if schema is not None:
            text += " AND owner = :schema"
            params['schema'] = schema

        rp = connection.execute(sql.text(text), **params).scalar()
        if rp:
            if util.py2k:
                rp = rp.decode(self.encoding)
            return rp
        else:
            return None


class _OuterJoinColumn(sql.ClauseElement):
    __visit_name__ = 'outer_join_column'

    def __init__(self, column):
        self.column = column
