This enables the subquery eager loader strategy to apply a DISTINCT
to the innermost SELECT subquery, to assist in the case where
duplicate rows are generated by the innermost query which corresponds
to this relationship (there's not yet a general solution to the issue
of dupe rows within subquery eager loading, however, when joins outside
of the innermost subquery produce dupes).  When the flag
is set to ``True``, the DISTINCT is rendered unconditionally, and when
it is set to ``None``, DISTINCT is rendered if the innermost relationship
targets columns that do not comprise a full primary key.
The option defaults to False in 0.8 (e.g. off by default in all cases),
None in 0.9 (e.g. automatic by default).   Thanks to Alexander Koval
for help with this. [ticket:2836]

query string to override those defaults set up in the connect,
including "buffered" and "raise_on_warnings".
[ticket:2515]

rendered in the column list of a CREATE INDEX statement.
[ticket:2742]

mssql to ensure that any literal SQL expression values are
rendered directly as literals, instead of as bound parameters,
within a CREATE INDEX statement. [ticket:2742]
- don't need expression_as_ddl(); literal_binds and include_table
take care of this functionality.

scalar attributes here and not relationships, else we get an error if there's no
actual scalars to load

or object method as an argument, in addition to a standalone function;
will properly detect if the "context" argument is accepted or not.

events are called, so that attachment events can be used to dynamically
generate a name for the index based on the parent table and/or
columns. [ticket:2835]

all known cases is provided by :class:`.DefaultDialect`, has been
tightened to expect ``include_columns`` and ``exclude_columns``
arguments without any kw option, reducing ambiguity - previously
``exclude_columns`` was missing. [ticket:2748]