Added new option to horizontal sharding API
:class:`_horizontal.set_shard_id` which sets the effective shard identifier
to query against, for both the primary query as well as for all secondary
loaders including relationship eager loaders as well as relationship and
column lazy loaders.

Modernize sharding examples with new-style mappings, add new asyncio example.

Fixes: #7226
Fixes: #7028
Change-Id: Ie69248060c305e8de04f75a529949777944ad511

Fixed stringify for a the :class:`.CreateSchema` DDL construct, which would
fail with an ``AttributeError`` when stringified without a dialect.

Fixes: #7664
Change-Id: Ifc1769604bc5219c060f5112f7bdea0f780f1a1c

Change-Id: I625af65b3fb1815b1af17dc2ef47dd697fdc3fb1

The :meth:`.SQLCompiler.construct_params` method, as well as the
:attr:`.SQLCompiler.params` accessor, will now return the
exact parameters that correspond to a compiled statement that used
the ``render_postcompile`` parameter to compile.   Previously,
the method returned a parameter structure that by itself didn't correspond
to either the original parameters or the expanded ones.

Passing a new dictionary of parameters to
:meth:`.SQLCompiler.construct_params` for a :class:`.SQLCompiler` that was
constructed with ``render_postcompile`` is now disallowed; instead, to make
a new SQL string and parameter set for an alternate set of parameters, a
new method :meth:`.SQLCompiler.construct_expanded_state` is added which
will produce a new expanded form for the given parameter set, using the
:class:`.ExpandedState` container which includes a new SQL statement
and new parameter dictionary, as well as a positional parameter tuple.

Fixes: #6114
Change-Id: I9874905bb90f86799b82b244d57369558b18fd93

Fixed a series of issues regarding positionally rendered bound parameters,
such as those used for SQLite, asyncpg, MySQL and others. Some compiled
forms would not maintain the order of parameters correctly, such as the
PostgreSQL ``regexp_replace()`` function as well as within the "nesting"
feature of the :class:`.CTE` construct first introduced in :ticket:`4123`.

Fixes: #8827
Change-Id: I9813ed7c358cc5c1e26725c48df546b209a442cb

We can cache the annotated cache key for Table, but
for selectables it's not safe, as it fails to pass the
anon_map along and creates many redudant structures in
observed test scenario.  It is likely safe for a
Column that's mapped to a Table also, however this is
not implemented here.   Will have to see if that part
needs adjusting.

Fixed critical memory issue identified in cache key generation, where for
very large and complex ORM statements that make use of lots of ORM aliases
with subqueries, cache key generation could produce excessively large keys
that were orders of magnitude bigger than the statement itself. Much thanks
to Rollo Konig Brock for their very patient, long term help in finally
identifying this issue.

Also within TypeEngine objects, when we generate elements
for instance variables, skip the None elements at least.
this also saves on tuple complexity.

Fixes: #8790
Change-Id: I448ddbfb45ae0a648815be8dad4faad7d1977427

The PostgreSQL dialect now supports reflection of expression based indexes.
The reflection is supported both when using
:meth:`_engine.Inspector.get_indexes` and when reflecting a
:class:`_schema.Table` using :paramref:`_schema.Table.autoload_with`.
Thanks to immerrr and Aidan Kane for the help on this ticket.

Fixes: #7442
Change-Id: I3e36d557235286c0f7f6d8276272ff9225058d48

Added very experimental feature to the :func:`_orm.selectinload` and
:func:`_orm.immediateload` loader options called
:paramref:`_orm.selectinload.recursion_depth` /
:paramref:`_orm.immediateload.recursion_depth` , which allows a single
loader option to automatically recurse into self-referential relationships.
Is set to an integer indicating depth, and may also be set to -1 to
indicate to continue loading until no more levels deep are found.
Major internal changes to :func:`_orm.selectinload` and
:func:`_orm.immediateload` allow this feature to work while continuing
to make correct use of the compilation cache, as well as not using
arbitrary recursion, so any level of depth is supported (though would
emit that many queries).  This may be useful for
self-referential structures that must be loaded fully eagerly, such as when
using asyncio.

A warning is also emitted when loader options are connected together with
arbitrary lengths (that is, without using the new ``recursion_depth``
option) when excessive recursion depth is detected in related object
loading. This operation continues to use huge amounts of memory and
performs extremely poorly; the cache is disabled when this condition is
detected to protect the cache from being flooded with arbitrary statements.

Fixes: #8126
Change-Id: I9f162e0a09c1ed327dd19498aac193f649333a01