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   Fixes #SNOW-689531

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Optimise reflection performance (SNOW-689531)

Summary

Adds SQLAlchemy 2.x get_multi_* bulk reflection hooks and per-table SHOW … IN TABLE / DESC TABLE reflection paths. Schema-wide queries run once per MetaData.reflect() pass instead of once per table.
Single-table Inspector calls use fast per-table queries that work correctly for all table types including temporary and dynamic tables.

Changes

Performance

• Add get_multi_columns, get_multi_pk_constraint, get_multi_unique_constraints, get_multi_foreign_keys — SA 2.x bulk hooks that each issue one schema-wide query per reflection pass instead of one
query per table.
• cache_column_metadata=True opt-in enables per-table SHOW … IN TABLE queries for PK, UK, FK, and indexes on both SA versions.

Correctness

• Fix FK referred_schema resolution: both per-table and schema-wide paths now include the explicitly reflected schema in the same-schema set, preventing incorrect cross-schema references when
reflecting a non-default schema.
• Fix unique constraint keying: constraints are now keyed by (table_name, constraint_name) instead of constraint_name alone, preventing silent overwrites when two tables have identically-named
constraints.
• Add _always_quote_join helper that always quotes denormalised identifiers, ensuring correct SQL for case-sensitive table and schema names.
• Replace SHOW TABLES LIKE with SHOW INDEXES IN TABLE for single-table index reflection, eliminating SQL LIKE wildcard false-positives and case-sensitivity bugs.

Robustness

• Narrow exception handling in all per-table reflection methods (_get_table_primary_keys, _get_table_unique_constraints, _get_table_foreign_keys, _get_table_indexes) from sa_exc.DBAPIError to
sa_exc.ProgrammingError — connection and operational errors now propagate instead of being silently swallowed.
• Narrow _execute_desc from bare except Exception to sa_exc.ProgrammingError — only SQL-level errors (e.g. table dropped by another session) are swallowed; network/permission errors propagate with
actionable diagnostics.
• Add logger.debug to all swallowed exceptions for diagnosability.

Code quality

• Add shared row-parsing helpers (_parse_pk_rows, _parse_uk_rows, _parse_fk_rows, _parse_index_rows) so correctness fixes propagate to both per-table and schema-wide paths.
• Hoist _StructuredTypeInfoManager allocation outside the fallback loop in get_multi_columns.
• Fix misleading docstrings on get_table* methods that claimed "results are not cached" when the calling methods apply @reflection.cache.

Documentation

• README: document SA 2.x vs 1.4 reflection routing, cache_column_metadata opt-in, performance implications, and best practices for different schema sizes.
• README: note that Inspector.get_columns() on SA 2.x uses DESC TABLE, which includes Snowflake's resolved default sizes in type str() representations (e.g. BINARY(8388608)). Type objects are
functionally identical; use isinstance() for type checks.

Behavioural notes

• SA 2.x Inspector.get_columns() now uses DESC TABLE instead of information_schema. Reflected type objects are functionally identical, but str() output may include explicit default sizes (e.g.
BINARY(8388608) vs BINARY). MetaData.reflect() and Alembic are unaffected — they use get_multi_columns which still queries information_schema.
• SA 2.x get_columns() for missing tables returns [] instead of raising NoSuchTableError, aligning with the SA 2.0 convention of returning falsy values. reflect_table handles the empty-columns
case separately.
• Per-table reflection methods now let non-ProgrammingError exceptions (network failures, permission errors) propagate instead of returning empty metadata silently.