feat(csharp): implement SEA metadata — GetObjects, GetTableSchema, GetTableTypes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: msrathore-db

csharp/doc/sea-metadata-design.md

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+# SEA Metadata Architecture
+
+## Overview
+
+The Databricks ADBC driver supports two protocols for metadata retrieval:
+- **Thrift** (HiveServer2): Uses Thrift RPC calls to fetch metadata from the server
+- **SEA** (Statement Execution API): Uses SQL commands (`SHOW CATALOGS`, `SHOW SCHEMAS`, etc.) via the REST API
+
+Both protocols share the same Arrow result structure for `GetObjects`, `GetColumns`, `GetPrimaryKeys`, and other metadata operations. This document describes how shared code is structured to avoid duplication while allowing each protocol to fetch data from its respective backend.
+
+## Shared Interface: IGetObjectsDataProvider
+
+```
+AdbcConnection.GetObjects()  [sync ADBC API]
+         │
+         ▼
+GetObjectsResultBuilder.BuildGetObjectsResultAsync()  [async orchestrator]
+         │
+         ├─ provider.GetCatalogsAsync()
+         ├─ provider.GetSchemasAsync()
+         ├─ provider.GetTablesAsync()
+         └─ provider.PopulateColumnInfoAsync()
+         │
+         ▼
+    BuildResult() → HiveInfoArrowStream  [Arrow structure construction]
+```
+
+`IGetObjectsDataProvider` is the abstraction between "how to fetch metadata" and "how to build Arrow results":
+
+- **GetObjectsResultBuilder** knows how to construct the nested Arrow structures (catalog → schema → table → column) required by the ADBC GetObjects spec.
+- **IGetObjectsDataProvider** implementations know how to retrieve the raw data from their protocol.
+
+### Thrift Implementation (HiveServer2Connection)
+- Calls Thrift RPCs: `GetCatalogsAsync()`, `GetSchemasAsync()`, `GetTablesAsync()`, `GetColumnsAsync()`
+- Server returns typed result sets with precision, scale, column size
+- `SetPrecisionScaleAndTypeName` override handles per-connection type mapping
+
+### SEA Implementation (StatementExecutionConnection)
+- Executes SQL: `SHOW CATALOGS`, `SHOW SCHEMAS IN ...`, `SHOW TABLES IN ...`, `SHOW COLUMNS IN ...`
+- Server returns type name strings only — metadata is computed locally via `ColumnMetadataHelper`
+- `ColumnMetadataHelper.PopulateTableInfoFromTypeName` derives data type codes, column sizes, decimal digits from type names
+
+## Async Design
+
+The ADBC base class defines `GetObjects()` as synchronous:
+```csharp
+public abstract IArrowArrayStream GetObjects(GetObjectsDepth depth, ...);
+```
+
+Internally, the interface and builder are async:
+```csharp
+interface IGetObjectsDataProvider {
+    Task<IReadOnlyList<string>> GetCatalogsAsync(...);
+    // ...
+}
+
+static async Task<HiveInfoArrowStream> BuildGetObjectsResultAsync(
+    IGetObjectsDataProvider provider, ...) { ... }
+```
+
+The sync ADBC boundary blocks once at the top level:
+```csharp
+public override IArrowArrayStream GetObjects(...) {
+    return BuildGetObjectsResultAsync(this, ...).GetAwaiter().GetResult();
+}
+```
+
+This avoids nested `.Result` blocking calls on every Thrift RPC while maintaining the sync ADBC API contract.
+
+## Shared Schema Factories
+
+`MetadataSchemaFactory` (in hiveserver2) provides schema definitions used by both protocols:
+
+| Factory Method | Used By |
+|---|---|
+| `CreateCatalogsSchema()` | DatabricksStatement, StatementExecutionStatement |
+| `CreateSchemasSchema()` | DatabricksStatement, StatementExecutionStatement |
+| `CreateTablesSchema()` | DatabricksStatement, StatementExecutionStatement |
+| `CreateColumnMetadataSchema()` | DatabricksStatement, FlatColumnsResultBuilder |
+| `CreatePrimaryKeysSchema()` | MetadataSchemaFactory builders |
+| `CreateCrossReferenceSchema()` | MetadataSchemaFactory builders |
+| `BuildGetInfoResult()` | HiveServer2Connection, StatementExecutionConnection |
+
+## Type Mapping
+
+### Thrift Path
+```
+Server result → SetPrecisionScaleAndTypeName (per-connection override)
+    ├─ SparkConnection: parses DECIMAL/CHAR precision from type name
+    └─ HiveServer2ExtendedConnection: uses server-provided values
+
+For flat GetColumns, EnhanceGetColumnsResult (on HiveServer2Statement) adds
+a BASE_TYPE_NAME column and optionally overrides precision/scale by calling
+SetPrecisionScaleAndTypeName per row. This is Thrift-only — SEA builds the
+complete result from scratch via FlatColumnsResultBuilder.
+```
+
+### SEA Path
+```
+SHOW COLUMNS response → ColumnMetadataHelper.PopulateTableInfoFromTypeName
+    └─ Computes: data type code, column size, decimal digits, base type name
+```
+
+### Shared GetArrowType
+`HiveServer2Connection.GetArrowType()` (internal static) converts a column type ID to an Apache Arrow type. Both Thrift and SEA use this — SEA derives the type ID via `ColumnMetadataHelper.GetDataTypeCode()` first.
+
+## SQL Command Builders
+
+SEA metadata uses `MetadataCommandBase` with command subclasses:
+
+| Command | SQL Generated |
+|---|---|
+| `ShowCatalogsCommand` | `SHOW CATALOGS [LIKE 'pattern']` |
+| `ShowSchemasCommand` | `SHOW SCHEMAS IN \`catalog\` [LIKE 'pattern']` |
+| `ShowTablesCommand` | `SHOW TABLES IN CATALOG \`catalog\` [SCHEMA LIKE ...] [LIKE ...]` |
+| `ShowColumnsCommand` | `SHOW COLUMNS IN CATALOG \`catalog\` [SCHEMA LIKE ...] [TABLE LIKE ...] [LIKE ...]` |
+| `ShowKeysCommand` | `SHOW KEYS IN CATALOG ... IN SCHEMA ... IN TABLE ...` |
+| `ShowForeignKeysCommand` | `SHOW FOREIGN KEYS IN CATALOG ... IN SCHEMA ... IN TABLE ...` |
+
+Pattern conversion: ADBC `%` → Databricks `*`, ADBC `_` → Databricks `.`
+
+## GetObjects RPC Count
+
+Each IGetObjectsDataProvider method makes one server call. Total RPCs by depth:
+
+| Depth | Methods Called | RPCs |
+|---|---|---|
+| Catalogs | GetCatalogsAsync | 1 |
+| DbSchemas | + GetSchemasAsync | 2 |
+| Tables | + GetTablesAsync | 3 |
+| All | + PopulateColumnInfoAsync | 4 |