Tiered storage: X-Arc-Database header queries fail for cold-only data

[xe-nvdk]

Bug Description

When data has been migrated to cold storage via tiered storage, queries using the X-Arc-Database header fail because the query path still resolves to local (hot) storage instead of checking tiered storage metadata.

Symptoms

1. X-Arc-Database header queries fail for cold-only data:

   POST /api/v1/query
   X-Arc-Database: benchmark_logs
   
   SELECT * FROM logs LIMIT 100;
   

   Error: IO Error: No files found that match the pattern "/Users/.../data/arc/benchmark_logs/logs/**/*.parquet"

2. Qualified name queries work:

   SELECT * FROM benchmark_logs.logs LIMIT 10;

   ✅ Works correctly, returns data from cold tier

3. Database listing doesn't show cold-only databases:

   • GET /api/v1/databases only returns databases with hot tier data
   • Databases migrated entirely to cold storage are invisible

Root Cause Analysis

Issue 1: Fast path bypasses tiering metadata

The optimized fast path for header-based queries (convertSingleTableQuery()) calls getStoragePath() which returns the hot tier path directly, before tiering metadata can be consulted.

Flow comparison:

X-Arc-Database header path (BROKEN):
  convertSQLToStoragePathsWithHeaderDB()
  → convertSingleTableQuery() [fast path]
  → h.getStoragePath() [returns HOT TIER ONLY]
  → buildReadParquetExpr() [tiering check happens too late]
  → ❌ Fails for cold-only data

Qualified name path (WORKS):
  convertSQLToStoragePaths() [regex-based]
  → Extracts database/measurement before path resolution
  → buildReadParquetExpr() [proper tiering check]
  → buildMultiTierReadParquet()
  → ✅ Works for cold tier data

Issue 2: Database listing only scans hot tier

DatabasesHandler.listDatabasesWithMeasurementCounts() only queries the hot tier storage backend. It should also query tiering metadata to include cold-only databases.

Key Code Locations

Issue	File	Function
Fast path tiering bypass	internal/api/query.go	convertSingleTableQuery()
Database listing missing cold tier	internal/api/databases.go	listDatabasesWithMeasurementCounts()
Missing tiering integration	internal/api/databases.go	DatabasesHandler struct

Expected Behavior

1. Queries with X-Arc-Database header should check tiering metadata and route to the appropriate tier
2. GET /api/v1/databases should return all databases, including those with data only in cold storage
3. Both query syntaxes should work identically for tiered data

Logs

2026-01-26T11:38:23-03:00 INF Migration cycle completed component=tiering-manager migrated=66
2026-01-26T11:39:21-03:00 ERR Query failed error="IO Error: No files found that match the pattern..."