Chooser: add storage-size fallback when stats are missing #78

Context

PR #75 introduced a cost-based Volcano/Vectorized chooser that uses RowEstimator::estimate_scan_rows() to decide whether to use the Vectorized path. This function returns table.num_rows when stats are available, or 0 otherwise.

Problem

For newly created tables or tables that have not had ANALYZE TABLE run, num_rows is 0. The current chooser logic treats estimated_rows > kVectorizedRowThreshold as the Vectorized trigger. When estimated_rows == 0 (unknown), use_vectorized stays false, falling back to Volcano — which is correct for truly empty tables but misses an opportunity for large scans where stats are simply stale/missing.

A table that has 5 million physical rows on disk but hasn't been ANALYZEd would incorrectly fall back to the (slower) Volcano path.

Proposed Solution

When estimate_scan_rows() returns 0 or when the table has no stats, derive a fallback estimate from storage size:

1. Add a method to StorageManager or BufferPoolManager that returns the on-disk size or block count for a given table:

   uint64_t StorageManager::estimate_table_size(const std::string& filename) const;

2. Alternatively, HeapTable::tuple_count() could be used as a non-stats-based row estimate — it traverses all pages and counts non-deleted tuples (expensive but accurate).

3. In the chooser block, when estimated_rows == 0 || !has_stats, fall back to:

   • Call storage size → compute rows as size_bytes / estimated_row_width (row width can be estimated from schema)
   • Or simply: treat missing stats as "assume large scan" → use_vectorized = true if table has any data

4. Compare the fallback estimate against kVectorizedRowThreshold to set use_vectorized.

Files Affected

• src/executor/query_executor.cpp — chooser block (lines ~405-419)
• include/storage/storage_manager.hpp — possibly new method

References

• PR Query Optimizer Phase 1: ANALYZE TABLE + Volcano/Vectorized Chooser #75: Query Optimizer Phase 1
• QueryExecutor::execute_select() — chooser logic
• RowEstimator::estimate_scan_rows() — returns 0 when no stats