fix: count stored rows, not sealed row_count, in search and stats (#534)

[DrTrippel]

Fixes #534

Problem

On Milvus Server, memsearch search and memsearch stats report a collection as empty for about 10 minutes after memsearch index succeeds. The data is not lost. memsearch just doesn't see it.

MilvusStore.search() guards against the #306 BM25 crash by returning [] when get_collection_stats()["row_count"] == 0, and count() returns the same value. row_count only counts sealed/flushed segments. Freshly upserted rows sit in a growing segment until the DataNode syncs them (dataNode.segment.syncPeriod, 600s by default). Until then:

• row_count is 0, so search returns nothing without querying;
• stats prints Total indexed chunks: 0.

Collections that already have an older flushed segment are not affected: the guard passes, and hybrid_search sees the growing rows immediately. That is why the bug only shows up on new or freshly re-created collections.

Milvus Lite flushes eagerly, which masks the problem, so #534 looked like a missing flush.

Repro (Milvus v3.0.1 standalone in Docker, memsearch 0.4.20, pymilvus 3.0.1)

Step	Result
memsearch index <dir> --collection ms_probe	Indexed 1 chunks.
get_collection_stats("ms_probe")["row_count"]	0
query("ms_probe", filter="", output_fields=["count(*)"], consistency_level="Strong")	1
memsearch search <canary> --collection ms_probe	No results found. until 608s after indexing, then found
Existing 23-row collection, stats / search	0 / nothing; after MilvusClient.flush(...), 23 and a score-1.0 hit

Fix

• count() counts stored rows with query(filter="", output_fields=["count(*)"], consistency_level="Strong").
• search() guards on self.count() == 0, so the Milvus crash on hybrid_search: Assert "std::isfinite(element.val)" failed → "Invalid sparse row: NaN or Inf value" #306 protection for truly empty collections stays. It also stops counting deleted-but-not-compacted rows.

Tests

New tests

• test_unsealed_rows_are_counted_and_searchable simulates Milvus Server by patching get_collection_stats to report row_count: 0 after an upsert. It fails on main (assert 0 == 1) and passes with this change.
• test_search_empty_collection_returns_empty keeps the Milvus crash on hybrid_search: Assert "std::isfinite(element.val)" failed → "Invalid sparse row: NaN or Inf value" #306 guard covered.

Local runs

• uv run python -m pytest tests/test_store.py: 19 passed.
• uv run python -m pytest: 423 passed, 7 skipped, 6 failed. The same 6 fail on unmodified main (test_claude_stop_indexes_only_server[*], test_session_start_and_data_commands_are_scoped_to_host_project[*]), so they are unrelated to this change.
• ruff check and ruff format --check: clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Ng6tyJJrBWCrSZNKGUbBdc