Improved debugging for HNSW locks [skip ci]

README.md

@@ -581,6 +581,10 @@ or choose to store vectors inline:
 ALTER TABLE items ALTER COLUMN embedding SET STORAGE PLAIN;
 ```
 
+#### Why are there less results for a query after adding an HNSW index?
+
+Results are limited by the size of the dynamic candidate list (`hnsw.ef_search`). There may be even less results due to dead tuples or filtering conditions in the query. We recommend setting `hnsw.ef_search` to at least twice the `LIMIT` of the query. If you need more than 500 results, use an IVFFlat index instead.
+
 #### Why are there less results for a query after adding an IVFFlat index?
 
 The index was likely created with too little data for the number of lists. Drop the index until the table has more data.
@@ -589,6 +593,8 @@ The index was likely created with too little data for the number of lists. Drop
 DROP INDEX index_name;
 ```
 
+Results can also be limited by the number of probes (`ivfflat.probes`).
+
 ## Reference
 
 ### Vector Type

src/hnsw.c

@@ -17,7 +17,10 @@
 #endif
 
 int			hnsw_ef_search;
-int			hnsw_lock_tranche_id;
+int			hnsw_entry_lock_tranche_id;
+int			hnsw_allocator_lock_tranche_id;
+int			hnsw_flush_lock_tranche_id;
+int			hnsw_element_lock_tranche_id;
 static relopt_kind hnsw_relopt_kind;
 
 /*
@@ -36,16 +39,27 @@ HnswInitLockTranche(void)
 	bool		found;
 
 	LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
-	tranche_ids = ShmemInitStruct("hnsw LWLock ids",
-								  sizeof(int) * 1,
+	tranche_ids = ShmemInitStruct("hnsw LWLock ids v2",
+								  sizeof(int) * 4,
 								  &found);
 	if (!found)
+	{
 		tranche_ids[0] = LWLockNewTrancheId();
-	hnsw_lock_tranche_id = tranche_ids[0];
+		tranche_ids[1] = LWLockNewTrancheId();
+		tranche_ids[2] = LWLockNewTrancheId();
+		tranche_ids[3] = LWLockNewTrancheId();
+	}
+	hnsw_entry_lock_tranche_id = tranche_ids[0];
+	hnsw_allocator_lock_tranche_id = tranche_ids[1];
+	hnsw_flush_lock_tranche_id = tranche_ids[2];
+	hnsw_element_lock_tranche_id = tranche_ids[3];
 	LWLockRelease(AddinShmemInitLock);
 
 	/* Per-backend registration of the tranche ID */
-	LWLockRegisterTranche(hnsw_lock_tranche_id, "HnswBuild");
+	LWLockRegisterTranche(hnsw_entry_lock_tranche_id, "HnswEntry");
+	LWLockRegisterTranche(hnsw_allocator_lock_tranche_id, "HnswAllocator");
+	LWLockRegisterTranche(hnsw_flush_lock_tranche_id, "HnswFlush");
+	LWLockRegisterTranche(hnsw_element_lock_tranche_id, "HnswElement");
 }
 
 /*

src/hnsw.h

@@ -113,7 +113,10 @@
 
 /* Variables */
 extern int	hnsw_ef_search;
-extern int	hnsw_lock_tranche_id;
+extern int	hnsw_entry_lock_tranche_id;
+extern int	hnsw_allocator_lock_tranche_id;
+extern int	hnsw_flush_lock_tranche_id;
+extern int	hnsw_element_lock_tranche_id;
 
 typedef struct HnswElementData HnswElementData;
 typedef struct HnswNeighborArray HnswNeighborArray;

src/hnswbuild.c

@@ -548,7 +548,7 @@ InsertTuple(Relation index, Datum *values, bool *isnull, ItemPointer heaptid, Hn
 	HnswPtrStore(base, element->value, valuePtr);
 
 	/* Create a lock for the element */
-	LWLockInitialize(&element->lock, hnsw_lock_tranche_id);
+	LWLockInitialize(&element->lock, hnsw_element_lock_tranche_id);
 
 	/* Insert tuple */
 	InsertTupleInMemory(buildstate, element);
@@ -611,9 +611,9 @@ InitGraph(HnswGraph * graph, char *base, long memoryTotal)
 	graph->flushed = false;
 	graph->indtuples = 0;
 	SpinLockInit(&graph->lock);
-	LWLockInitialize(&graph->entryLock, hnsw_lock_tranche_id);
-	LWLockInitialize(&graph->allocatorLock, hnsw_lock_tranche_id);
-	LWLockInitialize(&graph->flushLock, hnsw_lock_tranche_id);
+	LWLockInitialize(&graph->entryLock, hnsw_entry_lock_tranche_id);
+	LWLockInitialize(&graph->allocatorLock, hnsw_allocator_lock_tranche_id);
+	LWLockInitialize(&graph->flushLock, hnsw_flush_lock_tranche_id);
 }
 
 /*