Fix bug where SQL query in version match check would execute outside …

…of an outer snapshot or portal (#266)

src/hnsw/utils.c

@@ -114,6 +114,17 @@ float4 *ToFloat4Array(ArrayType *arr)
 // This is used to prevent interacting with the index when the two don't match
 bool VersionsMatch()
 {
+    // If a parallel worker runs as a result of query execution, executing the SQL query below will lead to the
+    // error "ERROR:  cannot execute SQL without an outer snapshot or portal." Instead of loading in a snapshot, we
+    // simply return if one doesn't exist, the idea being that in the case of a parallel worker running this
+    // function, the original worker will have already run this function (after which all the parallel workers run
+    // this function, invoked by _PG_init). We return true so that we suppress any version mismatch messages from
+    // callers of this function
+    if(!ActiveSnapshotSet()) {
+        version_checked = versions_match = false;
+        return true;
+    }
+
     if(likely(version_checked)) {
         return versions_match;
     } else {

test/expected/missing_outer_snapshot_portal.out

@@ -0,0 +1,36 @@
+-- This test is to check that Lantern properly handles missing outer snapshots or portals when it is loaded
+-- So far, this test checks that Lantern handles cases of initializing parallel workers properly.
+-- Specifically, we test that Lantern performs the version match check by only performing its initialization SQL queries when a proper outer snapshot or portal exists. 
+-- This is to prevent the following error: "ERROR:  cannot execute SQL without an outer snapshot or portal"
+-- Note: Dropping and loading the extension again is necessary to test the desired missing outer snapshot/portal behavior
+DROP EXTENSION lantern;
+CREATE EXTENSION lantern;
+CREATE TABLE IF NOT EXISTS ourtable (
+     id SERIAL,
+     v REAL[128]
+);
+\copy ourtable (v) FROM '/tmp/lantern/vector_datasets/siftsmall_base_arrays.csv' with csv;
+SET max_parallel_workers_per_gather TO 4;
+SET max_parallel_workers TO 8;
+--- Make parallel plans more favorable ---
+SET min_parallel_table_scan_size TO '8kB';
+SET parallel_setup_cost TO 10;
+SET parallel_tuple_cost TO 0.001;
+SET seq_page_cost TO 10; 
+-- This query should have a parallel plan
+EXPLAIN (COSTS false) SELECT COUNT(*) FROM ourtable;
+                   QUERY PLAN                    
+-------------------------------------------------
+ Finalize Aggregate
+   ->  Gather
+         Workers Planned: 4
+         ->  Partial Aggregate
+               ->  Parallel Seq Scan on ourtable
+(5 rows)
+
+SELECT COUNT(*) FROM ourtable;
+ count 
+-------
+ 10000
+(1 row)
+

test/schedule.txt

@@ -3,6 +3,6 @@
 # - every test that needs to be run iff pgvector is installed appears in a 'test_pgvector:' line
 # - 'test' lines may have multiple space-separated tests. All tests in a single 'test' line will be run in parallel
 
-test: hnsw_config hnsw_correct hnsw_create hnsw_create_expr hnsw_dist_func hnsw_insert hnsw_select hnsw_todo hnsw_index_from_file hnsw_cost_estimate ext_relocation hnsw_ef_search hnsw_failure_point hnsw_operators hnsw_blockmap_create hnsw_create_unlogged hnsw_insert_unlogged hnsw_logged_unlogged
+test: hnsw_config hnsw_correct hnsw_create hnsw_create_expr hnsw_dist_func hnsw_insert hnsw_select hnsw_todo hnsw_index_from_file hnsw_cost_estimate ext_relocation hnsw_ef_search hnsw_failure_point hnsw_operators hnsw_blockmap_create hnsw_create_unlogged hnsw_insert_unlogged hnsw_logged_unlogged missing_outer_snapshot_portal
 test_pgvector: hnsw_vector
 test_extras: hnsw_extras

test/sql/missing_outer_snapshot_portal.sql

@@ -0,0 +1,33 @@
+-- This test is to check that Lantern properly handles missing outer snapshots or portals when it is loaded
+
+-- So far, this test checks that Lantern handles cases of initializing parallel workers properly.
+-- Specifically, we test that Lantern performs the version match check by only performing its initialization SQL queries when a proper outer snapshot or portal exists. 
+-- This is to prevent the following error: "ERROR:  cannot execute SQL without an outer snapshot or portal"
+
+-- Note: Dropping and loading the extension again is necessary to test the desired missing outer snapshot/portal behavior
+DROP EXTENSION lantern;
+CREATE EXTENSION lantern;
+
+CREATE TABLE IF NOT EXISTS ourtable (
+     id SERIAL,
+     v REAL[128]
+);
+
+\copy ourtable (v) FROM '/tmp/lantern/vector_datasets/siftsmall_base_arrays.csv' with csv;
+
+SET max_parallel_workers_per_gather TO 4;
+SET max_parallel_workers TO 8;
+
+--- Make parallel plans more favorable ---
+SET min_parallel_table_scan_size TO '8kB';
+
+SET parallel_setup_cost TO 10;
+SET parallel_tuple_cost TO 0.001;
+
+SET seq_page_cost TO 10; 
+
+-- This query should have a parallel plan
+EXPLAIN (COSTS false) SELECT COUNT(*) FROM ourtable;
+SELECT COUNT(*) FROM ourtable;
+
+