sql: fix name resolution to avoid empty-name db lookups

The internal executor has a current DB of "", and it also executes
against queries against system tables using names like `system.users`.
The name resolution logic turned this into a lookup of "".system.users,
which was not desired.

Now we explicitly only allow the "" DB name if the schema name
corresponds to a virtual schema.

Release note: none

pkg/bench/ddl_analysis/testdata/benchmark_expectations

@@ -66,5 +66,5 @@ min	max	benchmark
 8	8	VirtualTableQueries/select_crdb_internal.invalid_objects_with_1_fk
 1	1	VirtualTableQueries/select_crdb_internal.tables_with_1_fk
 3	3	SystemDatabaseQueries/select_system.users_without_schema_name
-6	6	SystemDatabaseQueries/select_system.users_with_empty_database_name
+2	2	SystemDatabaseQueries/select_system.users_with_empty_database_name
 2	2	SystemDatabaseQueries/select_system.users_with_schema_name

pkg/sql/sem/tree/name_resolution.go

@@ -308,19 +308,22 @@ func ResolveExisting(
 		// Two parts: D.T.
 		// Try to use the current database, and be satisfied if it's sufficient to find the object.
 		//
-		// Note: we test this even if curDb == "", because CockroachDB
-		// supports querying virtual schemas even when the current
-		// database is not set. For example, `select * from
-		// pg_catalog.pg_tables` is meant to show all tables across all
-		// databases when there is no current database set.
-
-		if found, objMeta, err := r.LookupObject(ctx, lookupFlags, curDb, scName, u.Object()); found || err != nil {
-			if err == nil {
-				namePrefix.CatalogName = Name(curDb)
-				namePrefix.SchemaName = Name(scName)
+		// Note: CockroachDB supports querying virtual schemas even when the current
+		// database is not set. For example, `select * from pg_catalog.pg_tables` is
+		// meant to show all tables across all databases when there is no current
+		// database set. Therefore, we test this even if curDb == "", as long as the
+		// schema name is for a virtual schema.
+
+		if _, isVirtualSchema := sessiondata.VirtualSchemaNames[scName]; isVirtualSchema || curDb != "" {
+			if found, objMeta, err := r.LookupObject(ctx, lookupFlags, curDb, scName, u.Object()); found || err != nil {
+				if err == nil {
+					namePrefix.CatalogName = Name(curDb)
+					namePrefix.SchemaName = Name(scName)
+				}
+				return found, namePrefix, objMeta, err
 			}
-			return found, namePrefix, objMeta, err
 		}
+
 		// No luck so far. Compatibility with CockroachDB v1.1: try D.public.T instead.
 		if found, objMeta, err := r.LookupObject(ctx, lookupFlags, u.Schema(), PublicSchema, u.Object()); found || err != nil {
 			if err == nil {

pkg/sql/sem/tree/name_resolution_test.go

@@ -529,6 +529,7 @@ func newFakeMetadata() *fakeMetadata {
 				{"public", []tree.Name{"foo", "bar"}},
 				{"pg_temp_123", []tree.Name{"foo", "baz"}},
 			}},
+			{"system", []knownSchema{{"public", []tree.Name{"users"}}}},
 		},
 	}
 }
@@ -624,6 +625,7 @@ func TestResolveTablePatternOrName(t *testing.T) {
 		{`kv`, ``, mpath("public", "pg_catalog"), true, ``, ``, ``, `prefix or object not found`},
 		{`pg_tables`, ``, mpath("public", "pg_catalog"), true, `pg_tables`, `"".pg_catalog.pg_tables`, `.pg_catalog[0]`, ``},
 		{`pg_tables`, ``, mpath(), true, `pg_tables`, `"".pg_catalog.pg_tables`, `.pg_catalog[0]`, ``},
+		{`system.users`, ``, mpath(), true, `system.public.users`, `system.public.users`, `system.public[0]`, ``},
 
 		{`blix`, ``, mpath("public"), false, ``, ``, ``, `prefix or object not found`},
 		{`blix`, ``, mpath("public", "pg_catalog"), false, `blix`, `"".pg_catalog.blix`, `.pg_catalog`, ``},

pkg/sql/sessiondata/search_path.go

@@ -44,6 +44,14 @@ const PgTempSchemaName = "pg_temp"
 // when installing an extension, but must be stored as a separate schema in CRDB.
 const PgExtensionSchemaName = "pg_extension"
 
+// VirtualSchemaNames is a set of all virtual schema names.
+var VirtualSchemaNames = map[string]struct{}{
+	PgCatalogName:          {},
+	InformationSchemaName:  {},
+	CRDBInternalSchemaName: {},
+	PgExtensionSchemaName:  {},
+}
+
 // DefaultSearchPath is the search path used by virgin sessions.
 var DefaultSearchPath = MakeSearchPath(
 	[]string{UserSchemaName, PublicSchemaName},