Merge #142068

142068: sql: fix CDC expression planning for tables without columns r=asg0451 a=massimo-ua

Fixes an issue in the CDC expression planning that was causing an error when planning for tables without columns. The error occurred because the planner
returned an assertion failure when it couldn't determine any result columns. The issue happens when using a CDC expression with a table that has no user-defined columns (e.g. created with CREATE TABLE empty()) and running a query like SELECT * FROM empty. Since there are no columns to select, the presentation list is empty, which triggered the assertion failure. This fix removes the check that returns an error when the presentation is empty, allowing CDC expressions to work with tables that have no user-defined columns. The planner now correctly handles this edge case and returns a valid plan with an empty presentation.
Fixes #114058.
Fixes https://cockroachlabs.atlassian.net/browse/CRDB-33318.
Release note (bug fix): Fixed an issue where changefeed expressions on tables without columns would fail with an internal error: "unable to determine result columns".

Co-authored-by: maksym.hryshkov <[email protected]>

Author: craig[bot]

pkg/ccl/changefeedccl/changefeed_test.go

@@ -11072,3 +11072,28 @@ func assertEqualTSNSHLCWalltime(t *testing.T, tsns int64, tshlc string) {
 	tsHLCWallTimeNano := parseTimeToHLC(t, tshlc).WallTime
 	require.EqualValues(t, tsns, tsHLCWallTimeNano)
 }
+
+// TestChangefeedAsSelectForEmptyTable verifies that a changefeed
+// yields a proper user error on an empty table and in the same
+// allows hidden columns to be selected.
+func TestChangefeedAsSelectForEmptyTable(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	testFn := func(t *testing.T, s TestServer, f cdctest.TestFeedFactory) {
+		sqlDB := sqlutils.MakeSQLRunner(s.DB)
+		sqlDB.Exec(t, `CREATE TABLE empty()`)
+		sqlDB.Exec(t, `INSERT INTO empty DEFAULT VALUES`)
+		// Should fail when no columns are selected.
+		// Use expectErrCreatingFeed which handles sinkless feeds correctly by
+		// attempting to read from the feed if no error occurs at creation time
+		expectErrCreatingFeed(t, f, `CREATE CHANGEFEED AS SELECT * FROM empty`, `SELECT yields no columns`)
+
+		// Should succeed when a rowid column is explicitly selected.
+		feed, err := f.Feed(`CREATE CHANGEFEED AS SELECT rowid FROM empty`)
+		require.NoError(t, err)
+		defer closeFeed(t, feed)
+	}
+
+	cdcTest(t, testFn)
+}

pkg/sql/distsql_plan_changefeed.go

@@ -127,8 +127,16 @@ func PlanCDCExpression(
 
 	// The top node contains the list of columns to return.
 	presentation := planColumns(p.curPlan.main.planNode)
+
+	// SELECT statement provided by the user yields zero columns.
+	// If some user actually wants to have a changefeed for the
+	// hidden rowid column of a zero-column table, then they can be explicit
+	// about it: CREATE CHANGEFEED ... SELECT rowid ....
 	if len(presentation) == 0 {
-		return CDCExpressionPlan{}, errors.AssertionFailedf("unable to determine result columns")
+		return CDCExpressionPlan{}, errors.WithHintf(
+			errors.New("SELECT yields no columns"),
+			"Specify at least one column in your SELECT statement or use SELECT rowid if you need a changefeed for the hidden rowid column of a zero-column table.",
+		)
 	}
 
 	// Walk the plan, perform sanity checks and extract information we need.