What causes "err" in vitessio/vitess?

Rewrite the query so the ORDER BY column exists identically in every UNION branch.

err

Error message

err

What it means

Union.AddWSColumn converts addWeightStringToOffset's error into a panic. When appending a weight_string() column on top of a UNION (needed for ORDER BY collation handling), the helper failed — e.g. the requested offset has no matching source column or branches are inconsistent — so Vitess aborts planning. The message is the bare underlying error ('err'), making it opaque without a stack trace.

Source

Thrown at go/vt/vtgate/planbuilder/operators/union.go:172

func (u *Union) GetSelectFor(source int) *sqlparser.Select {
	src := u.Sources[source]
	for {
		switch op := src.(type) {
		case *Horizon:
			return getFirstSelect(op.Query)
		case *Route:
			src = op.Source
		default:
			panic(vterrors.VT13001("expected all sources of the UNION to be horizons"))
		}
	}
}

func (u *Union) AddWSColumn(ctx *plancontext.PlanningContext, offset int, underRoute bool) int {
	outputOffset, err := u.addWeightStringToOffset(ctx, offset)
	if err != nil {
		panic(err)
	}
	return outputOffset
}

func (u *Union) AddColumn(ctx *plancontext.PlanningContext, reuse bool, gb bool, expr *sqlparser.AliasedExpr) int {
	if reuse {
		offset := u.FindCol(ctx, expr.Expr, false)
		if offset >= 0 {
			return offset
		}
	}

	switch e := expr.Expr.(type) {
	case *sqlparser.ColName:
		cols := u.GetColumns(ctx)
		// here we deal with pure column access on top of the union
		offset := slices.IndexFunc(cols, func(column *sqlparser.AliasedExpr) bool {
			if column.ColumnName() == expr.ColumnName() {

View on GitHub (pinned to 01a25a7d17)

Solutions

Rewrite the query so the ORDER BY column exists identically in every UNION branch.
Avoid ORDER BY expressions requiring weight_string on UNION results (e.g. order in each branch or use an explicit compatible column).
If valid SQL triggers it, file a Vitess issue including the query and schema.

Example fix

// before
SELECT a FROM t1 UNION SELECT b FROM t2 ORDER BY a COLLATE utf8_bin;
// after: order by a column present in all branches
SELECT a FROM t1 UNION SELECT b AS a FROM t2 ORDER BY a;

Defensive patterns

Strategy: try-catch

Try / catch

defer func() {
    if r := recover(); r != nil {
        err = fmt.Errorf("union weight_string planning failed: %v", r)
    }
}()

Prevention

ORDER BY columns on UNIONs must exist identically in all branches.
Prefer ordering by a select-list alias over complex expressions.
Test collation-sensitive ORDER BY queries against UNIONs before deploying.

When it happens

Trigger: Calling Union.AddWSColumn(ctx, offset, underRoute) where addWeightStringToOffset returns an error, which is immediately panicked. Reached when planning ORDER BY on a UNION that requires weight_string columns.

Common situations: Queries ordering a UNION by a column requiring collation-aware sorting (weight_string) where the UNION branches' columns don't line up; typically hit on unhandled query shapes during development.

Related errors

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/f72922d954f012dc. Report an issue: GitHub.