add comment explaining the code

Signed-off-by: Olga Shestopalova <oshestopalova@hubspot.com>
HubSpot · Sep 26, 2023 · 30e21a4 · 30e21a4
1 parent 90b4bff
commit 30e21a4
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Showing 2 changed files with 12 additions and 2 deletions.
diff --git a/go/vt/vttablet/tabletmanager/vreplication/table_plan_builder.go b/go/vt/vttablet/tabletmanager/vreplication/table_plan_builder.go
@@ -425,6 +425,11 @@ func (tpb *tablePlanBuilder) analyzeExpr(selExpr sqlparser.SelectExpr) (*colExpr
 		references: make(map[string]bool),
 	}
 	if expr, ok := aliased.Expr.(*sqlparser.ConvertUsingExpr); ok {
+		// Here we find the actual column name in the convert, in case
+		// this is a column rename and the AS is the new column.
+		// For example, in convert(c1 using utf8mb4) as c2, we want to find
+		// c1, because c1 exists in the current table whereas c2 is the renamed column
+		// in the desired table.
 		var colName sqlparser.IdentifierCI
 		err := sqlparser.Walk(func(node sqlparser.SQLNode) (kontinue bool, err error) {
 			switch node := node.(type) {
@@ -437,7 +442,7 @@ func (tpb *tablePlanBuilder) analyzeExpr(selExpr sqlparser.SelectExpr) (*colExpr
 			return true, nil
 		}, aliased.Expr)
 		if err != nil {
-			return nil, fmt.Errorf("failed to find column name for convert using expression: %v", sqlparser.String(aliased.Expr))
+			return nil, fmt.Errorf("failed to find column name for convert using expression: %v, %v", sqlparser.String(aliased.Expr), err)
 		}
 		selExpr := &sqlparser.ConvertUsingExpr{
 			Type: "utf8mb4",

diff --git a/go/vt/vttablet/tabletserver/vstreamer/planbuilder.go b/go/vt/vttablet/tabletserver/vstreamer/planbuilder.go
@@ -727,6 +727,11 @@ func (plan *Plan) analyzeExpr(vschema *localVSchema, selExpr sqlparser.SelectExp
 			FixedValue: sqltypes.NewInt64(num),
 		}, nil
 	case *sqlparser.ConvertUsingExpr:
+		// Here we find the actual column name in the convert, in case
+		// this is a column rename and the AS is the new column.
+		// For example, in convert(c1 using utf8mb4) as c2, we want to find
+		// c1, because c1 exists in the current table whereas c2 is the renamed column
+		// in the desired table.
 		var colName sqlparser.IdentifierCI
 		err := sqlparser.Walk(func(node sqlparser.SQLNode) (kontinue bool, err error) {
 			switch node := node.(type) {
@@ -739,7 +744,7 @@ func (plan *Plan) analyzeExpr(vschema *localVSchema, selExpr sqlparser.SelectExp
 			return true, nil
 		}, aliased.Expr)
 		if err != nil {
-			return ColExpr{}, fmt.Errorf("failed to find column name for convert using expression: %v", sqlparser.String(aliased.Expr))
+			return ColExpr{}, fmt.Errorf("failed to find column name for convert using expression: %v, %v", sqlparser.String(aliased.Expr), err)
 		}
 		colnum, err := findColumn(plan.Table, colName)
 		if err != nil {