[PLAT-396] allow dict optimization when lookup dim is in select clause (#8740)

* allow dict optimization when lookup dim is in select clause

* parse validation

* simplify

* lint

Author: pjain1

runtime/metricsview/ast.go

@@ -110,6 +110,7 @@ type FieldNode struct {
 	Expr        string
 	Unnest      bool
 	TreatNullAs string // only used for measures
+	LookupMeta  *lookupMeta
 }
 
 // ExprNode represents an expression for a WHERE clause.
@@ -118,6 +119,13 @@ type ExprNode struct {
 	Args []any
 }
 
+type lookupMeta struct {
+	table    string
+	keyExpr  string
+	keyCol   string
+	valueCol string
+}
+
 // And returns a new node that is the AND of the current node and the given expression.
 func (n *ExprNode) And(expr string, args []any) *ExprNode {
 	if expr == "" {
@@ -220,11 +228,28 @@ func NewAST(mv *runtimev1.MetricsViewSpec, sec MetricsViewSecurity, qry *Query,
 			return nil, fmt.Errorf("failed to compile dimension %q expression: %w", dim.Name, err)
 		}
 
+		var lkMeta *lookupMeta
+		if dim.LookupTable != "" {
+			var keyExpr string
+			if dim.Column != "" {
+				keyExpr = ast.Dialect.EscapeIdentifier(dim.Column)
+			} else {
+				keyExpr = dim.Expression
+			}
+			lkMeta = &lookupMeta{
+				table:    dim.LookupTable,
+				keyExpr:  keyExpr,
+				keyCol:   dim.LookupKeyColumn,
+				valueCol: dim.LookupValueColumn,
+			}
+		}
+
 		f := FieldNode{
 			Name:        dim.Name,
 			DisplayName: dim.DisplayName,
 			Expr:        expr,
 			Unnest:      dim.Unnest,
+			LookupMeta:  lkMeta,
 		}
 
 		if dim.Unnest {

runtime/metricsview/astexpr.go

@@ -677,7 +677,7 @@ func (b *sqlExprBuilder) sqlForName(name string) (expr string, unnest bool, look
 			if f.Name == name {
 				// Note that we return "false" even though it may be an unnest dimension because it will already have been unnested since it's one of the dimensions included in the query.
 				// So we can filter against it as if it's a normal dimension.
-				return f.Expr, false, nil, nil
+				return f.Expr, false, f.LookupMeta, nil
 			}
 		}
 
@@ -692,19 +692,13 @@ func (b *sqlExprBuilder) sqlForName(name string) (expr string, unnest bool, look
 			return "", false, nil, fmt.Errorf("invalid dimension reference %q: %w", name, err)
 		}
 
-		if dim.Unnest && dim.LookupTable != "" {
-			return "", false, nil, fmt.Errorf("dimension %q is unnested and also has a lookup. This is not supported", name)
-		}
-
 		var lm *lookupMeta
 		if dim.LookupTable != "" {
 			var keyExpr string
 			if dim.Column != "" {
 				keyExpr = b.ast.Dialect.EscapeIdentifier(dim.Column)
-			} else if dim.Expression != "" {
-				keyExpr = dim.Expression
 			} else {
-				return "", false, nil, fmt.Errorf("dimension %q has a lookup table but no column or expression defined", name)
+				keyExpr = dim.Expression
 			}
 			lm = &lookupMeta{
 				table:    dim.LookupTable,
@@ -754,13 +748,6 @@ func convertLikeExpressionToRegexExpression(like *Expression) (*Expression, erro
 	return &Expression{Value: pattern}, nil
 }
 
-type lookupMeta struct {
-	table    string
-	keyExpr  string
-	keyCol   string
-	valueCol string
-}
-
 // skipMetricsViewSecurity implements the MetricsViewSecurity interface in a way that allows all access.
 type skipMetricsViewSecurity struct{}
 

runtime/parser/parse_metrics_view.go

@@ -379,6 +379,9 @@ func (p *Parser) parseMetricsView(node *Node) error {
 			if strings.Contains(dim.Expression, "dictGet") {
 				return fmt.Errorf("dictGet expression and lookup fields cannot be used together")
 			}
+			if dim.Unnest {
+				return fmt.Errorf("unnest cannot be used with lookup fields")
+			}
 		}
 
 		// Validate the dimension name is unique