Adding support for ALTER TABLE ... DROP CONSTRAINT IF EXISTS

Author: fulghum

sql/analyzer/constraints.go

@@ -74,6 +74,12 @@ func resolveDropConstraint(ctx *sql.Context, a *Analyzer, n sql.Node, scope *pla
 			}
 		}
 
+		if dropConstraint.IfExists {
+			newAlterDropCheck := plan.NewAlterDropCheck(rt, dropConstraint.Name)
+			newAlterDropCheck.IfExists = true
+			return newAlterDropCheck, transform.NewTree, nil
+		}
+
 		return nil, transform.SameTree, sql.ErrUnknownConstraint.New(dropConstraint.Name)
 	})
 }
@@ -82,6 +88,11 @@ func resolveDropConstraint(ctx *sql.Context, a *Analyzer, n sql.Node, scope *pla
 func validateDropConstraint(ctx *sql.Context, a *Analyzer, n sql.Node, scope *plan.Scope, sel RuleSelector, qFlags *sql.QueryFlags) (sql.Node, transform.TreeIdentity, error) {
 	switch n := n.(type) {
 	case *plan.DropCheck:
+		// Don't bother validating that the constraint exists if the IfExists flag is set
+		if n.IfExists {
+			return n, transform.SameTree, nil
+		}
+
 		rt := n.Table
 
 		ct, ok := rt.Table.(sql.CheckTable)

sql/plan/alter_check.go

@@ -41,8 +41,9 @@ var _ sql.CollationCoercible = (*CreateCheck)(nil)
 
 type DropCheck struct {
 	ddlNode
-	Table *ResolvedTable
-	Name  string
+	Table    *ResolvedTable
+	Name     string
+	IfExists bool
 }
 
 var _ sql.Node = (*DropCheck)(nil)
@@ -172,7 +173,8 @@ func NewCheckDefinition(ctx *sql.Context, check *sql.CheckConstraint) (*sql.Chec
 // not known, and is determined during analysis.
 type DropConstraint struct {
 	UnaryNode
-	Name string
+	Name     string
+	IfExists bool
 }
 
 var _ sql.Node = (*DropConstraint)(nil)

sql/planbuilder/ddl.go

@@ -737,6 +737,7 @@ func (b *Builder) buildAlterConstraint(inScope *scope, ddl *ast.DDL, table *plan
 			outScope.node = &plan.DropConstraint{
 				UnaryNode: plan.UnaryNode{Child: table},
 				Name:      c.name,
+				IfExists:  ddl.ConstraintIfExists,
 			}
 		default:
 			err := sql.ErrUnsupportedFeature.New(ast.String(ddl))

sql/rowexec/ddl_iters.go

@@ -1859,6 +1859,27 @@ func (b *BaseBuilder) executeDropCheck(ctx *sql.Context, n *plan.DropCheck) erro
 		return err
 	}
 
+	checkTable, ok := chAlterable.(sql.CheckTable)
+	if !ok {
+		return plan.ErrNoCheckConstraintSupport.New(chAlterable.Name())
+	}
+
+	checks, err := checkTable.GetChecks(ctx)
+	if err != nil {
+		return err
+	}
+
+	exists := false
+	for _, check := range checks {
+		if strings.EqualFold(check.Name, n.Name) {
+			exists = true
+		}
+	}
+
+	if !exists && n.IfExists {
+		return nil
+	}
+
 	return chAlterable.DropCheck(ctx, n.Name)
 }