sql: add support for NOT VALID Foreign Key constraints

Implemented the support of not validating an ALTER TABLE foreign key
constraint to a table. Within alter_table.go we set the validity of
the foreign key reference to be unvalidated, and within shema_changer.go
we then check the case where the foreign key being added is unvalidated
and do not check its validity.

This allows users to create a FK reference between two tables whose rows
don't comply with the constraint when it's first added.

Release note (sql change): Support NOT VALID for Foreign Key constraints.

Update logic tests to more thoroughly test the added support of NOT VALID.

address PR comments, update a few tests and comments

pkg/ccl/importccl/read_import_mysql.go

@@ -494,7 +494,7 @@ type delayedFK struct {
 func addDelayedFKs(ctx context.Context, defs []delayedFK, resolver fkResolver) error {
 	for _, def := range defs {
 		if err := sql.ResolveFK(
-			ctx, nil, resolver, def.tbl, def.def, map[sqlbase.ID]*sqlbase.MutableTableDescriptor{}, sql.NewTable,
+			ctx, nil, resolver, def.tbl, def.def, map[sqlbase.ID]*sqlbase.MutableTableDescriptor{}, sql.NewTable, tree.ValidationDefault,
 		); err != nil {
 			return err
 		}

pkg/ccl/importccl/read_import_pgdump.go

@@ -268,7 +268,7 @@ func readPostgresCreateTable(
 					continue
 				}
 				for _, constraint := range constraints {
-					if err := sql.ResolveFK(evalCtx.Ctx(), nil /* txn */, fks.resolver, desc, constraint, backrefs, sql.NewTable); err != nil {
+					if err := sql.ResolveFK(evalCtx.Ctx(), nil /* txn */, fks.resolver, desc, constraint, backrefs, sql.NewTable, tree.ValidationDefault); err != nil {
 						return nil, err
 					}
 				}

pkg/sql/alter_table.go

@@ -273,7 +273,7 @@ func (n *alterTableNode) startExec(params runParams) error {
 					} else {
 						tableState = NonEmptyTable
 					}
-					err = params.p.resolveFK(params.ctx, n.tableDesc, d, affected, tableState)
+					err = params.p.resolveFK(params.ctx, n.tableDesc, d, affected, tableState, t.ValidationBehavior)
 				})
 				if err != nil {
 					return err

pkg/sql/create_table.go

@@ -384,8 +384,9 @@ func (p *planner) resolveFK(
 	d *tree.ForeignKeyConstraintTableDef,
 	backrefs map[sqlbase.ID]*sqlbase.MutableTableDescriptor,
 	ts FKTableState,
+	validationBehavior tree.ValidationBehavior,
 ) error {
-	return ResolveFK(ctx, p.txn, p, tbl, d, backrefs, ts)
+	return ResolveFK(ctx, p.txn, p, tbl, d, backrefs, ts, validationBehavior)
 }
 
 func qualifyFKColErrorWithDB(
@@ -421,8 +422,8 @@ const (
 // It may, in doing so, add to or alter descriptors in the passed in `backrefs`
 // map of other tables that need to be updated when this table is created.
 // Constraints that are not known to hold for existing data are created
-// "unvalidated", but when table is empty (e.g. during creation), no existing
-// data imples no existing violations, and thus the constraint can be created
+// "unvalidated", but when table is empty (e.g. during creating on), no existing
+// data implies no existing violations, and thus the constraint can be created
 // without the unvalidated flag.
 //
 // The caller should pass an instance of fkSelfResolver as
@@ -437,6 +438,10 @@ const (
 //
 // The passed Txn is used to lookup databases to qualify names in error messages
 // but if nil, will result in unqualified names in those errors.
+//
+// The passed validationBehavior is used to determine whether or not preexisting
+// entries in the table need to be validated against the foreign key being added.
+// This only applies for existing tables, not new tables.
 func ResolveFK(
 	ctx context.Context,
 	txn *client.Txn,
@@ -445,6 +450,7 @@ func ResolveFK(
 	d *tree.ForeignKeyConstraintTableDef,
 	backrefs map[sqlbase.ID]*sqlbase.MutableTableDescriptor,
 	ts FKTableState,
+	validationBehavior tree.ValidationBehavior,
 ) error {
 	for _, col := range d.FromCols {
 		col, _, err := tbl.FindColumnByName(col)
@@ -586,7 +592,11 @@ func ResolveFK(
 	}
 
 	if ts != NewTable {
-		ref.Validity = sqlbase.ConstraintValidity_Validating
+		if validationBehavior == tree.ValidationSkip {
+			ref.Validity = sqlbase.ConstraintValidity_Unvalidated
+		} else {
+			ref.Validity = sqlbase.ConstraintValidity_Validating
+		}
 	}
 	backref := sqlbase.ForeignKeyReference{Table: tbl.ID}
 
@@ -1282,7 +1292,7 @@ func MakeTableDesc(
 			desc.Checks = append(desc.Checks, ck)
 
 		case *tree.ForeignKeyConstraintTableDef:
-			if err := ResolveFK(ctx, txn, fkResolver, &desc, d, affected, NewTable); err != nil {
+			if err := ResolveFK(ctx, txn, fkResolver, &desc, d, affected, NewTable, tree.ValidationDefault); err != nil {
 				return desc, err
 			}