release-20.2: sql: add a hint for timestamptz -> string error

pkg/sql/logictest/testdata/logic_test/computed

@@ -307,6 +307,20 @@ CREATE TABLE y (
   b TIMESTAMP AS (a::TIMESTAMP) STORED
 )
 
+# Make sure the error has a hint that mentions AT TIME ZONE.
+statement error context-dependent operators are not allowed in computed column.*\nHINT:.*AT TIME ZONE
+CREATE TABLE y (
+  a TIMESTAMPTZ,
+  b STRING AS (a::STRING) STORED
+)
+
+# Make sure the error has a hint that mentions AT TIME ZONE.
+statement error context-dependent operators are not allowed in computed column.*\nHINT:.*AT TIME ZONE
+CREATE TABLE y (
+  a TIMESTAMPTZ,
+  b TIMESTAMP AS (a::TIMESTAMP) STORED
+)
+
 statement error context-dependent operators are not allowed in computed column
 CREATE TABLE y (
   a TIMESTAMPTZ,

pkg/sql/sem/tree/casts.go

@@ -39,6 +39,11 @@ type castInfo struct {
 	to         types.Family
 	volatility Volatility
 
+	// volatilityHint is an optional string for VolatilityStable casts. When set,
+	// it is used as an error hint suggesting a possible workaround when stable
+	// casts are not allowed.
+	volatilityHint string
+
 	// Telemetry counter; set by init().
 	counter telemetry.Counter
 
@@ -164,7 +169,10 @@ var validCasts = []castInfo{
 	{from: types.GeographyFamily, to: types.StringFamily, volatility: VolatilityImmutable},
 	{from: types.BytesFamily, to: types.StringFamily, volatility: VolatilityStable},
 	{from: types.TimestampFamily, to: types.StringFamily, volatility: VolatilityImmutable},
-	{from: types.TimestampTZFamily, to: types.StringFamily, volatility: VolatilityStable},
+	{
+		from: types.TimestampTZFamily, to: types.StringFamily, volatility: VolatilityStable,
+		volatilityHint: "TIMESTAMPTZ to STRING casts depend on the current timezone; consider using (t AT TIME ZONE 'UTC')::STRING instead.",
+	},
 	{from: types.IntervalFamily, to: types.StringFamily, volatility: VolatilityImmutable},
 	{from: types.UuidFamily, to: types.StringFamily, volatility: VolatilityImmutable},
 	{from: types.DateFamily, to: types.StringFamily, volatility: VolatilityImmutable},
@@ -240,11 +248,17 @@ var validCasts = []castInfo{
 
 	// Casts to TimestampFamily.
 	{from: types.UnknownFamily, to: types.TimestampFamily, volatility: VolatilityImmutable},
-	{from: types.StringFamily, to: types.TimestampFamily, volatility: VolatilityStable},
+	{
+		from: types.StringFamily, to: types.TimestampFamily, volatility: VolatilityStable,
+		volatilityHint: "STRING to TIMESTAMP casts are context-dependent because of relative timestamp strings like 'now'; use parse_timestamp() instead.",
+	},
 	{from: types.CollatedStringFamily, to: types.TimestampFamily, volatility: VolatilityStable},
 	{from: types.DateFamily, to: types.TimestampFamily, volatility: VolatilityImmutable},
 	{from: types.TimestampFamily, to: types.TimestampFamily, volatility: VolatilityImmutable},
-	{from: types.TimestampTZFamily, to: types.TimestampFamily, volatility: VolatilityStable},
+	{
+		from: types.TimestampTZFamily, to: types.TimestampFamily, volatility: VolatilityStable,
+		volatilityHint: "TIMESTAMPTZ to TIMESTAMP casts depend on the current timezone; consider using AT TIME ZONE 'UTC' instead",
+	},
 	{from: types.IntFamily, to: types.TimestampFamily, volatility: VolatilityImmutable},
 
 	// Casts to TimestampTZFamily.

pkg/sql/sem/tree/datum_invariants_test.go

@@ -27,7 +27,7 @@ func TestAllTypesCastableToString(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
 	for _, typ := range types.Scalar {
-		if ok, _, _ := isCastDeepValid(typ, types.String); !ok {
+		if err := resolveCast("", typ, types.String, true /* allowStable */); err != nil {
 			t.Errorf("%s is not castable to STRING, all types should be", typ)
 		}
 	}
@@ -37,7 +37,7 @@ func TestAllTypesCastableFromString(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
 	for _, typ := range types.Scalar {
-		if ok, _, _ := isCastDeepValid(types.String, typ); !ok {
+		if err := resolveCast("", types.String, typ, true /* allowStable */); err != nil {
 			t.Errorf("%s is not castable from STRING, all types should be", typ)
 		}
 	}

pkg/sql/sem/tree/type_check.go

@@ -409,26 +409,47 @@ func (expr *CaseExpr) TypeCheck(
 	return expr, nil
 }
 
-func isCastDeepValid(castFrom, castTo *types.T) (bool, telemetry.Counter, Volatility) {
+// resolveCast checks that the cast from the two types is valid. If allowStable
+// is false, it also checks that the cast has VolatilityImmutable.
+//
+// On success, any relevant telemetry counters are incremented.
+func resolveCast(context string, castFrom, castTo *types.T, allowStable bool) error {
 	toFamily := castTo.Family()
 	fromFamily := castFrom.Family()
 	switch {
 	case toFamily == types.ArrayFamily && fromFamily == types.ArrayFamily:
-		ok, c, v := isCastDeepValid(castFrom.ArrayContents(), castTo.ArrayContents())
-		if ok {
-			telemetry.Inc(sqltelemetry.ArrayCastCounter)
+		err := resolveCast(context, castFrom.ArrayContents(), castTo.ArrayContents(), allowStable)
+		if err != nil {
+			return err
 		}
-		return ok, c, v
+		telemetry.Inc(sqltelemetry.ArrayCastCounter)
+		return nil
+
 	case toFamily == types.EnumFamily && fromFamily == types.EnumFamily:
-		// Casts from ENUM to ENUM type can only succeed if the two enums
-		return castFrom.Equivalent(castTo), sqltelemetry.EnumCastCounter, VolatilityImmutable
-	}
+		// Casts from ENUM to ENUM type can only succeed if the two types are the
+		// same.
+		if !castFrom.Equivalent(castTo) {
+			return pgerror.Newf(pgcode.CannotCoerce, "invalid cast: %s -> %s", castFrom, castTo)
+		}
+		telemetry.Inc(sqltelemetry.EnumCastCounter)
+		return nil
 
-	cast := lookupCast(fromFamily, toFamily)
-	if cast == nil {
-		return false, nil, 0
+	default:
+		cast := lookupCast(fromFamily, toFamily)
+		if cast == nil {
+			return pgerror.Newf(pgcode.CannotCoerce, "invalid cast: %s -> %s", castFrom, castTo)
+		}
+		if !allowStable && cast.volatility >= VolatilityStable {
+			err := NewContextDependentOpsNotAllowedError(context)
+			err = pgerror.Wrapf(err, pgcode.InvalidParameterValue, "%s::%s", castFrom, castTo)
+			if cast.volatilityHint != "" {
+				err = errors.WithHint(err, cast.volatilityHint)
+			}
+			return err
+		}
+		telemetry.Inc(cast.counter)
+		return nil
 	}
-	return true, cast.counter, cast.volatility
 }
 
 func isEmptyArray(expr Expr) bool {
@@ -487,22 +508,16 @@ func (expr *CastExpr) TypeCheck(
 	}
 
 	castFrom := typedSubExpr.ResolvedType()
-
-	ok, c, volatility := isCastDeepValid(castFrom, exprType)
-	if !ok {
-		return nil, pgerror.Newf(pgcode.CannotCoerce, "invalid cast: %s -> %s", castFrom, exprType)
+	allowStable := true
+	context := ""
+	if semaCtx != nil && semaCtx.Properties.required.rejectFlags&RejectStableOperators != 0 {
+		allowStable = false
+		context = semaCtx.Properties.required.context
 	}
-	if err := semaCtx.checkVolatility(volatility); err != nil {
-		err = pgerror.Wrapf(err, pgcode.InvalidParameterValue, "%s::%s", castFrom, exprType)
-		// Special cases where we can provide useful hints.
-		if castFrom.Family() == types.StringFamily && exprType.Family() == types.TimestampFamily {
-			err = errors.WithHint(err, "string to timestamp casts are context-dependent because "+
-				"of relative timestamp strings like 'now'; use parse_timestamp() instead.")
-		}
+	err = resolveCast(context, castFrom, exprType, allowStable)
+	if err != nil {
 		return nil, err
 	}
-
-	telemetry.Inc(c)
 	expr.Expr = typedSubExpr
 	expr.Type = exprType
 	expr.typ = exprType