sql: remove UNKNOWN from castMap

The UNKNOWN type, which is given to expressions that statically evaluate
to NULL, can be cast to any type. Codifying all possible casts from with
an UNKNOWN source type in the `castMap` is cumbersome and it is easy to
accidentally omit some target types. This commit removes the UNKNOWN
casts from the `castMap` and adds logic in `ValidCast` that allows
UNKNOWN types to be cast to any other type in all contexts.

Release note: None

pkg/sql/sem/tree/cast.go

@@ -91,12 +91,6 @@ const (
 	// [1] https://www.postgresql.org/docs/13/catalog-pg-cast.html#CATALOG-PG-CAST
 	// [2] https://www.postgresql.org/docs/13/sql-createcast.html#SQL-CREATECAST-NOTES
 	contextOriginAutomaticIOConversion
-	// contextOriginNullConversion specifies that a cast's maximum context is
-	// not included in Postgres's pg_cast table. This is only used for casts
-	// where the source is the unknown type. The unknown type is only used for
-	// expressions that statically evaluate to NULL. NULL can be implicitly
-	// converted to any type.
-	contextOriginNullConversion
 )
 
 // cast includes details about a cast from one OID to another.
@@ -120,8 +114,10 @@ type cast struct {
 	origin contextOrigin
 }
 
-// castMap defines all possible casts. It maps from a source OID to a target OID
-// to a cast struct that contains information about the cast.
+// castMap defines valid casts. It maps from a source OID to a target OID to a
+// cast struct that contains information about the cast. Some possible casts,
+// such as casts from the UNKNOWN type and casts from a type to the identical
+// type, are not defined in the castMap and are instead codified in ValidCast.
 //
 // Validation is performed on the map in init().
 //
@@ -629,43 +625,6 @@ var castMap = map[oid.Oid]map[oid.Oid]cast{
 		oid.T_text:    {maxContext: CastContextAssignment, origin: contextOriginAutomaticIOConversion},
 		oid.T_varchar: {maxContext: CastContextAssignment, origin: contextOriginAutomaticIOConversion},
 	},
-	oid.T_unknown: {
-		// Unknown is the type of an expression that statically evaluates to
-		// NULL. NULL can be implicitly cast to any type.
-		oid.T_bit:          {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_bool:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_bpchar:       {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oidext.T_box2d:     {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_bytea:        {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_char:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_date:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_float4:       {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_float8:       {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oidext.T_geography: {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oidext.T_geometry:  {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_inet:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_int2:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_int4:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_int8:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_interval:     {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_jsonb:        {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_name:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_numeric:      {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_oid:          {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_regclass:     {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_regnamespace: {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_regproc:      {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_regprocedure: {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_regrole:      {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_regtype:      {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_text:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_time:         {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_timestamp:    {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_timestamptz:  {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_timetz:       {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_varbit:       {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-		oid.T_varchar:      {maxContext: CastContextImplicit, origin: contextOriginNullConversion},
-	},
 	oid.T_uuid: {
 		// Automatic I/O conversions to string types.
 		oid.T_bpchar:  {maxContext: CastContextAssignment, origin: contextOriginAutomaticIOConversion},
@@ -800,27 +759,6 @@ func init() {
 					srcStr, tgtStr,
 				))
 			}
-
-			// Casts from NULL are implicit.
-			if src == oid.T_unknown {
-				if ent.maxContext != CastContextImplicit {
-					panic(fmt.Sprintf(
-						"cast from %s to %s must be an implicit cast",
-						srcStr, tgtStr,
-					))
-				}
-				if tgt != oid.T_unknown && ent.origin != contextOriginNullConversion {
-					panic(fmt.Sprintf(
-						"cast from T_unknown to %s must have an origin of contextOriginNullConversion",
-						tgtStr,
-					))
-				}
-			}
-
-			// Only casts from NULL should use contextOriginNullConversion.
-			if ent.origin == contextOriginNullConversion && src != oid.T_unknown {
-				panic("contextOriginNullConversion should only be set for casts from unknown")
-			}
 		}
 	}
 }
@@ -843,6 +781,12 @@ func ValidCast(src, tgt *types.T, ctx CastContext) bool {
 		return true
 	}
 
+	// Unknown is the type given to an expression that statically evaluates to
+	// NULL. NULL can be cast to any type in any context.
+	if src.Oid() == oid.T_unknown {
+		return true
+	}
+
 	srcFamily := src.Family()
 	tgtFamily := tgt.Family()