sql: ban alter column type on view-depended cols

Previously, it was legal to edit the column type of columns that were
depended on by views. This is not a good idea for various reasons, and
is not Postgres compatible to boot.

Now, it is illegal to alter the column type of columns that are depended
on views.

Release note (sql change): prevent column type modification of columns
that are depended on by views.

pkg/sql/alter_column_type.go

@@ -62,6 +62,19 @@ func AlterColumnType(
 	cmds tree.AlterTableCmds,
 	tn *tree.TableName,
 ) error {
+	for _, tableRef := range tableDesc.DependedOnBy {
+		found := false
+		for _, colID := range tableRef.ColumnIDs {
+			if colID == col.ID {
+				found = true
+			}
+		}
+		if found {
+			return params.p.dependentViewError(
+				ctx, "column", col.Name, tableDesc.ParentID, tableRef.ID, "alter type of",
+			)
+		}
+	}
 
 	typ, err := tree.ResolveType(ctx, t.ToType, params.p.semaCtx.GetTypeResolver())
 	if err != nil {

pkg/sql/logictest/testdata/logic_test/alter_column_type

@@ -546,3 +546,20 @@ INSERT INTO t30 VALUES (e'a\\01');
 
 statement error pq: could not parse "a\\\\01" as type bytes: bytea encoded value ends with incomplete escape sequence
 ALTER TABLE t30 ALTER COLUMN x TYPE BYTES
+
+# Ensure that dependent views prevent column type modification.
+
+statement ok
+CREATE VIEW v AS SELECT x FROM t29
+
+statement error cannot alter type of column "x" because view "v" depends on it
+ALTER TABLE t29 ALTER COLUMN x TYPE INT2
+
+statement ok
+DROP VIEW v
+
+statement ok
+CREATE MATERIALIZED VIEW v AS SELECT x FROM t29
+
+statement error cannot alter type of column "x" because view "v" depends on it
+ALTER TABLE t29 ALTER COLUMN x TYPE INT2