Add null as a valid constant SELECT value for vtexplain.

go/vt/vtexplain/testdata/multi-output/selectsharded-output.txt

@@ -72,12 +72,12 @@ select name, count(*) from user group by name /* scatter aggregate */
 1 ks_sharded/c0-: select name, count(*) from user group by name limit 10001 /* scatter aggregate */
 
 ----------------------------------------------------------------------
-select 1, "hello", 3.14 from user limit 10 /* select constant sql values */
+select 1, "hello", 3.14, null from user limit 10 /* select constant sql values */
 
-1 ks_sharded/-40: select 1, 'hello', 3.14 from user limit 10 /* select constant sql values */
-1 ks_sharded/40-80: select 1, 'hello', 3.14 from user limit 10 /* select constant sql values */
-1 ks_sharded/80-c0: select 1, 'hello', 3.14 from user limit 10 /* select constant sql values */
-1 ks_sharded/c0-: select 1, 'hello', 3.14 from user limit 10 /* select constant sql values */
+1 ks_sharded/-40: select 1, 'hello', 3.14, null from user limit 10 /* select constant sql values */
+1 ks_sharded/40-80: select 1, 'hello', 3.14, null from user limit 10 /* select constant sql values */
+1 ks_sharded/80-c0: select 1, 'hello', 3.14, null from user limit 10 /* select constant sql values */
+1 ks_sharded/c0-: select 1, 'hello', 3.14, null from user limit 10 /* select constant sql values */
 
 ----------------------------------------------------------------------
 select * from (select id from user) s /* scatter paren select */

go/vt/vtexplain/testdata/selectsharded-queries.sql

@@ -11,7 +11,7 @@ select count(*) from user where id = 1 /* point aggregate */;
 select count(*) from user where name in ('alice','bob') /* scatter aggregate */;
 select name, count(*) from user group by name /* scatter aggregate */;
 
-select 1, "hello", 3.14 from user limit 10 /* select constant sql values */;
+select 1, "hello", 3.14, null from user limit 10 /* select constant sql values */;
 select * from (select id from user) s /* scatter paren select */;
 
 select name from user where id = (select id from t1) /* non-correlated subquery as value */;
@@ -31,4 +31,4 @@ select id, case when name = 'alice' then 'ALICE' when name = 'bob' then 'BOB' en
 select id, case when name = 'alice' then 'ALICE' when name = 'bob' then 'BOB' else 'OTHER' end as name from user where id = 1 /* select case */;
 select id, case when substr(name, 1, 5) = 'alice' then 'ALICE' when name = 'bob' then 'BOB' else 'OTHER' end as name from user where id = 1 /* select case */;
 
-select id, 'abc' as test from user where id = 1 union all select id, 'def' as test from user where id = 1 union all select id, 'ghi' as test from user where id = 1 /* union all */;
+select id, 'abc' as test from user where id = 1 union all select id, 'def' as test from user where id = 1 union all select id, 'ghi' as test from user where id = 1 /* union all */;

go/vt/vtexplain/vtexplain_vttablet.go

@@ -628,6 +628,9 @@ func inferColTypeFromExpr(node sqlparser.Expr, colTypeMap map[string]querypb.Typ
  colNames, colTypes = inferColTypeFromExpr(node.Expr, colTypeMap, colNames, colTypes)
  case *sqlparser.CaseExpr:
  colNames, colTypes = inferColTypeFromExpr(node.Whens[0].Val, colTypeMap, colNames, colTypes)
+ case *sqlparser.NullVal:
+ colNames = append(colNames, sqlparser.String(node))
+ colTypes = append(colTypes, querypb.Type_NULL_TYPE)
  default:
  log.Errorf("vtexplain: unsupported select expression type +%v node %s", reflect.TypeOf(node), sqlparser.String(node))
  }