sql: fix semantics of JSON ? operator on strings

Fixes cockroachdb#35001.

Release note (sql change): The semantics of the JSON '?' operator when
applied to JSON strings has been changed to match Postgres. Now, a JSON
string '?'s a string when the two are equal.

pkg/sql/logictest/testdata/logic_test/json

@@ -133,6 +133,28 @@ SELECT bar FROM foo WHERE bar ? 'a'
 ----
 {"a": "b"}
 
+query BBBBBBB
+VALUES (
+  '"hello"'::JSONB   ? 'hello',
+  '"hello"'::JSONB   ? 'goodbye',
+  '"hello"'::JSONB   ? 'ello',
+  '"hello"'::JSONB   ? 'h',
+  'true'::JSONB      ? 'true',
+  '1'::JSONB         ? '1',
+  'null'::JSONB      ? 'null'
+)
+----
+true  false  false  false  false  false  false
+
+query T
+SELECT bar FROM foo WHERE bar ? 'hello'
+----
+"hello"
+
+query T
+SELECT bar FROM foo WHERE bar ? 'goodbye'
+----
+
 query T
 SELECT bar FROM foo WHERE bar ?| ARRAY['a','b']
 ----

pkg/util/json/encoded.go

@@ -606,8 +606,13 @@ func (j *jsonEncoded) Exists(key string) (bool, error) {
 				return true, nil
 			}
 		}
+	default:
+		s, err := j.decode()
+		if err != nil {
+			return false, err
+		}
+		return s.Exists(key)
 	}
-	return false, nil
 }
 
 func (j *jsonEncoded) FetchValKeyOrIdx(key string) (JSON, error) {

pkg/util/json/json.go

@@ -1434,7 +1434,11 @@ func (jsonNull) Exists(string) (bool, error)   { return false, nil }
 func (jsonTrue) Exists(string) (bool, error)   { return false, nil }
 func (jsonFalse) Exists(string) (bool, error)  { return false, nil }
 func (jsonNumber) Exists(string) (bool, error) { return false, nil }
-func (jsonString) Exists(string) (bool, error) { return false, nil }
+
+func (j jsonString) Exists(s string) (bool, error) {
+	return string(j) == s, nil
+}
+
 func (j jsonArray) Exists(s string) (bool, error) {
 	for i := 0; i < len(j); i++ {
 		if elem, ok := j[i].(jsonString); ok && string(elem) == s {

pkg/util/json/json_test.go

@@ -695,7 +695,7 @@ func TestJSONExists(t *testing.T) {
 			{`baz`, false},
 		},
 		`["a"]`: {{``, false}, {`0`, false}, {`a`, true}},
-		`"a"`:   {{``, false}, {`0`, false}, {`a`, false}},
+		`"a"`:   {{``, false}, {`0`, false}, {`a`, true}},
 		`1`:     {{``, false}, {`0`, false}, {`a`, false}},
 		`true`:  {{``, false}, {`0`, false}, {`a`, false}},
 	}